Most of us know that the risk of heart disease can be modified by lifestyle. For more than fifty years, that’s what we’ve been taught by the people we trust, scientists, medical professionals and public health officials.
But the doubters have always been out there, and they ask questions. How do these people know what’s good and bad for us? Well, of course we’re aware that their evidence is based on scientific data. But, is the data reliable and has it been interpreted correctly?
For decades we’ve been told that saturated fat, the type found in meat, butter and cheese, raises the risk of heart disease. Health officials have urged the public to avoid saturated fat as much as possible, saying it should be replaced with unsaturated fats like that found in nuts, fish, seeds and vegetable oils. In fact, many consider this to be conventional wisdom, which basically mean that it’s undebatable. However, recent research has challenged this view, and the debate on the risk associated with consuming saturated fat has grown louder by the years.
One of the main reason saturated fat has historically had a bad reputation is that it increases LDL-cholesterol, the type of cholesterol that is supposed to raise the risk for heart attacks. But the effects of saturated fat on blood cholesterol are probably more complex than that. For example saturated fat also increases HDL-cholesterol, the so-called good cholesterol. However, the effect of polyunsaturated fat (PUFA) on blood cholesterol and traditional lipid profile is generally considered more favorable than the effect of saturated fat.
A few days ago, a scientific study published in the Annals of Internal Medicine caught the headlines of news media around the world. The main reason for the attention is the fact that the study did not find that people who ate more saturated fat had more heart disease than those who ate less. Furthermore, the study did not find less heart disease in those eating higher amounts of omega-6 PUFA or those eating more monounsaturated fat.
The study deserves our attention, not only because it challenges current wisdom and modern dietary guidelines, but because it summarizes a very large amount of data from three different types of studies. However, because it’s not an open access paper, and because it’s a landmark study, I thought it would be proper to summarize the main findings as I see them.
The Study Design
The study was performed by researchers from the UK, USA and the Netherlands and was led by a team at the University of Cambridge in the UK. It was a systematic a review and meta-analysis of available scientific data addressing the association between dietary fatty acid intake, fatty acid biomarkers (measured in blood or adipose tissue), or fatty acid supplementation and the risk of coronary heart disease (CHD).
The investigators performed a systematic review of the scientific literature in order to find studies that fitted for their meta-analysis. The studies selected were both observational studies as well as randomized controlled clinical trials. To be eligible, the studies had to fulfill certain criteria. For example, observational studies were eligible if they were prospective in design with at least one year follow-up and involved participants from a general population (not people with known heart disease). Intervention studies were eligible if they were randomized and recorded CHD outcomes. A total 72 studies of more than 600 thousand individuals were selected for the final meta-analysis.
In order to grasp the results of the paper, it is important to understand that the meta-analysis consisted of three parts. Firstly, the association between the intake of different dietary fats with CHD was studied. Secondly, the association of measurements of fatty acid biomarkers with CHD was addressed. Thirdly the association between fatty acid supplements and CHD was studied.
Saturated fat, Omega-6 and Monounsaturated Fat Don’t Affect Risk
There was no statistically significant association between dietary saturated fatty acid intake and the risk of CHD. Furthermore, intake of omega-6 PUFA was not associated with the risk of CHD. The authors of the paper therefore conclude: “Our findings do not clearly support cardiovascular guidelines that promote high consumption of omega-6 polyunsaturated fatty acids and suggest reduced consumption of total saturated fatty acids”.
When studying measurements of biomarkers, it is important to keep in mind that there are many types of saturated fatty acids and many types of PUFA’s. Palmitic and stearic acids are saturated fats that were not significantly associated with the risk of CHD. However, margaric acid was significantly associated with lower risk of CHD. Margaric acid is an odd-chain saturated fatty acid. It’s levels are moderately correlated with milk and dairy consumption. The findings support the possibility that odd-chain saturated fats reflecting milk and dairy consumption, may be less harmful in terms of risk for CHD. Arachidonic acid was the only omega-6 fatty acid that correlated with lower risk of CHD.
Studies of dietary intake and biomarker studies did not find any significant association between monounsaturated fat and the risk of CHD.
Dietary Long Chain Omega-3 PUFA’s Are Protective
Dietary long-chain omega-3 PUFA was associated with lower risk of CHD. These findings were supported by the blood biomarker studies which showed some evidence that circulating levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (the two main types of omega-3 PUFA) are associated with lower risk of CHD. Alfa-linolenic acid was neutral in terms of risk.
On the other hand, meta-analysis of omega-3 and omega-6 PUFA supplements suggests that supplementation with these nutrients does not significantly affect the risk of CHD. However, the authors point out that more data is needed because the available data is limited. There is a large ongoing trial on the effects of omega-3 PUFA in primary prevention (VITAL). This study will also address the efficacy of vitamin D.
Trans-Fats Increase the Risk of Heart Disease
Not surprisingly, dietary trans-fats were associated with increased risk of heart disease. However, only five published prospective cohort studies contributed to this analysis.
The Changing Landscape
For the last five years, a number of reports (1, 2, 3, 4, 5) have concluded that there is a weak association between the consumption of saturated fat or major foods that contain saturated fatty acids (meat and milk) and the risk of CHD. The above study certainly adds strength to these conclusions. It appears that the advice to encourage high consumption of polyunsaturated fatty acids and low consumption of saturated fats is not based on solid scientific evidence, and needs to be reconsidered. In fact, the study suggests that dietary fat composition me play a much smaller role for cardiovascular risk than previously thought.
Finally, a few questions need to be asked. Firstly, is this the final verdict? Well, I guess not. But certainly these new results have to be taken seriously. Secondly, how did public health authorities manage to get it wrong for fifty years? Was it because the available scientific data was unreliable, or was it because the data was wrongly interpreted? Was it because the pieces of the puzzle had to fit into a preconceived notion? And thirdly, how will we get it right? How will the new landscape on dietary fats and heart disease be introduced to the public, and how will this landscape affect the food industry?
I know for sure that many experts will cover their ass by suggesting that the macro-nutrient approach is outdated, and they’re probably right. Dietary recommendations focusing on how much to eat of different types of fats and how much of our daily energy intake should be carbohydrates, protein or fat, are both misleading and impractical. So maybe we will soon see a paradigm shift in the way medical professionals and public health officials educate people about the effect of diet and nutrition on health and disease.