Heart healthy diets: evidence, not opinion
A new publication in this week's Annals of Internal Medicine illustrates the importance of distinguishing evidence-based reviews from more political conclusions by expert groups as employed, for example, in the creation of the US Dietary Guidelines or the World Health Organization's diet recommendations. Andew Mente, et al released "A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease ." They followed an examplary procedure, defining how they would analyze the evidence before actually digging into the subject content of any of the studies. First they identifed 5,705 medical journal articles on diet and heart disease. They then applied pre-established rules on their strength, consistency, temporality (exposure before outcome) and coherence, and then considered biological gradient, experimental evidence, specificity (avoiding multiple risk exposures), biological plausibility and the avoidance of analogy. These comprise the rigorous Bradfor Hill Criteria for Assessing Causation.
The "blind" screening process netted 146 prospective cohort studies and 94 randomized controlled trials that were then analyzed to answer the question: what does medical science tell us about diet and heart disease? The results of the study, in the authors' words:
Strong evidence supports valid associations (4 criteria satisfied) of protective factors, including intake of vegetables, nuts, and "Mediterranean" and high-quality dietary patterns with CHD, and associations of harmful factors, including intake of trans–fatty acids and foods with a high glycemic index or load. Among studies of higher methodologic quality, there was also strong evidence for monounsaturated fatty acids and "prudent" and "western" dietary patterns. Moderate evidence (3 criteria) of associations exists for intake of fish, marine -3 fatty acids, folate, whole grains, dietary vitamins E and C, beta carotene, alcohol, fruit, and fiber. Insufficient evidence (2 criteria) of association is present for intake of supplementary vitamin E and ascorbic acid (vitamin C); saturated and polyunsaturated fatty acids; total fat; -linolenic acid; meat; eggs; and milk. Among the dietary exposures with strong evidence of causation from cohort studies, only a Mediterranean dietary pattern is related to CHD in randomized trials.
Some interpreted the finding as disappointing because the evidence found only a limited number of dietary causes of heart disease -- unlike the huge number of alleged relationships read about in the newspapers almost daily. As USA Today summed it up: "What we know for sure about diet and what protects the heart is a relatively short list." The Systematic Review validated the Salt Institute's dietary advocacy: a quality diet embodying the Mediterranean Diet without regard to any concern about salt intake is the best thing for heart health. No evidence implicated salt as a cause of coronary heart disease.
Rather than disappointment, we should cheer the conclusion that "evidence supports a valid association of a limited number of dietary factors and dietary patterns with CHD." Since we've been largely unsuccessful in changing Americans' dietary patterns, having greater agreement on a limited number of interventions that are proven effective sounds like a real breakthrough to us.
Dr. Steven R. Covey, management guru, teaches as one of his "Seven Habits" to "begin with the end in mind." The key here is that the focus of science and health is the end of preserving the integrity of the science by rigorously following quality scientific methods. The "end" is not finding evidence to support a pre-determined policy preference. Thank you, doctors, for this outstanding reminder that process can determine outcomes and that data-driven conclusions are far more valuable than expert opinion.
Stay tuned to see how the experts on the Dietary Guidelines greet this study.
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