Scientists in Canada reviewing the research so far on links between different diets and heart disease found strong evidence that diets high in vegetables and nuts, and those that follow a Mediterranean pattern rich in fruit, vegetables and fish were strongly associated with lower heart disease risk than those that rely on food with a high glycemic index or high in trans-fatty acids. High glycemic index food includes rice, pasta and refined carbohydrates like white bread, and foods high in trans-fatty acids include fried foods, baked goods and snacks.
The study, which reviewed 50 years of research, was the work of scientists from McMaster University, Hamilton, Ontario, Canada, and is published online in the 13 April issue of the Archives of Internal Medicine. The first author was Dr Andrew Mente, of the Population Health Research Institute.
Mente and colleagues wrote that despite the wealth of literature on the subject, nobody had yet done a systematic review of the strength of the evidence from studies examining links between various diets and coronary heart disease risk.
For their review they did a systematic search of published literature in the MEDLINE database, and picked out nearly 150 prospective cohort studies and over 40 randomized trials that investigated links between different diets and coronary heart disease.
They then reviewed the evidence in the studies using recognized tests to determine which ones had the strongest methods and arguments to back their claims. For instance, they used the Bradford Hill guidelines to work out a causation score based on four criteria: strength, consistency, temporality, and coherence, and looked for consistency with the evidence of randomized trials.
The results showed that:
* There was strong evidence (that satisfied all 4 criteria) of a protective link between intake of vegetables, nuts and “Mediterranean” and high-quality dietary patterns and coronary heart disease.
* There was equally strong evidence (that satisfied all 4 criteria) of a harmful link between intake of trans-fatty acids and foods with a high glycemic index or load.
* Among studies whose methods were considered to be of high quality, there was also evidence of strong protective links for diets rich in monounsaturated fatty acids and harmful links for those that followed a “western” dietary pattern (higher in red and processed meats, high fat dairy products, and refined foods).
* There was moderate evidence (satisfied 3 criteria) of links between intakes of fish, omega-3 fatty acids from marine sources, foods with vitamins E and C, beta carotene, folate, whole grains, fruit, fiber, and alcohol.
* There was not enough evidence to say if individual factors such as intake of vitamin E or C supplements, saturated and poly-unsaturated fats, meat, eggs and milk were linked to heart disease (satisfied only 1 or 2 criteria).
* Only the Mediterranean dietary pattern had strong evidence of lowering heart disease risk in randomized trials.
The authors concluded that:
“The evidence supports a valid association of a limited number of dietary factors and dietary patterns with CHD [coronary heart disease].”
They said more cohort studies and randomized trials should be done to evaluate dietary patterns, including their nutrient and food components.
“A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease.”