In part one of this series I provided a bit of history on the Lipid and Diet Heart hypotheses as well as some basic background information on saturated fat and cholesterol to allow for a better understanding of the remainder of the series. Part 2 will focus on Diet Heart Hypothesis; specifically, we will look at the evidence regarding saturated fat and cholesterol’s relationship with elevated blood cholesterol and the risk of cardiovascular disease. Evidence for both sides of the debate are presented.
Dietary Saturated Fat, Serum Cholesterol and Cardiovascular Disease Risk
As mentioned in Part 1, a belief that dietary saturated fat intake is negatively associated with serum cholesterol and cardiovascular disease risk has been present in the scientific literature since the 1950s; since this time, there has been much evidence presented both for and against this hypothesis.
Diet Heart Hypothesis Support
Evidence indicating that saturated fat is a causative factor in high serum LDL started in the 1970s with the seven countries study by Ancel Keys (1, 2), while there is controversy over the data collection and results of this study, the results did indicate that the higher a country’s saturated fat intake the higher that country’s risk of cardiovascular disease. Since this time there have been multiple studies indicating that an increase in saturated fat intake can negatively impact serum cholesterol levels (3, 4). One study (3) from 2003 reviewed 60 controlled experimental trials from 1970 to 2003 and came to the conclusion that dietary saturated fat was associated with unfavorable alterations in total serum LDL, total cholesterol:HDL ratio, and LDL particle size, all of which were trusted markers of cardiovascular risk at the time of the study. This study further indicated that replacing dietary saturated fats with unsaturated fats resulted in a decrease in the noted serum lipid markers of cardiovascular disease risk. Another meta-analysis study from 1992 (4) also provided evidence that the replacement of saturated fat with carbohydrate, monounsaturated fats, and polyunsaturated fats would reduce LDL levels. However, it is important to note that a limitation in these reviews was that they do not include long term studies in the primary references.
In review of the scientific literature, there is more evidence linking saturated fat intake to an observed risk of cardiovascular disease than to elevated serum cholesterol. One epidemiological review study from 2008 (5) linked saturated fat intake to an increased risk of cardiovascular disease, and indicated that the replacement of saturated fats with mono and polyunsaturated fats would improve cardiovascular disease risk. Another study (6) found that a diet higher in saturated and mono unsaturated fat increased the risk of cardiovascular disease in individuals under the age of 60, but no relationship was found in individuals above 60 years. A further review study from 2013 (7) suggested that the current evidence supported reducing saturated fat consumption while achieving a proper Omega 6:3 ratio of 2:1 was ideal for cardiovascular disease risk reduction. Notwithstanding the above information, many studies have indicated that the interaction between saturated fat and cardiovascular disease risk is much more complicated than a direct relationship without confounding variables. Most recent studies indicate that total fat consumption is not as important as the quality of dietary fat selection or simply the choice of macronutrient substitution. For instance, some studies indicate that replacing saturated with monounsaturated or polyunsaturated fats are best for reducing cardiovascular disease risk while other studies indicate that the replacement of saturated fat with carbohydrates has a stronger protective effect (8, 9, 10, 11). Furthermore, there is also evidence that the type of dietary saturated fat can have different impacts on cardiovascular disease risk. For instance, stearic acid has been shown to have a neutral effect on risk, while lauric, myristic and palmitic acids have been shown to increase risk of cardiovascular disease (11).
In Discord with Diet Heart Hypothesis
Notwithstanding the above information, limitations in the evidence linking saturated fat to high blood cholesterol and cardiovascular disease are present. For instance, there is no consensus as to what the most appropriate dietary substitution (mono, polyunsaturated fats, or carbohydrates) for saturated fat reduces the risk of cardiovascular disease the most, what type of saturated fat increases the risk the most, or whether other factors such as Omega 6:3 ratio may play a more important role. More importantly, for the diet heart hypothesis to hold true, dietary saturated fat should increase cardiovascular disease risk across all populations and cohorts, and this is not shown in the literature. Furthermore, there is significant evidence indicating that saturated fat is not associated with an increase in serum cholesterol or observed cardiovascular disease risk.
With regard to the effect of saturated fat on cholesterol and other serum lipid measures of cardiovascular disease, plenty of evidence indicates that there is no clear association between these factors. For instance, 2 studies from the late 1970s (12, 13) did not find any relationship between saturated fat consumption and serum cholesterol levels. In addition, a more recent study from 2009 (14) also found no significant relationship between the amount of saturated fat in the diet and serum LDL cholesterol numbers. Interestingly, the study did find that higher saturated fat intake increased serum HDL cholesterol, a protective marker of heart disease. In 2012, a meta-analysis (15) reviewed multiple studies on the effects of low carbohydrate diets on markers of chronic disease, including serum LDL cholesterol levels and came to the conclusion that low carbohydrate diets have no effect on serum LDL cholesterol and actually have positive impacts on other markers of cardiovascular disease such as blood pressure, serum triglycerides, and inflammation. Keep in mind that the majority of calories in a low carbohydrate diet come from fat and protein; thus, low carbohydrate diets are considered high fat diets.
Along with limited evidence that saturated fat increases serum LDL cholesterol, the finding that saturated fat may actually increase protective measures of cardiovascular disease leads to the conclusion that saturated fat may not be related to an increased risk of cardiovascular disease, and there is much evidence in support of such a theory. Aside from a direct relation to serum cholesterol levels, there is a large body of evidence indicating that saturated fat consumption is not associated with the risk of cardiovascular disease. In fact, most recent studies do not find a correlation between saturated fat and an increased risk of cardiovascular disease (16-23). Studies have shown that palm oil and diary fat, both of which are high in saturated fat, intake are not associated with cardiovascular disease risk (16, 17) and saturated fat intake is not associated with increased cardiovascular disease in diabetics (18, 21). Furthermore, a reduction in saturated fat intake may actually increase disease risk in non-healthy populations (21). Additionally, five meta-analysis studies from 2011 thru 2013 all come to the conclusion that saturated fat intake is not associated with the risk of cardiovascular disease (19-23), one study even indicated that the dietary recommendations from the United States and European advisory committees were not based on fully inclusive scientific evidence; as such, these dietary recommendations may not be accurate (23).
Population based studies have also cast doubt on the theory that saturated fat increases serum cholesterol and the risk of cardiovascular disease. For instance, the French are known for a notoriously rich diet, high in saturated fat, yet they experience low rates of cardiovascular disease, leading to the term the French Paradox (24). Many studies have tried to explain this paradox while upholding the Diet Heart Hypothesis by suggesting that moderate red wine and cheese consumption have protective effects (24). However, given the above evidence, it may simply be the case that saturated fat is not related to cardiovascular disease. To further this point, many other countries have shown a similar paradox. One study (25) charted the intake of saturated fat with cardiovascular disease across multiple countries and found that the higher the saturated fat content, the lower the risk of heart disease. Other population studies from traditional cultures have also reported diets high in saturated fat but very low in cardiovascular disease. This is the case for the traditional male Masai diet that is exclusively compromised of meat and milk yet there is no evidence of elevated serum cholesterol or cardiovascular disease (26). This also holds true for the native Tokelau population who consumed approximately 50% of their dietary energy as saturated fat, mainly from coconut (27). In light of these studies, the French paradox should no longer be considered a paradox.
If we consider the Lipid Hypothesis to be true (see Part 1), the Diet Heart Hypothesis states the following:
- Dietary saturated fat increases serum cholesterol
- Dietary cholesterol increases serum cholesterol
- Thus, dietary saturated fat and cholesterol increase the risk of cardiovascular disease.
However, in order for the Diet Heart Hypothesis to hold true, there must be consistent scientific evidence for each of the above points. Even though this theory has been utilized to develop public health initiatives and pharmacological treatments there is significant discord in the scientific literature.
As outlined above, dietary saturated fat and cholesterol have not been consistently shown to be associated with elevated LDL and total cholesterol levels across multiple populations and cohorts, and have also not been associated with an increased risk of cardiovascular disease across populations and cohorts. Therefore, the Diet Heart Hypothesis cannot be considered true.
The next post in this series will focus on the Lipid Hypothesis in attempt to determine if LDL and total cholesterol are reliable predictors of cardiovascular disease, and what measures may be better.
The Barefoot Golfer
2.) Keys, A. Coronary heart disease in seven countries. Circulation 1970;41(suppl. I):1-195