Foods of the Oldways Cart

While it is important to keep abreast of the latest nutrition research, it shouldn’t feel like watching a pingpong match. “Fundamentals and current understanding do NOT change every time a new study makes headlines,” according to the Oldways Common Ground scientists. Despite the subtle variations and interpretations of healthy eating, decades of research give us a fairly well-rounded picture of a nutritious diet.

The “gold standard” of nutrition research is the randomized clinical trial, in which participants are randomly assigned to various diets and then followed to see how the diet affects their health. These studies are very expensive, as they require a large enough group of people to be able to see changes, and must be conducted over several weeks, months, years, or even decades, depending on what scientists are trying to track.

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Despite the challenges, clinical data proves that diets featuring fruits, vegetables, whole grains, and other minimally processed plant foods are healthy. The PREDIMED trial was a groundbreaking randomized clinical trial where adults at risk of heart disease were assigned to various diet groups (Mediterranean Diet + nuts, Mediterranean Diet + olive oil, or a lowfat control group). The Mediterranean Diet groups fared so well that the study ended early, as it was deemed unethical to prevent the control group from switching to a Mediterranean Diet. The republished PREDIMED data, which has stronger randomization protocols in place, further strengthens the evidence base on the health benefits of a Mediterranean diet.

Similarly, trials in which people are randomly assigned to either whole grains or refined grains demonstrate that whole grains may help improve cholesterol, inflammation, and insulin action. While large clinical trials are not as common as other types of research (due to big expenses and time committments), they add an additional layer of credibility to the large body of observational research available.

Observational studies, in which people are followed over a long period of time while scientists study their eating habits and health to look for relationships, are easier to carry out and are more common in nutrition research. However, observational studies can only show correlation — that one factor is associated with another — not that one factor caused another. Nonetheless, correlation is a useful scientific principle, and dismissing correlation entirely (by dismissing observational studies) is a misguided understanding of science. For example, the tobacco industry abused this principle to argue that smoking does not cause lung cancer, because correlation is not causation. Yet even though no one will ever do a clinical trial requiring one group to smoke and the other to abstain, we know that quitting smoking is a healthy move, and we know that based on observational studies.

While correlation alone does not prove causation, multiple correlations from well-designed studies that all reach the same conclusion can be sufficient — especially when combined with biological plausibility. (For example, if studies show whole grains are associated with heart health, and lab tests show whole grains make blood vessels more flexible, this offers a solid biological basis for the correlation.) Indeed, biology (adaptation, evolution, plausibility) and heritage (cultural traditions) are relevant sources of real-world information that can enhance our understanding of the long-term feasibility and health effects of diet.


It is very difficult to prove cause-effect relationships in scientific studies, especially in nutrition studies. After all, unlike drug trials (in which you either take the drug or you don’t), humans eat all day, creating endless food choices and lifestyle decisions. Additionally, there are a countless number of nutrition philosophies. Studying the lifetime effects of every fad diet simply isn’t feasible. Nonetheless, the current body of research shows very strong relationships between certain eating patterns (fruits, vegetables, whole grains, and other wholesome plant foods) and health.

Science, by its very definition, is not supposed to ever be final—but we can act now. As Dr. David Katz (one of our Oldways Common Ground co-chairs) often says, we already know how to prevent 80% of disease, so let’s not waste time arguing. Rather than being paralyzed by not knowing every minute detail about health, pitting one nutrient against another, let’s act on what we do know. And what we do know is a balanced diet with plenty of fruits, vegetables, whole grains, nuts, seeds, beans, and other minimally processed foods is a wise (and wonderfully delicious) place to start.

Bonus: For a week’s worth of healthy recipes and groceries that make up a balanced diet, try using the Oldways Cart.

Kelly Toups, MLA, RD, LDN, Oldways Director of Nutrition

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