by Lewis Chang, PhD
A plant-based diet emphasizes a high intake of plant foods and limits or eliminates the intake of animal products. Research has demonstrated various health benefits of plant-based diets, such as lower risks of obesity, high blood pressure, and type 2 diabetes. However, studies that examined their effects on all-cause mortality have yielded inconsistent results. Differences in study population, study duration and, as importantly, the chosen definition of “plant-based diets” can all influence study findings.
Not all plant foods are created equal. If a study’s dietary assessment method considers all plant foods as one category—including less healthy options such as French fries, refined grains, and soft drinks containing corn syrup—then the study likely would yield mixed results. Hence, researchers from Johns Hopkins Bloomberg School of Public Health (Baltimore, MD) employed a research method that distinguished between “healthy” and “less healthy” plant foods in their investigation of the association between plant-based diets and all-cause mortality.1
Study participants included 11,879 adults from the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994). Participants’ diet was assessed at baseline using a detailed food frequency questionnaire. The investigators constructed an overall plant-based diet index (PDI), which assigned positive scores for plant foods and negative scores for animal foods. Then, they derived a healthy PDI (hPDI) in which only healthy plant foods received positive scores, and an unhealthy PDI (uPDI) in which only less healthy plant foods received positive scores. The use of these diet indices took into account the quality of the plant-based diets, thus providing more meaningful and specific data analyses. Participants’ vital status and cause of death were tracked and documented over approximately two decades.1
The study investigators found that neither PDI nor uPDI were associated with all-cause mortality. However, among individuals with hPDI above the median, higher hPDI scores were significantly associated with a lower risk of all-cause mortality in the overall study population and among women. Specifically, a 10-point increase in hPDI was linked to a 5% and 6% lower risk of all-cause mortality in the overall study population and in women, respectively.1 This non-linear association reflects that not all plant foods are healthy. It also suggests that, in order to achieve long-term health benefits, there may be a minimum level of healthy plant foods needed in the diet.
This is the first study of its kind that used a nationally representative sample with a long-term follow-up, making the results more relevant to the general public. However, the study investigators acknowledged that some important limitations need to be addressed in future studies. For example, with a single diet assessment at baseline, the study could not evaluate the potential impact of the duration consuming the plant-based diets. Also, the diet indices reflected mainly the quality, not quantity, of the plant-based diets.
The study results were published in Journal of Nutrition.
Why is this Clinically Relevant?
- Plant-based diets are associated with numerous health benefits
- However, not all plant foods are healthy
- Whole fruit, vegetables, and whole grains are much healthier than French fries, refined grains, and soft drinks containing corn syrup
- Consumption of healthy plant foods, as opposed to unhealthy plant foods, is associated with lower risk of all-cause mortality
References
- Kim H, Caulfield LE, Rebholz CM. Healthy plant-based diets are associated with lower risk of all-cause mortality in US adults. J Nutr. 2018;148(4):624-631.
