Why Do We Eat? A Neurobiological Perspective. Part III

In the first post, I explained that all voluntary actions are driven by a central action selection system in the mesolimbic area (the reward system).  This is the part of you that makes the decision to act, or not to act.  This system determines your overall motivation to obtain food, based on a variety of internal and external factors, for example hunger, the effort required to obtain food, and the sensory qualities of food/drink.  These factors are recognized and processed by a number of specialized 'modules' in the brain, and forwarded to the reward system where the decision to eat, or not to eat, is made.  Researchers divide food intake into two categories: 1) eating from a true energy need by the body (homeostatic eating), e.g. hunger, and 2) eating for other reasons (non-homeostatic eating), e.g. eating for social reasons or because the food tastes really good.

In the second post of the series, we explored how the brain regulates food intake on a meal-to meal basis based on feedback from the digestive system, and how food properties can influence this process.  The integrated gut-brain system that accomplishes this can be called the satiety system.

In this post, we'll explore the energy homeostasis system, which regulates energy balance (energy in vs. energy out) and body fatness on a long term basis.

The Energy Homeostasis System

Read more »

Diet-Heart: Saturated Fat and Blood Cholesterol

Several hundred metabolic ward experiments have established beyond plausible doubt that isocaloric substitution of saturated fat for carbohydrate, polyunsaturated fat and monounsaturated fat raises serum (blood) LDL and total cholesterol (Fig. 1).1 Many of these experiments were carried out in locked facilities where the participants mode of life, energy balance and body weight were maintained, with the focus purely on isocaloric exchange of fat for starch or one kind of fat for another.2 This is one of the main pillars that the diet-heart hypothesis was built on.

Figure 1. Serum total cholesterol and dietary saturated fat in 395 solid food dietary and 32 liquid formula dietary experiments, by different experimental design

In Part I of this review, I addressed a misleading meta-analysis of prospective cohort studies examining the association between saturated fat intake and the risk of cardiovascular disease authored by Siri-Tarino et al. and Stephan Guyenet’s interpretation of this meta-analysis. In this part of the review I will address the evidence regarding the influence that saturated fat has on serum LDL and total cholesterol in humans and some of the primary arguments against the evidence that saturated fat raises serum cholesterol that Guyenet presented in a review of the literature that I previously commented on.3

One of the primary arguments against the evidence that saturated fat raises serum cholesterol that Guyenet presented is in regards to the duration of most of the metabolic ward experiments that are cited in the literature. For example, Guyenet stated: 

When references are provided, they nearly always point to the same type of study: short-term controlled diet trials, in which volunteers are fed different fats for 2-13 weeks and their blood cholesterol measured 

Guyenet’s statement is misleading as he appears to be emphasizing that there is a lack of long term controlled trials with findings that conform to the expectations from shorter term metabolic ward experiments. There have actually been a number of randomized controlled dietary trials with a duration of between one year and up to and beyond eight years that have demonstrated a significant decrease in serum LDL and total cholesterol, independent of weight loss when saturated fat was replaced with either carbohydrates or unsaturated fats. Examples include the National Diet-Heart Study and the Veterans Admin study, among many other trials included in a recent Cochrane review.4 It would be incorrect to suggest that detailed reviews neglect these long-term trials, although researchers may be more inclined to cite shorter-term metabolic ward experiments because of their more rigorous design. 

In the great majority of the metabolic ward experiments practically all of the serum cholesterol response to a change in dietary fat was completed within two to three weeks, and any further fluctuations in serum cholesterol can mainly be explained by intra- and interindividual variation, seasonal trends or laboratory error.2 Furthermore, in long term controlled trials the observed changes in serum LDL and total cholesterol and saturated fat intake actually conform to the expectations from the metabolic wards experiments that Guyenet attempts to downplay.5 6 This explains the universal agreement that intake of saturated fat (lauric, myristic and palmitic acids in specific) raises serum LDL and total cholesterol. 

Another one of the primary arguments against the evidence that saturated fat raises serum cholesterol that Guyenet presented is in regards to the null findings from certain observational studies. For example, Guyenet stated: 

…why do researchers almost never cite observational studies to support the idea that dietary saturated fat increases blood cholesterol? … One reason may be that in most instances, when researchers have looked for a relationship between habitual saturated fat intake and blood cholesterol, it has been very small or nonexistent.

There are actually several different types of observational studies that have studied the influence that saturated fat has on serum cholesterol. For example, secular trends have demonstrated a clear association between changes in saturated fat intake and changes in serum cholesterol within nations all-throughout the world, independent of other factors [reviewed previously]. 

The observational studies that Guyenet described in his review of the literature used cross-sectional methodology in which saturated fat intake and serum cholesterol are typically only measured at one specific point in time, and not repeatedly over a prolonged period over time. Guyenet’s suggestion that these studies that he cited examined ‘habitual saturated fat intake’ is unfounded and provides very scant evidence to suggest as he did that the findings from short term metabolic ward studies cannot be extrapolated to the long term. As Guyenet should already be aware, the shortcomings of the methodology used in the cross-sectional studies he cites was thoroughly addressed several decades ago and should no longer be considered controversial.7 8 9 10 11 A number of these shortcomings will be addressed here as Guyenet has conveniently failed to address them.

The Problem of Interindividual Variability

One of the greatest shortcoming of cross-sectional studies which has contributed towards unnecessary confusion has been caused by the lack of consideration  of the interindividual (between-individual) variability of serum lipids in response to diet. Steinburg has described how this shortcoming can obscure the findings in cross-sectional studies:12 

One explanation is that the variability from person to person in terms of response to dietary factors is so great that it dilutes out the correlation, as in one of Ahrens’ metabolic ward protocols, almost all would show the same qualitative responses but to different degrees... [Jacobs et al.] pointed out the large variability encountered from individual to individual in responses to changes in dietary fat. Jacobs et al. used a mathematical model to show that a zero correlation is what you would actually expect. They also applied their model to a set of experimental data and showed that, again, a zero correlation might be expected. In short, a zero correlation in a population study does not necessarily negate the possible role of dietary fat in helping to determine plasma cholesterol levels in individuals.

Table 1 provides a good example of the significant between-individual variability of serum lipids in response to diet in reference to the National Diet-Heart Study.12 

Table 1. Percentage of subjects with indicated percentage drop in serum cholesterol in the National Diet-Heart Study

This shows that even if the investigators know the exact composition of a person’s diet they will not be able to predict their level of serum cholesterol, but will be able to predict whether that person’s serum cholesterol will go up or down if that person modifies their dietary intake. As the association between diet and coronary heart disease in part relies upon serum lipids, this suggests that the between-individual variability of serum lipids in response to diet would likely have obscured the findings from the prospective cohort studies addressed in Part I of this review.8 

The Problem of Dietary Modification

Another important shortcoming of cross-sectional studies that can obscure the association between diet and serum cholesterol in observational studies can be caused by the participants decreasing their intake of saturated fat in response to unfavorable serum lipid concentrations. This has actually been shown to bias the cross-sectional association between saturated fat and serum cholesterol towards a negative association. This problem was addressed by Shekelle et al. in a paper from the Chicago Western Electric Company study:9

In the Chicago Western Electric Company study, diet was assessed at the initial examination, in 1957-1958, of 1900 middle-aged men and again at their second examination about one year later. At the first examination, lipid composition of the diet, as summarized by a score based on the formula of Keys, Anderson and Grande, was positively associated with level of serum cholesterol. Between the first and second examinations, however, hypercholesterolemic men were more likely than others to have reduced intake of dietary saturated fatty acids and cholesterol. As a result, at the second examination the cross-sectional linear association between the diet score and serum cholesterol concentration was significantly positive for men with initial levels of serum cholesterol less than 250 mg/dl, significantly negative for men with initial levels of 250 mg/dl or higher and not significantly different from zero for all men together.

Guyenet failed to mention this limitation despite referring to the Chicago Western Electric Company study in his review. Guyenet claimed that although this study found a positive association between saturated fat and serum cholesterol, there was no association with heart attack deaths. Guyenet failed to mention that the lack of a significant association with heart attack deaths may be explained at least in part by the fact that the researchers actually adjusted for serum cholesterol, and because the participants with elevated serum cholesterol were likely to have reduced their saturated fat intake. This study nevertheless found that saturated fat was associated with an 11% non-significant increased risk, and dietary cholesterol and the Keys score with a statistically significant increased risk of  fatal coronary heart disease.13 Furthermore, this study also found that dietary cholesterol was associated with a statistically significant increased risk of all-cause mortality, a finding consistent with several other studies including the Nurses’ Health Study.14 15 16

The purpose of the paper from the Health Professionals Follow-up Study that Guyenet cited was to address the influence of saturated fat intake on the risk of coronary heart disease, not on serum cholesterol. The researchers were aware that the health professionals, of which most had previously measured their serum cholesterol may have made dietary modifications if they were previously aware of having elevated cholesterol, and therefore excluded any participants with a baseline history of elevated cholesterol.17 The fact that electric company employees modified their diet in response to unfavorable serum lipids in the 1950’s strongly suggests that health professionals in the 1980’s would also have made dietary modifications, thus explaining the unexpected distribution of saturated fat intake of participants with baseline history of elevated serum cholesterol in this study.

The Problem of Intraindividual Variability

Another important shortcoming of cross-sectional studies that can obscure the association between diet and serum cholesterol is caused by intraindividual (within-individual) variability of serum lipids and dietary intake. Hegsted and Nicolosi showed that the spontaneous within-individual variability of serum cholesterol even when consuming a constant diet is relatively large, typically varying by 5 to 10%.11 It has been estimated that three samples of blood are required in order estimate the true serum cholesterol of an individual, assuming that standardized methods are being used. Even just sitting from a standing position has been shown to decrease serum cholesterol concentrations by 6%.18

Balogh et al. concluded that in order to estimate within 20% of the actual dietary intake of 90% of a studied population, there is a requirement of at least 22 days of 24-hour dietary recalls for saturated fat.19 Inaccurately measuring within-individual variability in dietary intake has been shown to lead to a miss-classification of subjects into ranges of usual dietary intakes, biasing correlation coefficients towards null.20 Liu et al. showed that just a single measure of a dietary variable and a single measure of serum cholesterol as was done in the Tecumseh Study reduces the correlation coefficient by 50%, and is therefore almost meaningless.7 It should be noted that almost half of the cross-sectional studies that Guyenet cited used only a single 24-hour recall to assess dietary intake, including the Tecumseh Study as well as the Evans Country study, the Bogalusa Heart Study, and the Japan-Honolulu-San Francisco study.20 21

Jacobs et al. demonstrated the problem of intraindividual variability on a graph (Fig. 2), showing how the predicted association between diet and serum cholesterol becomes obscured by the introduction of typical variations in serum cholesterol and diet.8

Figure 2. A: The open circles represent the relationship that exists in five typical individuals between diet score and predicted serum cholesterol. Vertical arrows represent random variations in serum cholesterol and horizontal arrows random variations in diet scores. B: The open circles demonstrate how the perfect correlation in A is completely obscured by the introduction of typical sources of intraindividual variability.

Excluding the Health Professional Follow-up Study, in the two cross-sectional studies that Guyenet cited with the largest number of participants, the Israel Ischemic Heart Disease Project (n=8829) and the Japan-Honolulu-San Francisco study (n=9844), when the researchers considered the participants as groups rather than as individuals in order to minimize intraindividual variability from obscuring the results, saturated fat was highly significantly and positively associated with serum cholesterol.22 23 Similar results were also found in the Seven Countries Study (n=12,700).18 The findings from these studies actually conform closely to the expectations from the metabolic ward experiments that Guyenet attempts to downplay.

It should be noted that each of these studies alone studied a significantly larger number of participants than all of the remaining cross-sectional studies cited by Guyenet combined. Of these remaining studies the Chicago Western Electric study which made up approximately half of the sample size (n=1900) also found a statistically significant association. Furthermore, Guyenet failed to cite at least half a dozen other cross-sectional studies that studied between 650 and 23,000 participants that found a significant association between saturated fat and serum cholesterol.18 24 25 Guyenet has since updated his post to note that he was made aware of a number of studies that he missed but nevertheless claimed that they are largely consistent with his conclusions.

In the review, Guyenet made several misleading statements in regards to the data from the Framingham study: 

One of the longest-running, most comprehensive and most highly cited observational studies, the Framingham study was organized by Harvard investigators and continues to this day. When investigators analyzed the relationship between saturated fat intake, serum cholesterol and heart attack risk, they were so disappointed that they never formally published the results. We know from multiple sources that they found no significant relationship between saturated fat intake and blood cholesterol or heart attack risk

Guyenet cited a 1976 paper from the Tecumseh study as a reference for his findings from the Framingham study, but failed to mention that the Framingham study has produced many formal publications since this paper was published. In 1992 Posner et al. published a paper from the Framingham study addressing the association between saturated fat and heart attack risk, however this study used a single 24-hour food recall and therefore is of limited value. In this study the younger but not older cohort saturated fat was associated with a marginally significant increased risk of coronary heart disease.26

The findings described in the paper from the Tecumseh study of the failure to find a significant association between diet and serum cholesterol in the Framingham study may have addressed an early publication of Framingham which excluded participants with very high or low serum cholesterol concentrations which could have been influenced by diet. A reanalysis of all of the men’s data in 1992 resulted in a larger sample size and range of serum cholesterol concentrations found a significant association between dietary cholesterol and total fat intake and serum cholesterol.18 However, the more recent analysis of women from the Framingham study produced contrasting findings for dietary cholesterol, which resulted in much confusion that has been provoked by the likes of Atkins and Sugar Busters. Dr. William Castelli, a former director of the Framingham Heart Study responded to this misuse of the Framingham study data:27

The data are diet history data. Very weak science!!!... Better science, where I lock you up in a metabolic ward has taught us that lowering the saturated fat, the cholesterol in the diet lowers cholesterol.

The Framingham study is well respected in part for enhancing the medical communities knowledge of risk factors for coronary heart disease including the Framingham Risk Score, not for the use of high quality dietary assessment methods. As suggested by Castelli, the dietary data from the Framingham study should not be misused as Guyenet has done in order to attempt to downplay the findings from metabolic ward experiments.

In response to my comments regarding the concerns about the quality of dietary assessment methods used in the cross-sectional studies that Guyenet cited in his review, he replied stating: 

You like good measurement methods, so let's focus on the two studies that did 7-day weighed food records, the Bankers study and the Caerphilly study. The first found no association between animal fat intake and serum cholesterol across a several-fold variation in intake. The second found a significant but extremely weak association between SFA intake and serum cholesterol. "The percentage of variance in the plasma lipid concentrations which could be explained solely by the dietary variables was very small, ranging from 1 to 7 per cent". That sounds quite consistent with the conclusions of my post.

As already explained, a near null finding is what the expected result would be in these cross-sectional studies regardless of the quality of the dietary and lipid assessment methods used due to the problem of between-individual variability. However, a significant limitation of the British bank men study (n=99), the smaller of the two studies Guyenet refers to was that there was a prolonged time period between the dietary survey and the blood sample in a portion of the studied men. In Table 2 the first column shows the data on all the men in the survey, and column two shows the data only on those men whose blood samples were taken either during or within one day of completing the dietary survey.7 

Table 2. Serum cholesterol and dietary factors in all men compared to men whose cholesterol was measured close to the dietary survey in the British bank men study

It is clear that the correlation between animal fat and serum cholesterol was a lot stronger in those men whose blood samples were taken during or within one day of completing the dietary survey, suggesting that the results from the ‘All Men’ group was obscured by dietary changes made in the prolonged time period between the dietary survey and the blood sample in some of the men. Furthermore, it was estimated that the combined within-person variability for dietary factors and serum cholesterol reduced the correlation by 35-40%.7 

In regards to the Caerphilly study (n=653), the larger of these two studies, Guyenet appears to be extrapolating the conclusions of the authors that the between-individual variability in diet explained only a small portion of the between-individual variability in the serum cholesterol levels of the studied population to claim that the association between saturated fat and serum cholesterol although statistically significant was ‘extremely weak’. The authors of this study argued in a later paper that studies on homogeneous populations consuming a diet uniformly high in saturated animal fat such as their own, the lack of a contribution of between-individual variability in saturated animal fat intake to coronary heart disease may merely reflect 'the absence of clear differences in mean intakes of saturated fatty acids'.28 As the between-individual variability in animal fat intake was small it is to be expected that animal fat would only explain a small portion of the between-individual variability in serum lipids, and that other factors including genetics may explain a larger portion of the variance. In the Seven Countries Study where saturated fat ranged from 3% to about 22% of calories, saturated fat intake explained approximately 89% of the variance in serum cholesterol between the 16 cohorts.18 Although animal fat intake may not explain much of the variation in serum cholesterol between individuals in the Caerphilly study, it still may have largely explained the uniformly high serum cholesterol of the studied population as a whole.

When critically examined, the findings from these studies are 'quite consistent' with the diet-heart hypothesis, not with Guyenet’s unfounded conclusions. Furthermore, these were not the only studies that I informed Guyenet of that used a 7-day food record. The EPIC-Norfolk study included a 7-day food record from 6,416 participants which according to the researchers found a similar association between diet and serum lipids as the food frequency questionnaire from 22,914 participants which was used to gain sufficient power for the analysis by separate genotypes. This study found that saturated fat was associated with a significantly higher serum LDL and total cholesterol for all four individual genotypes studied.24 

If Guyenet is to suggest that observational studies that use multiday food records are the highest quality form of observational evidence to test the diet-heart hypothesis, then high quality observational studies support both the hypothesis that saturated fat raises serum cholesterol and increases the risk of coronary heart disease. As Stamler described in regards to the Siri-Tarino et al. meta-analysis of prospective cohort studies and the risk of saturated addressed in Part I of this review:29

Five used dietary history or multiday food record; the RR was >1.00 in all 5 studies, even though 3 were adjusted for serum or dietary lipids.

Plant Positive has produced an informative video addressing measurement error in the newly released YouTube series Nutrition Past and Future, which provides further details addressing how intra- and interindividual variability can obscure the findings between diet and risk factors and heart disease in reference to the hand-picked observational studies that Gary Taubes included in his book, Good Calories Bad Calories. Coincidentally the observational studies addressing saturated fat and serum cholesterol included in Good Calories Bad Calories are mostly the same studies that Guyenet included in his review.

Cholesterol Confusion 7 The Measurement Problem

Longitudinal Observational Studies

Several prospective cohort studies that measured diet and serum cholesterol multiple times over several years have also found that changes in saturated fat intake was associated with changes in serum cholesterol.30 31 These studies are more informative than cross-sectional studies which typically only measure diet and serum cholesterol at a single point of time making them more prone to being obscured by intra- and interindividual variability.

The Blue Mountain Eyes Study was a prospective cohort study that measured diet and serum cholesterol three times throughout the 10 year follow-up, and found a statistically significant association between changes in saturated fat intake and serum cholesterol. As expected due to the problem of intra- and interindividual variability, the cross-sectional analysis of only the baseline data showed a weak non-significant association between saturated fat intake and serum cholesterol.30 In response to my comments regarding the positive association between saturated fat and serum cholesterol in this study, Guyenet responded stating:

Contrary to what you said, the Blue Mountain Eye study did not find any association whatsoever between SFA intake and TC/LDL. What they found was an association between TC/LDL and a CHANGE in SFA intake over time. That result is difficult to interpret due to the fact that in the same study, people who consistently ate more SFA didn't have higher TC/LDL than those who consistently ate less. You seem to have a tendency to misrepresent findings to support your position. I want to be clear with you, I will not tolerate that on this blog.

It is clear that Guyenet has again confused the cross-sectional analyses that examined only baseline saturated fat intake with habitual intake. The majority of the participants actually modified their intake of saturated fat to a substantial degree during the two subsequent assessments of dietary intake, demonstrating that the baseline measurement of diet as being a very poor marker of ‘people who consistently ate more SFA’.30 It is unfortunate that Guyenet is unwelcome to genuine criticisms of his arguments against the diet-heart hypothesis and his stance on this matter suggests that any further debate on this topic would likely have proved to be meaningless.

Isocaloric Substitution of Macronutrients

In reference to the meta-analysis of feeding experiments by Katan et al. Guyenet also attempted to downplay the findings by stating: 

As a side note, many of these studies were of poor quality, and were designed in ways that artificially inflated the effects of saturated fat on blood lipids. For example, using a run-in period high in linoleic acid, or comparing a saturated fat-rich diet to a linoleic acid-rich diet, and attributing the differences in blood cholesterol to the saturated fat. Some of them used hydrogenated seed oils as the saturated fat.

The claim that the meta-analysis authored by Katan et al. used ‘hydrogenated seed oils as the saturated fat’ is unfounded. Katan et al. clearly stated that one of the inclusion criteria for the experiments was that 'Food intake had to be thoroughly controlled and described’ and were also clear that diets rich in hydrogenated fat were analyzed separately.32 In a later meta-analysis of controlled experiments, Katan et al. found that trans fats from industrial hydrogenated fats and from ruminant sources found in meat and dairy both raise the ratio of serum LDL to HDL cholesterol.33 Such findings however has not had any  appreciable effect on the dietary recommendations of the cholesterol skeptics.

As emphasized in Part I on this review, a decrease of saturated fat intake suggests an increase in intake of other sources of energy in order to maintain caloric balance, and therefore examining saturated fat intake is meaningless without considering what sources of energy it is replacing. This is clearly why metabolic ward experiments compare saturated fat with other macronutrients. The purpose of the experiments that Guyenet refers to as being of ‘poor quality’ was to examine the effect of the isocaloric substitution of saturated fat for linoleic acid, not to ‘artificially inflate the effects of saturated fat on blood lipids’. Metabolic ward experiments have also demonstrated that isocaloric substitution of saturated fat for monounsaturated fat and carbohydrates also raises serum cholesterol but less appreciatively than when replaced for linoleic acid.1 32

As shown in Figure 1, saturated fat raised serum cholesterol in each type of solid diet experimental design to a similar degree.1 It is not these tightly controlled metabolic ward experiments that were of poor quality,  but it is Guyenet’s interpretation of these experiments that is of poor quality and it is Guyenet who has selected studies that artificially deflate the effects of saturated fat on blood lipids.

Criticisms of the Diet-Heart Hypothesis

Stephan Guyenet specifically chose to limit the focus of his review of the literature to a study design that he should have been well aware has near zero statistical power to detect a relationship between diet and serum cholesterol even if one existed. Guyenet should also have been aware that the majority of his audience are unaware of these design flaws and are therefore are likely to regard his conclusions as fact. These criticisms presented here also apply to other cholesterol skeptics, including Michael Eades and Chris Kresser who cited Guyenet's review as evidence to downplay the diet-heart hypothesis.34 35

The criticisms of the diet-heart hypothesis largely rely on the misinterpretation of studies that have near zero statistical power to detect a statistically significant relationship or suffer from significant methodological issues.

Diet-Heart Posts

Part I - Diet-Heart: A Problematic Revisit
Part III - Diet-Heart: The Role of Vegetarian Diets in the Hypothesis
Part IV - Cracking Down on Eggs and Cholesterol
Part V - Cracking Down on Eggs and Cholesterol: Part II

Please post any comments in the Discussion Thread. 

Diet-Heart: A Problematic Revisit

In 2010, Jeremiah Stamler published the editorial Diet-heart: a problematic revisit in the American Journal of Clinical Nutrition addressing a number of very serious flaws in a meta-analysis paper supported by the National Dairy Council and authored by Siri-Tarino et al. that concluded that there was insufficient evidence from prospective cohort studies to suggest that the intake of saturated fat increases the risk of coronary heart disease and cardiovascular disease.1 A number of researchers including Stamler, who has played a prominent role in the diet-heart hypothesis for over 60 years found that a number of serious flaws in this meta-analysis would have likely biased the association between saturated fat and coronary heart disease towards null.1 2 3

In the editorial Stamler produced a meta-analysis based on the same papers included in the Siri-Tarino et al. meta-analysis and calculated that saturated fat was associated with a 32% increased risk of fatal coronary heart disease, an end point ignored, perhaps intentionally by the authors of the original meta-analysis.1 It would seem almost implausible for anyone citing the Siri-Tarino et al. meta-analysis with an interest in saving lives to fail to mention the findings for this fatal end point, the single most leading cause of death in the world.4 Perhaps the cholesterol skeptics do not share Stamler’s interest of saving lives, explaining why they have chosen to refrain from informing their audience of Stamler’s findings. 

Another shortcoming of the Siri-Tarino et al. meta-analysis paper was the lack of acknowledgement in the assessments and conclusions that major cross-population studies with a prospective (future looking) design, such as the Seven Countries Study found that saturated fat was associated with a significantly increased risk of fatal coronary heart disease (Fig. 1).1 5 Consistent with the trend of the findings from the Seven Countries Studies, the nomadic Kirghiz plainsmen who subsist on a diet of enormous amounts of organic grass-fed milk and meat experience severe vascular disease at a very young age [reviewed previously].

Figure 1. Saturated fat as % of calories and fatal coronary heart disease in 16 cohorts from the Seven Countries Study

In this series of posts I will review the diet-heart hypothesis and the arguments against the hypothesis raised by known cholesterol skeptics. Note that in this review the diet-heart hypothesis refers to the hypothesis that dietary change, such as the substitution of individual dietary fats for carbohydrate influences serum (blood) lipids (including serum total and LDL cholesterol), and therefore at the very least indirectly influences the risk of developing coronary heart disease.

Stephan Guyenet, the author of the Whole Health Source blog has produced some very informative posts dispelling Gary Taubes’s misleading claims regarding carbohydrate metabolism, insulin and obesity.6 Unfortunately, like Taubes rather than embracing the preponderance of evidence that has established the diet-heart hypothesis, Guyenet has chosen to confuse the subject in a series of blog posts. I have previously commented on Guyenet’s blog regarding one such concerning post in May 2011 where I raised my concerns regarding Guyenet’s arguments against the evidence that saturated fat raises serum cholesterol, and increases the risk of coronary heart disease.7  

One of my main concerns I presented in my comments on Guyenet’s blog was his lack of acknowledgement that saturated fat was associated with an increased risk of fatal coronary heart disease in the Health Professional’s Follow-up Study and in Stamler’s meta-analysis. Guyenet was less than appreciative of these comments, stating:

I find it disturbing that you continue to cite the Health Professionals follow-up study to support your position despite the fact that there was no statistically significant association between SFA intake and any measure of CHD after maximum adjustment. If there were really a relationship between the two factors, you wouldn't have to cite non-significant findings to support your position.

In the paper from the Health Professional’s Follow-up Study cited by Guyenet, for men in the top verses the lowest fifth of saturated fat intake the relative risk for fatal coronary heart disease was 1.72 (95% confidence interval 1.01 to 2.90) after maximum adjustment.8 In other words this study found that saturated fat intake was associated with a statistically significant 72% increased risk of fatal coronary heart disease for high compared to low intake. Guyenet avoided directly responding to my comments regarding the finding in Stamler’s meta-analysis for fatal coronary heart disease, and simply referred back to the Siri-Tarino et al. meta-analysis which failed to address this fatal end point.

In this particular post that I commented on, Guyenet made several misleading statements in reference to the findings from the Siri-Tarino et al. meta-analysis: 

Nearly every high-quality (prospective) observational study ever conducted found that saturated fat intake is not associated with heart attack risk. So if saturated fat increases blood cholesterol, and higher blood cholesterol is associated with an increased risk of having a heart attack, then why don't people who eat more saturated fat have more heart attacks?

Apart from the failure to acknowledge that Stamler demonstrated that the cohort studies included in this meta-analysis found that saturated fat intake was actually associated with an increased risk of fatal coronary heart disease, there are several other points in this statement that are problematic that will be addressed separately.

The Problem of Overadjustment 

Guyenet’s suggestion that the Siri-Tarino et al. meta-analysis should have found a positive association between saturated fat and coronary heart disease if saturated fat raises serum cholesterol and serum cholesterol increases the risk of coronary heart disease is misleading. One of the most serious flaws in this meta-analysis was the inclusion of overadjustments for serum lipids and dietary lipids, which would have obscured this diet-cholesterol-heart relationship that Guyenet referred to. Stamler addressed this flaw in the editorial:1

…the issue of whether SFA relates to CHD in univariate analyses is relevant. If findings on this subject are positive but the association is markedly reduced or ceases in multivariate analyses, this may be due to confounding (eg, by dietary cholesterol) and/or overadjustment (eg, by inclusion in analyses of serum total or LDL cholesterol, a major CHD risk factor influenced by SFA intake)… Of 15 studies that unequivocally concern the SFA-CHD relation, 4 did not include other dietary lipids or serum lipids among covariates. Their CHD relative risks (RRs) ranged from 1.22 to 2.77—ie, >1.07, which was the estimated CHD RR in the meta-analysis. Do these larger RRs reflect freedom from confounding and overadjustment?

As Stamler demonstrated, the studies that that did not include overadjustments for dietary and serum lipids were more likely to find a positive association between saturated fat and coronary heart disease, reaffirming that the influence that saturated fat has on coronary heart disease is partly dependent on serum lipids.

The Problem of Dietary Assessment Methods 

Guyenet’s suggestion that the Siri-Tarino et al. meta-analysis found that the majority of high-quality prospective studies failed to find an association between saturated fat and heart disease is also misleading. Another serious flaw in the meta-analysis was the overreliance on poor quality dietary assessment methods, which was addressed by Katan et al.:2

A major weakness of the meta-analysis is the imprecision of dietary assessment methods used in the underlying studies. About half of the studies used 1-d dietary assessments or some other unvalidated method. Food intake varies from day to day, and there is a substantial literature showing that a single 24-h recall provides a poor estimation of the usual dietary intake of an individual. Such methods cannot reliably rank individuals by their long-term intake, especially within populations with a uniformly high saturated fat intake. Such imprecision in the assessment of disease determinants systematically reduces the strength of association of determinants with the disease. This is referred to as attenuation or regression dilution bias.

Stamler noted that the studies included in the Siri-Tarino et al. meta-analysis that used more precise dietary assessment methods were more likely to find a positive association between saturated fat and coronary heart disease:1

...the meta-analysis reported its findings as independent of a quality score including diet assessment. Of the 16 CHD studies, 4 relied on one 24-h dietary recall; the SFA-CHD RR was >1.00 for only one of these studies. Seven used a food-frequency questionnaire (FFQ); the RR was >1.00 in 3 of these studies. Five used dietary history or multiday food record; the RR was >1.00 in all 5 studies, even though 3 were adjusted for serum or dietary lipids. These facts, which were unnoted in the meta-analysis, prompt the question: Did low-level reliability (reproducibility) of dietary SFA data drive RR values toward 1.00 (the regression-dilution bias problem)? No data on SFA reliability are given

It could actually be concluded from this data that the majority of the cohort studies that used ‘high-quality’ dietary assessment methods, in particular those that did not include overadjustments for dietary and serum lipids found that saturated fat was associated with an increased risk of coronary heart disease.

In the Seven Countries Study dietary intake was measured with high quality assessment methods including a seven day food record and for a subsample of participants the diets were also chemically analyzed.5 Another strength of the Seven Countries Study is that dietary intake was assessed between groups of individuals which has been shown to result in less measurement error than assessing dietary intakes between individuals as was done in the cohort studies included in the Siri-Tarino et al. meta-analysis.9 A further strength of the Seven Countries Study was that saturated fat intake ranged from 3% to about 22% of calories, a far greater range than the studies on mostly homogeneous populations included in the Siri-Tarino et al. meta-analysis, providing greater statistical power to detect a significant relationship. 

The Problem of Dietary Modification

Stamler also addressed the problem related to participants making voluntary dietary changes, including the reduction of saturated fat intake in response to elevated serum cholesterol that could have also obscured the findings of the Siri-Tarnio et al. meta-analysis:1

Also, the meta-analysis says nothing about the problem for the 16 studies of possible bias in SFA-CHD findings due to dietary change (eg, reduced SFA intake) in people with higher serum total cholesterol seeking to lower total cholesterol/CHD risk (as occurred for the earliest of the 16 studies).

Even over 50 years ago in the Chicago Western Electric Company study, the earliest of the studies included in the Siri-Tarino et al. meta-analysis, participants were reducing intake of saturated fat and dietary cholesterol in response to unfavourable serum cholesterol concentrations.10 In studies where participants measured their lipid profile and subsequently lowered saturated fat intake in response to unfavourable results before entering the study, the saturated fat intake of these potentially high risk participants measured during the study could have been significantly lower than their lifetime averages. This could have resulted in an artificial increase in number of coronary events in the groups of participants classified as having a low intake of saturated fat. Similarly, the participants who lowered saturated fat intake in response to unfavourable serum lipids after completing their dietary assessment for the study may have artificially lowered the number of coronary events in the groups of participants classified as having high intake.

Few studies included in the Siri-Tarino et al. meta-analysis adequately addressed this problem, with the Health Professionals Follow-up Study perhaps being one of these few.8 In addition to the problem of imprecise dietary assessment methods, this problem further obscures the classification of the participants ranges of usual saturated fat intake potentially biasing the findings further towards null.11 Another problem that could have potentially obscured the findings in these studies, especially those that lasted into the statin era is that participants with higher serum cholesterol as a result of a high saturated fat intake maybe more likely to have received aggressive medical intervention in order to prevent cardiovascular disease. It should be emphasized here that Stamler found in a meta-analysis that saturated fat was associated with a 32% increased risk of fatal coronary heart disease despite such problems.1

The Problem of the Comparison Group

Guyenet’s suggestion that the Siri-Tarino et al. meta-analysis addressed the ‘association’ between saturated fat and coronary heart disease independent of other caloric sources is also misleading, a point that was addressed by Katan et al.:2

First, the notion that there exists such a thing as “the effect of saturated fat” is flawed. A lower intake of saturated fat implies an increased intake of some other source of calories to maintain caloric balance. Different substitutions for saturated fat have different effects on risk of coronary heart disease (CHD) and need to be discussed separately.

One of the greatest contributors to unnecessary confusion in nutritional research has resulted from studies that failed to compare foods or macronutrients with other suitable sources of energy. The majority of participants studied in developed nations typically consume only negligible amounts of whole plant foods, and therefore a lower intake of one particular food typically results in a higher intake of other processed or animal foods.12 Without giving this important fact careful consideration most foods that are less than optimal for human health will appear harmless in studies as they are typically compared with other unhealthy foods. This problem was elaborated on in a research panel including Ronald Krauss, the senior researcher of the Siri-Tarino et al. meta-analysis:13

For example, it may not be useful, as is usually done, to compare a specific food to all other sources of energy, which are usually mainly refined starches, sugars, red meat, and fat-rich dairy products in typical Western diets.

Hu FB and Sun Q, two of the authors of the Siri-Tarino et al. meta-analysis also addressed this shortcoming of the meta-analysis in a paper they co-authored, describing what sources of energy saturated fat was substituted for:14

…however, in this meta-analysis saturated fat was compared with other calorie sources, primarily refined carbohydrates, and high intake of refined carbohydrates has been associated with a high risk of CHD.

As the Siri-Tarino et al. meta-analysis failed to find a lower risk of saturated fat compared primarily to foods rich in refined carbohydrates even after adjusting for serum lipids, these findings hardly justify increasing the intake of saturated fat any more than they do to increasing the intake of refined carbohydrates. As expected from these findings, meta-analysis and systematic reviews that compare foods to all other sources of energy combined have also failed to find a clear association between refined grains and cardiovascular disease and all-cause mortality, even without the inclusion of such significant overadjustments.15 16 If Guyenet and the other cholesterol skeptics applied the same methodology they use to judge the health properties of saturated fat to all foods, they would not be able to justify their recommendation of limiting intake of refined grains in order to reduce the risk of cardiovascular disease and other non-communicable diseases. 

In Guyenet’s post I commented in regards to a pooled analysis of 11 large prospective cohort studies which found that replacing 5% of energy from saturated fat with an equivalent of polyunsaturated fat was associated with a 26% decreased risk of coronary heart disease mortality.17 Here again Guyenet was less than appreciative of such comments, stating: 

That's not how epidemiology works. What you do is you examine if people who eat more SFA have more heart attacks than people who eat less, while controlling for other variables-- and the studies have nearly all found no association. That's how epi works in other disciplines. Moving the goalposts to Keys score, SFA/PUFA ratios and using fancy math to model nutrient substitutions will only fool people who don't know any better or are desperate to believe that there's an association.

It appears that Guyenet is either desperately trying to confuse his audience or is suggesting that a change in saturated fat intake in the general population will not influence the intake of any other sources of energy for which he has provided no evidence for. Examining saturated fat intake is meaningless without considering what sources of energy it is replacing, which is why models of macronutrients substitution is preferred. The study of nutrition epidemiology is different than the study of other exposures such as tobacco smoke in the respect that energy is required in order to maintain life, and therefore it essential in nutrition science to compare one source of calories with suitable alternatives. 

Additional Findings from Observational Studies

Even if one were to judge the health properties of saturated fat on the basis of findings from prospective cohort studies that compared saturated fat intake with all other sources of energy combined, it would still be implausible to conclude that saturated fat is not disease promoting. The finding from Stamler’s meta-analysis that saturated fat intake was associated with a 32% increased risk of fatal coronary heart disease by itself is a cause for concern, however other findings from cohort studies also raise significant concern. 

A meta-analysis of 12 cohort studies of 418,816 women found that saturated fat intake was associated with an increased risk of breast cancer, consistent with the findings from more recent cohort studies of 319,826 and 188,736 women.18 19 20 In addition, a pooled-analysis of 12 cohort studies of 523,217 women found that a high intake of saturated fat was associated with an increased risk of ovarian cancer.21 Furthermore, a large cohort of 525,473 men and women found that saturated fat intake, especially that from animal sources increased the risk of pancreatic cancer and a cohort of 494,000 men and women found that saturated fat intake was associated with a greatly increased risk of small intentional cancer.22 23 Another cohort study of 137,486 women found that saturated fat intake was associated with an increased risk of hip fracture, consistent with other lines of evidence [reviewed previously].24

Siri-Tarino et al. excluded cohort studies of type II diabetics patients from the meta-analysis which should be addressed. Two such studies found a very strong association between saturated fat and cardiovascular disease, including the Nurses’ Health Study which also found a significant association for dietary cholesterol and the Keys score.25 26 Furthermore, although typically considered lower in the hierarchy of evidence than prospective cohort studies, a number of case-control studies have also found a positive association between saturated fat and coronary heart disease.27 28 29 30

In regards to the association between saturated fat intake and the risk of stroke, the Siri-Tarino et al. meta-analysis failed to address the possible influence that blood pressure has on the association between saturated fat and the risk of stroke despite finds from large cohort studies including the Nurses’ Health Study that suggest the association is dependent on blood pressure. Without consideration of these important details the Siri-Tarino et al. meta-analysis should not be considered as providing a clear interpretation of the association between saturated fat intake and the risk of stroke. I have addressed this matter in further detail in Part I and Part II of a review addressing blood pressure, blood cholesterol, diet and the risk of stroke, which also addresses the Northern Manhattan Study which found that saturated fat was associated with a trend towards an increased risk of ischemic stroke that was excluded from the Siri-Tarino et al. meta-analysis despite apparently meeting the requirements for the inclusion criteria.31

The Problem of Reductionism

The disease promoting properties of saturated animal fat cannot be ascribed purely to the substitution of saturated fat for other macronutrients, but also to other nutritional factors including the content of dietary cholesterol, ruminant trans-fat and the lack of dietary fiber and other phytonutrients. For example, a study on an apparently health conscious population included in the Siri-Tarino meta-analysis found that while saturated fat was associated with 2.77 fold increased risk of coronary heart disease which was the value used in the meta-analysis, the association for animal fat was even stronger, a 3.29 fold increased risk.32

In the pooled analysis of 11 large cohort studies, compared to saturated fat, monounsaturated fats which was predominantly derived from animal fat was associated with the greatest increased risk of coronary events out of all the studied macronutrients. Furthermore this pooled analysis adjusted for dietary fiber, dietary cholesterol and possibly ruminant trans-fat, which also needs to be taken into consideration as this could have potentially underestimated the adverse effects of increasing saturated animal fat intake at the expense of whole plant foods.33

As foods contain not only macronutrients but also tens of thousands of different bioactive constituents which can potentially influence health, it would therefore be more informative to compare the effect of substituting different foods rather than isolated macronutrients on disease outcomes.34 35 Arguably the highest quality prospective cohort study to have published a paper addressing the substitution of foods on the risk of coronary heart disease was the Nurses’ Health Study (Fig. 2).14 As suggested by this study, the benefits of replacing animal foods with whole plant foods to lower the risk of coronary heart disease can be explained partly but not entirely by the displacement of saturated fat with other macronutrients.

Figure 2. Coronary heart disease associated with replacement of a major dietary protein source with another in the Nurses' Health Study

Another paper that also addressed the substitution of foods on the risk of coronary heart disease was from the Iowa Women’s Health Study, which found that substituting foods rich in refined carbohydrates with dairy was associated with a increased the risk of fatal coronary heart disease and substitution with red meat was associated with a increased risk of both fatal coronary heart disease and all-cause mortality.36 These findings raise significant doubt towards the cholesterol skeptics claims that certain animal foods appear disease promoting in studies only because they act as a marker of refined food intake. This study actually found that dairy and red meat are disease promoting even when compared to foods rich in refined carbohydrates. 

The Problem of Conflicts of Interests 

It is well documented that the conclusions of studies that receive industry funding, including from the dairy, soda and tobacco industries are far more likely to bias in favor of the invested industry than studies without apparent industry funding.37 The Siri-Tarino et al. meta-analysis was funded by the National Dairy Council and the senior researcher,  Ronald Krauss has reported receiving grants from the National Dairy Council, the National Cattleman’s Beef Association and the Robert C. and Veronica Atkins Foundation. Although such conflicts of interests do not necessarily prove that the meta-analysis is flawed, it does at the very least suggest however that the author’s lack of acknowledgement of the positive association between saturated fat and fatal coronary heart disease and of the very serious flaws in the meta-analysis may have been intentional. 

Like diet, it is notoriously difficult to accurately measure environmental tobacco smoke exposure which has obscured the findings for passive smoking and smoking related diseases in observational studies. In a similar fashion as the dairy industry has done to downplay and distort the relationship between saturated fat and cardiovascular disease, the tobacco industry has taken advantage of measurement error in order to scrutinize the association between passive smoking and lung cancer in part due to the fact that the majority of observational studies failed to find a statistically significant association.38 However, it is clear that when all of the evidence is considered there is convincing evidence that passive smoking increases the risk of lung cancer, just as the substitution of whole plant foods with saturated animal fat increases the risk cardiovascular disease.39

In 2003, tobacco affiliated researchers Enstrom and Kobat published findings from a 39 year follow-up of a prospective cohort study in the British Medical Journal and concluded that exposure to environmental tobacco smoke does not likely significantly influence the development of lung cancer and coronary heart disease. This paper received a lot of attention from the mass media, including the Wall Street Journal, and was used by the tobacco industry to criticize government sponsored ‘junk science’.40

This study was criticized by a number of researchers and by the American Cancer Society which addressed a number of the very serious flaws in the study.41 42 The prominent flaw that was emphasized was the lack of a suitable comparison group. The analysis only took into account whether never smokers who had a smoking spouse were more likely to develop lung cancer and coronary heart disease compared to never smokers without a smoking spouse, and did not account other forms of environmental tobacco smoke. This was an issue because in 1959 when the participants were enrolled there was tobacco smoke virtually everywhere leaving no group unexposed. Furthermore this study only measured the spouses smoking status at study baseline and did not account for whether the spouse quit smoking, ended the marriage or died during the follow-up period. 

To summarize some of the shortcomings of this tobacco industry influenced study, it suffered from a lack of suitable comparison group, lack of  high quality assessment methods to precisely measure exposure, and the lack of assessment of changes to exposure during the follow-up period. These shortcomings remarkably resemble those of the Siri-Tarino et al. meta-analysis. This paper has even been cited in a lawsuit against tobacco companies by the US District Court as ‘a prime example of how nine tobacco companies engaged in criminal racketeering and fraud to hide the dangers of tobacco smoke.’43 

This was unfortunately not the last time that the researchers of an industry influenced study would publish a paper that has the potential to jeopardize the health of so many. What is also unfortunate is that many cholesterol skeptics have also chosen to exploit these findings in an attempt to advocate disease promoting diets to an uninformed audience. Follow-up posts in this series will critically examine other lines of evidence of the diet-heart hypothesis that cholesterol skeptics have chosen to misinterpret and exploit in an attempt to confuse the general population. 

Diet-Heart Posts

Part II - Diet-Heart: Saturated Fat and Blood Cholesterol

Part III - Diet-Heart: The Role of Vegetarian Diets in the Hypothesis
Part IV - Cracking Down on Eggs and Cholesterol
Part V - Cracking Down on Eggs and Cholesterol: Part II


Please post any comments in the Discussion Thread. 

Whole 30 Recap

Well, my Whole 30 turned out to be a Whole 25. On Day 26, I caved! The almond flour I’d ordered unexpectedly arrived incredibly fast and just sat there, taunting me. All I could think about was making chocolate chip cookies. So, last Saturday, I did. And they were delicious.

I try not to be an all or nothing person, so I definitely find value in taking on this challenge even though I didn’t make it to the bitter end. It was a great exercise in discipline and showed me what I am capable of when the commitment is there. The attention to my eating coupled with working out more has me feeling leaner and more energetic. I did not weigh myself before, during, or after this challenge because I want to get away from the scale as my barometer. What if I didn’t lose a pound this month? I think seeing that would make me feel down, even though I am registering so many other signs of better body composition, not to mention the mental and emotional benefits. On the flip side, I’m sure I’d feel like a rock star if I saw some substantial scale movement. But, I’m willing to let that go in search of “better” goals. I’m not doing this to see a certain number. I’m doing this to feel fantastic, strong, and healthy. How I judge my progress should match up with those goals. Yes, they are quite nebulous and hard to quantify. But I’m OK with that. I’m in this for life – I’m “training for life” as Relentless Roger likes to say.

So, I mentioned the chocolate chip cookies. There were also snickerdoodles and there were pancakes. And pizza. Nothing crazy and all gluten free. Pretty tame as binges go… It does give me a hint about how I react to perceived restriction, though. There is the inevitable bounce-back. And I should also mention the “last supper” type of eating that happened just prior to the Whole 30. I think it’s just basic psychology and is not necessarily even a bad thing, per se. I just need to be aware of it. That’s not to say I won’t ever do another challenge again. I think it’s helpful to nix the sugar, in particular, for a stretch to reset my taste buds from time to time. I may go with the 21 Day Sugar Detox next time for the shorter time frame and slightly less restrictive mentality. My plan for now is to go forward more moderately, including a modest amount of treats within an overall diet that is Paleo and full of healthy meat, veggies, and fats. I will continue working out – I’m going to try out the high intensity/low volume training laid out in The Smarter Science of Slim. I really think that the hormonal shift from more exercise has helped quite a bit this month as well. Decent eating + punctuated intense exercise = Healthy Amelia, or at least, that’s the plan!

How about you? Did you participate in any sort of challenge this month? How did it go/is it going? What are the big takeaways?