Vegetarian and other plant-based diets

Plant-based diets consist of a diverse spectrum of dietary patterns, generally defined by limited consumption of animal foods. Vegetarian diets are a major category of plant-based diets ranging from partial to complete exclusion of animal products, such as vegan diets [133]. Small RCTs have shown that compared to participants’ usual diets, vegetarian diets significantly reduced blood pressure, total and LDL cholesterol levels, body weight, HbA1c, and other cardiometabolic risk factors [134]. Large cohort studies have shown that vegetarians tended to have lower risk of obesity, type 2 diabetes, and CHD than nonvegetarians [134]. In the Seventh-day Adventist Health Study, participants who followed vegetarian or vegan diets had reduced mortality and longer life expectancies compared to the general population [135]. In addition, participants following vegetarian diets had lower rates of certain cancers—particularly colon, breast, and prostate cancers [134]. Moreover, vegetarian diets were associated with better cognitive function and a lower risk of cognitive decline among Adventists [136]. Although the Seventh-day Adventist Health Study has provided valuable insights into the health benefits of vegetarian diets, caution is warranted when applying these findings to the broader population because the participants’ other lifestyle practices and religious benefits might have contributed to some of the observed health outcomes. In addition, it is important to pay attention to nutritional adequacy (vitamins B12 and D, iron, calcium, zinc, and long-chain omega-3 fatty acids) of vegetarian diets, particularly vegan diets that entirely exclude animal foods.

Traditionally, plant-based diets are quantified by the reduced amount and frequency of animal-based food consumption, often overlooking the quality of the plant-based foods consumed. However, there is large heterogeneity among plant foods in terms of their nutritional value and health effects. Therefore, we examined two distinct variations of plant-based dietary patterns [133]: A healthful plant-based index (hPDI) that favors high-quality nutrient-dense plant-based foods (whole grains, fruits, vegetables, nuts, and legumes) and minimizes less healthy plant foods (refined grains, potatoes, SSBs) and animal products; and an unhealthful plant-based index (uPDI) that is comprised mostly of less healthy plant foods (sugar and refined carbohydrates) and with less intake of healthy plant foods. In the NHS and HPFS, there were divergent associations for healthy versus unhealthy plant-based diet indices and risk of type 2 diabetes [137], CHD [138], and stroke [139]. For instance, an overall plant-based diet index was modestly inversely associated with incident CHD (HR comparing extreme deciles: 0.92; 95% CI: 0.83–1.01; p-trend = 0.003). This inverse association was substantially stronger for hPDI (HR comparing extreme deciles: 0.75; 95% CI: 0.68–0.83; p-trend < 0.001). However, the association was positive for uPDI (HR comparing extreme deciles: 1.32; 95% CI: 1.20–1.46; p-trend < 0.001). Adherence to a healthful plant-based diet was associated with improvements in both physical and mental dimensions of health-related quality of life [140]. In the VA Million Veteran Program [141], hPDI was inversely associated with total mortality (HR comparing extreme deciles = 0.64, 95% CI: 0.61–0.68), whereas uPDI was positively associated with total mortality (HR comparing extreme deciles = 1.41, 95% CI: 1.33–1.49). The associations between the plant-based diet indices and total mortality were consistent among African and European American participants.

Besides the healthful plant-based diets defined by hPDI, there are a variety of healthy plant-based dietary patterns defined by other dietary indices, such as the Healthy Eating Index 2015 (HEI-2015) based on Dietary Guidelines for Americans [142], Alternate Healthy Eating Index (AHEI) based on Harvard’s Healthy Eating Plate [143] (Fig. 3), the DASH score [144, 145], the Portfolio diet score [146, 147], empirical dietary inflammatory pattern score [148, 149], and the planetary health diet index [150] (see Table 1).

We recently conducted a comparative analysis of four healthy dietary patterns (HEI-2015, AHEI, hPDI, and aMED) in relation to mortality risk in the NHS and HPFS [151]. When comparing the highest with the lowest quintiles, the pooled multivariable-adjusted HRs of total mortality were 0.81 (95%CI, 0.79–0.84) for HEI-2015, 0.82 (95%CI, 0.79–0.84) for aMED score, 0.86 [95%CI, 0.83–0.89] for hPDI, and 0.80 [95%CI, 0.77–0.82] for AHEI [p < .001 for trend for all]. All dietary scores were significantly inversely associated with death from CVD, cancer, and respiratory disease. The aMED score and AHEI were inversely associated with mortality from neurodegenerative disease. The inverse associations between these scores and risk of mortality were consistent in different racial and ethnic groups, including Hispanic, non-Hispanic Black, and non-Hispanic White individuals. These findings support that multiple healthy eating patterns can be adapted to individual food preferences and cultural traditions.