April 13 (HealthDay News) -- Vegetables, nuts and the so-called "Mediterranean" diet are heart-friendly, while trans fats and foods with a high glycemic index can harm your heart, say researchers who reviewed 189 studies published between 1950 and 2007.
The studies included 146 prospective cohort studies (which examined past habits of participants) and 43 randomized controlled trials (volunteers were randomly assigned to consume a certain kind of diet).
"The relationship between dietary factors and coronary heart disease has been a major focus of health research for almost half a century," wrote Andrew Mente, of the Population Health Research Institute, and colleagues. But even though there are many published studies on the topic, "the strength of the evidence supporting valid associations has not been evaluated systemically in a single investigation."
When they pooled the findings from the studies and applied a predefined algorithm, Mente and his team identified "strong evidence of a causal relationship for protective factors, including intake of vegetables, nuts and monosaturated fatty acids and Mediterranean, prudent and high-quality dietary patterns, and harmful factors, including intake of trans-fatty acids and foods with a high glycemic index or load and a Western dietary pattern," the researchers wrote.
"Among these dietary exposures, however, only a Mediterranean dietary pattern has been studied in randomized controlled trials and significantly associated with coronary heart disease," they said.
The Mediterranean died is typically loaded with fruits, vegetables, grains and olive oil.
The researchers also found modest evidence of a causal relationship between heart health and several other foods and vitamins, such as fish, omega-3 fatty acids from marine sources, folate, whole grains, alcohol, fruits, fiber, dietary vitamins E and C and beta carotene. There was weak evidence of a causal relationship between heart health and vitamin E and C in supplement form, saturated and polyunsaturated fatty acids and total fats, alpha-linoleic acid, meat, eggs and milk.
"The modest or weak evidence of these dietary exposures is mostly consistent with the findings of randomized controlled trials, although randomized controlled trials have yet to be conducted for several factors," the study authors wrote.
"Taken together, these findings support a causal relationship between only a few dietary exposures and coronary heart disease, whereas the evidence of most individual nutrients or foods is too modest to be conclusive," the team said.
The review was published in the April 13 issue of the journal Archives of Internal Medicine.
"Although investigations of dietary components may help to shed light on mechanisms behind the benefits of dietary patterns, it is unlikely that modifying the intake of a few nutrients or foods would substantially influence coronary outcomes," Mente and colleagues concluded. "Our findings support the strategy of investigating dietary patterns in cohort studies and randomized controlled trials for common and complex chronic diseases such as coronary heart disease."
SOURCE: JAMA/Archives journals, news release, April 13, 2009