Most scientists agree that diet plays an important role in heart disease, but until now there has been no comprehensive analysis of which dietary factors most strongly affect disease outcome. A new meta-analysis published in the Archives of Internal Medicine reviews six decades of research (1950-2007) to assess how different dietary factors affect heart disease. Vegetables, nuts, “Mediterranean” and high-quality dietary patterns are strongly protective, while trans-fat, foods with high glycemic index or load and a “Western” dietary pattern were shown to be harmful.
This new study is unique for several reasons. First, the authors were only interested in factors that influenced heart disease directly, not simply heart disease risk factors such as cholesterol levels. Also, emphasis was placed on high-quality studies designed to identify strong dietary associations (cohort studies and randomized controlled trials) with long periods of follow up (at least one year). They asked whether the studies they reviewed were consistent with other data such as epidemiological reports, and sought to establish a causal link between diet and heart disease outcomes. Another important goal of the analysis was to identify factors that lack sufficient evidence to be conclusive and require further research.
In addition to identifying vegetables, nuts, high-quality and Mediterranean dietary patterns as being strongly protective against heart disease, they also found monounsaturated fats (e.g. olive oil), dietary folate (e.g. whole grains, not supplements), dietary vitamins C and E (not supplements), alcohol consumption (in any form) and omega-3 fatty acids from fish (not plants, e.g. flax) to be moderately protective.
Factors that were not associated with heart disease in this study were dietary supplements (e.g. vitamins C and E), total fat, saturated fats, polyunsaturated fats (from plants), meat, eggs and milk. It is important to note, however, that negative findings in this analysis are not necessarily indicative of a lack of causality. Rather, it may indicate insufficient data to observe a significant positive association.
The authors point out that “only overall healthy dietary patterns are significantly associated with coronary heart disease” in the controlled trials, while “evidence for most individual nutrients or foods is too modest to be conclusive.” They suggest that the reason an association exists for dietary patterns and not individual nutrients is that patterns “have the advantage of taking into account the complex interactions and cumulative effects of multiple nutrients within the entire diet.” The authors recommend future trials test various dietary patterns for disease outcome, including cardiovascular disease and cancer.
Taking this further, most dietary factors that were shown to be protective when consumed as part of a healthy diet were not protective when taken in supplement form. This finding bolsters the argument that overall diet rather than individual foods or nutrients are the best strategy for protecting against heart disease. The authors conclude that their findings suggest “investigating dietary patterns in cohort studies and randomized controlled trials for common and complex chronic diseases such as coronary heart disease.”
Based on their analysis, the dietary pattern that best protects against heart disease is rich in vegetables, nuts, fish, healthy fats, whole grains, and fruit. Likewise, the worst dietary pattern consists of refined carbohydrates and artificial trans-fats. The lesson: the best diet consists of plants, fish and whole foods, while processed foods contribute to heart disease.
What about red meat and saturated fats?
Interestingly, there was insufficient data to conclude that red meat or saturated fats are harmful for the heart. This is not terribly surprising, since the data has always been inconsistent. However, I would point out that many studies have looked at the role of red meat and saturated fat in coronary risk and the outcome always shows either harm or no result. And as explained above, no result can be indicative of a lack of statistical power rather than lack of causation. Importantly however, I cannot recall a single study suggesting that red meat and saturated fat is actually good for you.
From this the best we can conclude is that red meat or saturated fat may be involved in promoting heart disease, but if they are the effect is likely to be less harmful than a diet of processed foods. Practically this means small doses of saturated fat may not do much harm when eaten as a part of an overall healthy diet. This is a fairly compelling argument for exercising moderation.
Before you run out and order a ribeye, keep in mind that heart disease is not the only debilitating chronic disease that plagues our culture. Red meat is also associated with several kinds of cancer. Likewise, refined carbohydrates are highly correlated with type 2 diabetes. Vegetables and whole grains are protective against these other diseases as well, and fish may play a role in protecting against neurodegenerative diseases.
The take home lesson is that both diet and disease are complex systems that involve innumerable factors in several different regions of the body. When choosing what to eat it is important that you consider the context of your overall diet and do not get caught up is single foods or a single disease threat.