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).

Table 1: Components of the plant-based diet index
Impact of a healthy and unhealthy plant-based diet on health outcomes
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Consultant Haematologist and Lifestyle Medicine Physician. Founder and Director of Plant-based health professionals UK.