Much of what we know about diet, nutrition and health comes from prospective cohort studies, which follow large groups of people, usually healthy at baseline, for a number of years and analyse associations between dietary intake and health outcomes (incidence of chronic disease and causes of death). Dietary information is usually collected in the form of a food frequency questionnaire(FFQ), which is a list of foods and beverages with response categories to indicate usual frequency of consumption over the time period queried (usually the past 1 year). To assess the total diet, the number of foods and beverages assessed typically ranges from 80 to 120. There are a few prospective cohort studies that have a significant number of vegan and vegetarians such as the Adventist Health Studies and the EPIC-Oxford study. However, most prospective cohort studies do not include a significant proportion of vegan and vegetarians. Therefore, to study the effects of the consumption of whole plant foods on health, researchers have developed the provegetarian score or plant-based diet index (PDI) — a way of scoring the data collected from FFQs. This plant-based scoring system gives the consumption of plant foods positive marks and the consumption of any animal-derived foods negative markers. Unhealthy plant foods, such as refined grains and sugar and processed foods can also be given negative marks in what is termed an unhealthy plant-based diet index (uPDI).
The table above shows the positive and negative marks given to foods in the plant-based diet index. Healthy plant foods are given positive marks and unhealthy plant foods and animal-derived foods are given a negative mark. The first time this sort of scoring was used was in the reanalysis of the PREDIMED study (1), a randomised study of a Mediterranean-style diet high in olive oil or nuts compared to the control diet, which was supposed to be low in fat (but wasn’t). The reanalysis showed that the benefits of the Mediterranean-style diet were really down to the high consumption of whole plant foods and those that consumed a diet with a high plant-based diet score/index had the lowest risk of death from cardiovascular disease. Since that study, a number of prospective cohort studies have shown that participants whose diet has a high PDI have a significant reduction in risk of coronary heart disease (2), cancer (3), type 2 diabetes (4), kidney failure (5) and weight grain (6). These studies have also shown that a diet with a high uPDI is associated with an increased risk of all these chronic diseases, even if there is a low consumption of animal-derived foods. Table 2 shows that change in risk associated with high adherence to the PDI and whether it healthy or not healthy.
So, in summary, look out for studies that analyse diet data using the PDI to better understand the association between plant-based diets and health. This overcomes the issue of the limited number of studies available in vegans and vegetarians and also provides a way of analysing diet quality.
1) A provegetarian food pattern and reduction in total mortality in the Prevención con Dieta Mediterránea (PREDIMED) study. The American Journal of Clinical Nutrition, Volume 100, Issue suppl_1, 1 July 2014, Pages 320S–328S, https://doi.org/10.3945/ajcn.113.071431
2) Healthful and Unhealthful Plant-Based Diets and the Risk of Coronary Heart Disease in U.S. Adults. J Am Coll Cardiol. 2017;70(4):411–22. http://dx.doi.org/10.1016/j.jacc.2017.05.047
3) Association between a pro plant‐based dietary score and cancer risk in the prospective NutriNet‐santé cohort. International Journal of Cancer, 2018, Volume 143, Issue 9 pages 2168–2176. https://doi.org/10.1002/ijc.31593
4) Plant-Based Dietary Patterns and Incidence of Type 2 Diabetes in US Men and Women: Results from Three Prospective Cohort Studies.PLOS Medicine (2016) 13(6): e1002039. https://doi.org/10.1371/journal.pmed.1002039
5) Plant-Based Diets and Incident CKD and Kidney Function. CJASN May 2019, 14 (5) 682–691; DOI: https://doi.org/10.2215/CJN.12391018
6) Satija, A. et al. (2019) ‘Changes in intake of plant-based diets and weight change: results from 3 prospective cohort studies’, The American Journal of Clinical Nutrition, Volume 110, Issue 3, Pages 574–582, https://doi.org/10.1093/ajcn/nqz049