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Plant-based diet index

A tool for assessing the health impacts of a plant-based diet

by Dr Shireen Kassam, Consultant Haematologist and Lifestyle Medicine Physician

Much of what we know about diet and health outcomes come 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/or 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 past 1 year). To assess the total diet, the number of foods and beverages assessed typically ranges from 80 to 120 within different food categories.

There are a few prospective cohort studies that have a significant number of vegan and vegetarians such as the Adventist Health Studies, EPIC-Oxford study and Tzu Chi Health Study. However, most prospective cohort studies do not include a significant proportion of participants identifying as vegans or 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 a positive score and the consumption of any animal-sourced food a negative score. Unhealthy plant foods, such as refined grains, sugar and processed foods can also be given a negative score to derive what is termed the unhealthy plant-based diet index (uPDI).

​​Table 1 shows the three categories that comprise the plant-based diet index.

Table 1: Scoring in the plant-based diet index

Healthy plant foods Unhealthy plant foods Animal foods
Fruits Fruit juice Meat
Vegetables Refined grains Fish
Whole grains Potatoes Eggs
Nuts Sugar-sweetened beverages Dairy
Tea & coffee Sweets & desserts Animal fat
Vegetable oils    
The first time this scoring was used was in a reanalysis of the PREDIMED study, a randomised study of a Mediterranean-style diet high in olive oil or nuts compared to a 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 largely down to the higher consumption of whole plant foods and those that consumed a diet achieving a high plant-based diet score 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 score have a significant reduction in numerous chronic conditions, and even a lower risk of premature death (Table). These same studies have also shown that a diet with a high uPDI score is associated with a neutral or increased risk of these same chronic diseases, even if there is a low consumption of animal-sourced foods.

Table 2 – The impact of a healthy plant-based diet on disease risk

Disease Risk reduction
Coronary heart disease1 25%↓
Type 2 diabetes2 34%↓
Cancer risk3 15%↓
Stroke4 10%↓
Renal failure5 14%↓
Fatty liver6 24%↓
Parkinson’s disease7 22%↓
Sleep apnoea8 17%↓
Venous thromboembolism9 14%↓
COVID-19 incidence/severity10 10%↓/40↓%
All-cause mortality11,12 10-16%↓

1JACC 2017 https://doi.org/10.1016/j.jacc.2017.05.047 2PLOS Medicine 2016 https://doi.org/10.1371/journal.pmed.1002039 3International Journal of Cancer 2018 https://doi.org/10.1002/ijc.31593 4Neurology 2021 DOI: 10.1212/WNL.0000000000011713 5Clinical Journal of the American Society of Nephrology 2019 https://doi.org/10.2215/CJN.12391018 6Clinical nutrition 2019 https://doi.org/10.1016/j.clnu.2018.08.010 7Mov Disord. 2023 https://doi.org/10.1002/mds.29580 8ERJ Open Res 2023 https://doi.org/10.1183/23120541.00739-2023 9JACC 2024 https://doi.org/10.1016/j.jacadv.2024.101318 10Gut. 2021 Nov;70(11):2096-2104. https://gut.bmj.com/content/70/11/2096 11Circulation 2019 https://doi.org/10.1161/CIRCULATIONAHA.119.041014 12JAMA Netw Open.2023; doi:10.1001/jamanetworkopen.2023.4714

People always question why potatoes are scored negatively in the plant-based diet index. This is because in the studies led by researchers from Harvard University, such as the Nurses’ Health Study and the Health Professionals Follow-up study, consumption of potatoes, regardless of cooking methods, has been associated with an increased risk of overweight and obesity, high blood pressure and type 2 diabetes. However, other studies from different populations have not replicated these findings and a more recent meta-analysis finds that the detrimental impact of eating potatoes is due to cooking methods such as frying, rather than the potato itself. However, favouring whole grains rather than potatoes has additional health benefits. In general, potatoes of all colours can be included in a healthy plant-based diet, but of course the variety of plants in the diet is also important.

So, in summary, look out for studies that analyse dietary data using the PDI to better understand the association between plant-based diets and health. This overcomes some of the issues related to limited numbers of vegans and vegetarians within study cohorts and also provides a way of assessing the impact of diet quality.

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