Readers may have heard the response by the person warned by a family member that what he or she was eating was bad for their heart. “Wait a month and there will be a different study showing it is good for me.” What we know for sure about diet and the heart is actually a surprisingly short list.
This week comes the study, A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease, by Drs. Mente, de Koning, et al. , 2009 Arch Intern Med. 169(7):659-669. This review study did an analysis of nearly 200 studies involving millions of people.
The authors concluded that strong evidence supports valid associations of protective factors for vegetables, nuts, and “Mediterranean” diet patterns, but that associations of harmful factors included intake of trans–fatty acids and foods with a high glycemic index.
Significantly, insufficient evidence of association was present for intake of supplementary vitamin E and ascorbic acid (vitamin C); saturated and polyunsaturated fatty acids; total fat; meat; eggs; and milk.
One of the interesting features is that the authors used the traditional Bradford Hill guidelines on drawing causation conclusions from evidence of association to derive a causation score, basing it on 4 criteria (strength, consistency, temporality, and coherence) for each dietary exposure in cohort studies; they also examined the results for consistency with the findings of randomized trials.
Epidemiology involves judgment; it is not an absolute science, and the presence of confounding factors and the difficulty in finding an adequate control/comparison group for many food issues are similar to the issues seen in toxic tort litigation.
No surprise, then, that the authors conclude that future evaluation of dietary patterns, including their nutrient and food components, in cohort studies and randomized trials is recommended.