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Not all plant foods are equally healthy

April 29, 2021

by Dr. Mark Messina

Health authorities increasingly recommend that Westerners shift toward more plant-based diets for health and environmental reasons. Common types of vegetarian diets include lacto-ovo vegetarian and vegan diets. While most people will never be vegetarian, many may eat a diet that consists of more plants and plant protein. These plant-focused, but non-vegetarian, diets are often referred to as plant forward or flexitarian.

Increasingly, when examining diet-health relationships, epidemiologists look not just at the amount of plant foods consumed, but the quality of those plant foods. This approach to interpreting the scientific literature reveals large differences in health outcomes according to the type of plant food consumed.

Because of its size, the Adventist Health Study-2 (AHS-2) is one of only 2 large cohorts able to provide meaningful insight into the health status of vegetarians. In the AHS-2, the body mass index of vegans was shown to be lower than that of nonvegetarians and other types of vegetarians.1 Vegans were also shown to be less likely to develop diabetes than participants adhering to these other types of dietary patterns.2 On the other hand, a recent systematic review concluded only that “evidence weakly demonstrates associations between vegan diets and risk of CVDs, with the direction of associations varying with the specific CVD outcome tested.”3

Without denying the informative value of understanding differences in health status according to dietary pattern, this type of “all or nothing” approach — wherein one falls neatly into one dietary pattern or another — may obscure insights that can be gained by taking a more nuanced approach to evaluating the overall quality of the diet. After all, not all plant foods are equally healthy.4 Whole grains and refined grains are 100% vegetarian, but the former is thought to exert health benefits whereas the latter, not so much.5

The variation in the quality of plant-based diets may be just as great as the variation in quality of omnivorous diets. To this point, more than 20 years ago researchers identified two different dietary patterns among U.S. omnivores: the Western diet and the prudent (Western) diet.6 The former was characterized by processed and red meats, eggs, potatoes, and refined grains and the latter by cruciferous vegetables, greens, carrots, salads, and fresh fruits. This type of classification recognizes that omnivorous diets can differ markedly in nutrient content and overall quality.

More recently, researchers began to develop plant-based diet indices and to differentiate healthy plant-based diets from less healthy plant-based diets.7 In one analysis using this approach, higher adherence to an overall plant-based diet index was associated with an 8% decreased risk of coronary heart disease. However, further analysis revealed that adherence to a healthy plant-based diet index was associated with 25% reduction in risk whereas higher adherence to an unhealthy plant-based index was associated with a 32% increased risk of CHD.

These kinds of observations have been published by other researchers as well. For example, there are the results of a Korean cohort involving 5,636 normotensive adult men and women, which identified 2,244 cases of hypertension over the 14-year follow up period.8 Among those tightly adhering to an overall plant-based diet index and healthy plant-based diet index, risk of developing hypertension was reduced by 9% and 35%, respectively, whereas among those adhering to an unhealthy plant-based diet index, risk of developing hypertension was increased by 44%.

The results of this study, and the previous one, are not unexpected because the healthy and unhealthy plant-based indices are created based on knowledge of the association between foods and health outcomes. Nevertheless, these studies, and many others like them, serve to highlight the importance of adopting a more nuanced approach to understanding the impact of diet on health.

References

  1. Orlich MJ, Singh PN, Sabate J, et al. Vegetarian dietary patterns and mortality in Adventist Health Study 2. JAMA internal medicine. 2013;173:1230-8.
  2. Tonstad S, Stewart K, Oda K, et al. Vegetarian diets and incidence of diabetes in the Adventist Health Study-2. Nutrition, metabolism, and cardiovascular diseases : NMCD. 2013;23:292-9.
  3. Kaiser J, van Daalen KR, Thayyil A, et al. A systematic review of the association between vegan diets and risk of cardiovascular disease. J Nutr. 2021.
  4. Williams KA, Sr., Patel H. Healthy plant-based diet: What does it really mean? J Am Coll Cardiol. 2017;70:423-5.
  5. Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393:434-45.
  6. Tseng M, DeVillis R. Correlates of the “western” and “prudent” diet patterns in the us. Ann Epidemiol. 2000;10:481-2.
  7. Satija A, Bhupathiraju SN, Spiegelman D, et al. Healthful and unhealthful plant-based diets and the risk of coronary heart disease in U.S. adults. J Am Coll Cardiol. 2017;70:411-22.
  8. Kim J, Kim H, Giovannucci EL. Quality of plant-based diets and risk of hypertension: a Korean genome and examination study. Eur J Nutr. 2021.