Hoenselaar, R. 2012. Saturated fat and cardiovascular disease: The discrepancy between the scientific literature and dietary advice. Nutrition: 28: 118-123.
Meta-analysis: A study that combines the results of many studies and determines the overall conclusion all the studies.
This review article exams the Dietary Guidelines published by the US Department of Agriculture (USDA) and the US Department of Health and Human Services (USDHHS) in regards to saturated fat intake. The author chose to exam this topic due to previous criticisms of the scientific basis of these guidelines.
The USDA and USDHHS Dietary Guidelines tell Americans to “consume less than 10% of calories from saturated fatty acids by replacing them with monounsaturated and polyunsaturated fatty acids.”
Saturated fatty acids are fats with no double bonds in their structure, making them very stable. Monounsaturated and polyunsaturated fatty acids have one or more double bonds, respectively, making them less stable. Saturated fatty acids are common in butter, coconut oil, and animal fats. Monounsaturated fatty acids are common in olive oil and animal fats. Polyunsaturated fatty acids are common in seed oils (such as corn, canola, soy), fish oil, and nuts.
The review breaks down the scientific evidence vs. Dietary Guidelines for several aspects, ultimately coming to the conclusion that the Dietary Guidelines do not reflect the scientific evidence.
Saturated Fat Intake and Serum Cholesterol Levels
Scientific Evidence: A 2003 meta-analysis of 60 studies found that saturated fat increases both HDL (“good”) and LDL (“bad”) cholesterol compared with carbohydrates, but it did not change the ratio of LDL: HDL ratio (Mensink et al 2003). This ratio is important because total cholesterol: HDL cholesterol is the strongest predictor of heart disease mortality (Prospective Studies Collaboration 2007). Therefore, an increase in LDL is not harmful, as long as HDL is increased correspondingly. Additionally, the researchers noted that because all sources of natural fat contain both saturated fat (which do not affect LDL: HDL ratio) and unsaturated fat (which improves the LDL: HDL ratio), the net effect of fat is to improve the LDL:HDL ratio vs. replacing dietary fat with carbohydrates. Simply stated, eating natural sources of fat will have a net beneficial effect on the LDL: HDL ratio, while eating carbohydrates will have a net deleterious effect on this ratio.
USDA/USDHHS Guidelines: The USDA ignored this large, 60-study meta-analysis in preparing their Dietary Guidelines. Instead, they used 11 randomly selected studies. All of these studies found that saturated fat increases LDL cholesterol and two also noted that HDL cholesterol was increased. The USDA/USDHHS Dietary Guidelines included the effect on LDL cholesterol, yet failed to include the effect on HDL.
Had all the scientific evidence been include in the USDA/USDHHS’s Dietary Guidelines, it would have been difficult for them to justify a low saturated fat diet.
Prospective studies of saturated fat in relation to cardiovascular disease
Scientific Evidence: Three major meta-analyses have been performed examining the relationship between saturated fat intake and cardiovascular disease (CVD) (Skeaff and Miller 2009, Mente 2009, Siri-Tarino et al 2010). These studies showed a consistent lack of association between saturated fat intake and CVD or stroke.
Another meta-analysis of 11 studies compared the effects of replacing saturated fats with monounsaturated fats, polyunsaturated fats, or carbohydrates (Jakobsen et al 2009). They used “hazard ratios” to compare the effects of these changes on CVD risk. A hazard ratio of 1.0 indicates that the substitution produces no change in CVD risk, while a value of less than one indicates lowered CVD risk and a value greater than one indicates an increased CVD risk. For example, a hazard ratio of 2.0 would indicate the substitution doubles the CVD risk.
This meta-analysis found that substitution of saturated fat with polyunsaturated fat slightly decreased CVD risk (hazard ratio= 0.87), while carbohydrate and monounsaturated fat slightly increased CVD risk (hazard ratios= 1.07 and 1.19, respectively).
Overall, no investigators from the mega-analyses concluded that a reduction in saturated fat would affect the risk of CVD.
USDA/USDHHS Guidelines: The Dietary Guidelines chose not to include the first three meta-analyses discussed above, which showed a lack of association between saturated fat intake and CVD or stroke.
Instead, they only used the 11-study meta-analysis that showed a slightly decreased risk for CVD when substituting saturated fat for polyunsaturated fat. Again, this meta-analysis found an increased risk for CVD when substituting saturated fat for carbohydrates or monounsaturated fats. The Dietary Guidelines glossed over this point. In order to justify their Dietary Guidelines, they picked the only one of these 11 studies in the meta-analysis that suggested substitution of saturated fat by monounsaturated fat or carbohydrate decreased cardiovascular risk.
It seems the review board for the USDA and USDHHS were selectively picking the data to find a single study that suits their Dietary Guidelines, while overlooking the mountain of evidence opposing their “lower saturated fat intake” message.
As noted in the review article, “The results and conclusions about saturated fat intake in relation to CVD, from leading advisory committees, do not reflect the available scientific literature.
Going through this research, it was really incredible how far off the actual scientific evidence was from the USDA/USDHHS Dietary Guidelines. This is really unfortunate because while many Americans may not actually read the Dietary Guidelines, they do serve as the basis for “common sense” nutritional advice. The media bases their information on the U.S. Dietary Guidelines, the American Heart Association and American Diabetes Association bases, in large part, their guidelines on the U.S. Dietary Guidelines, and children are taught in schools how to eat “healthy” based on the U.S. Dietary Guidelines. With so many people depending on the U.S. Dietary Guidelines, it’s crucial that the Guidelines follow the scientific evidence and not the old status quo or desires of special interests.
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