Dissecting Dietary Fats and Heart Disease: The Story Continues
Cardiovascular disease and cancer are ‘catchall’ medical phrases because they encompass a wide variety of different diagnoses. Cardiovascular disease (CVD) refers to the failure of the heart muscle to pump properly (congestive heart failure), abnormal rhythms of the heart (arrhythmia) or heart valve leakages. CVD also includes hardening of the arteries (atherosclerosis) which causes blockages, stopping blood flow to the heart (ischemia) or the brain (stroke). WebMD describes all of these as CVD.
Researchers identified a relationship between dietary fat and CVD over 50 years ago. Subsequently, much has been learned about the relative importance of fat type (saturated, monounsaturated, polyunsaturated and trans fatty acids). It is known that the balance of omega-6 and omega-3 fatty acids in the diet affects the types of prostaglandins, eicosanoids and leukotrienes which are generated. Because elongation and desaturation of the omega-3 fatty acid, α-linolenic acid, is limited, it is important to consume eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
Just as the type of fatty acids consumed affects our biology, we need to remember that CVD is not a single medical condition. It is important for researchers to define focused CVD endpoints rather than composite outcomes. For example, by focusing on hypertension, Miller and colleagues conducted a meta-analysis of 70 randomized control trials (RCTs) and found EPA+DHA supplementation reduces systolic blood pressure. Yet, as Stephen Daniells of NutraIngredients noted, this report did not receive media attention.
So it is not very surprising when researchers collating data from a myriad of CVD studies (observational and RCTs) do not find significant relationships between fatty acid concentrations and composite CVD outcomes. However, it is surprising that the authors took such a strong stance against current dietary recommendations regarding polyunsaturated and saturated fats with respect to CVD. While it is the role of scientists to probe data and seek alternative explanations/associations, hopefully these hypotheses will be confined to discsussions among scientists and the scientific literature. As Duffy MacKay, senior vice president of scientific and regulatory affairs at the Council of Responsible Nutrition said, “There are thousands of studies and decades of recommendations from government, academic, nutritional and medical organizations and experts supporting the important heart health benefits associated with diets high in polyunsaturated fats, low in saturated fats, and avoidance of trans fats.” This meta-analysis should not lead to headlines that researcher find fat intake doesn’t affect heart disease tomorrow.
Nor should scientists be surprised when researchers scour data from an intervention involving older people at risk of eye-disease (AREDS2) and do not find a nutrient-CVD composite disease (myocardial infarction, stoke and cardiovascular death) relationship. The AREDS2 study was not designed with enough participants to reasonably detect a 25% reduction in composite CVD outcomes. They did, however, find a protective effect of EPA+DHA supplementation in those participating in AREDS2 with no previous history of hypertension. There may be two reasons for this observation. One, hypertension is a more precise outcome than a composite CVD outcome. Second, the AREDS2 individuals had higher EPA+DHA intakes than most Americans. The lowest quintile of EPA+DHA intake in AREDS2 was equal to that of the average American (0.090 g EPA+DHA/d). The median (0.200 g/d) and highest quintile intakes (0.368 g/d) are 2 and 4X higher than that of the average adult and closer to intakes (≥2 g/d) reported by Miller and colleagues to reduce systolic blood pressure.
In conclusion, CVD is not a single disease. Fats are composed of many fatty acid types. Over half a century ago, Dr Dorothy Rathman wrote that “there are important differences in the way that our bodies metabolize fats from different sources and that these differences may be associated with the development of coronary heart disease”. The terminology has changed. The science has advanced. Many of the same questions remain unanswered.
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Writing Group for the AREDS2 Research Group. Effect of long-chain ω-3 fatty acids and lutein + zeaxanthin supplements on cardiovascular outcomes. Results of the Age-Related Eye Disease Study 2 (AREDS2) Randomized Clinical Trial. 2014 JAMA Intern Med doi:10.1001/jamainternmed.2014.328
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Miller PE, Van Elswyk M, Alexander DD. Long-chain omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid and blood pressure: A meta-analysis of randomized controlled trials. 2014 Am J Hypertens doi: 10.1093/ajh/hpu024
The Age-Related Eye Disease Study2 (AREDS2) Research Group. Lutein + zeaxanthin and omega-3 fatty acids for age-related macular degeneration: The Age-Related Eye Disease Study 2 (AREDS2) randomized clinical trial. 2013 JAMA doi:10.001/jama.2013.4997