Some parts of the UK media made a fuss over the findings of a study of more than 200,000 US adults (Satija A, et al., 2017) casting doubt on the health benefits of a vegetarian diet. RAC member, Paul Appleby considers this study in more detail:
The abstract (summary) of the study reads as follows:
Background: Plant-based diets are recommended for coronary heart disease (CHD) prevention. However, not all plant foods are necessarily beneficial for health.
Objectives: This study sought to examine associations between plant-based diet indices and CHD incidence.
Methods: We included 73,710 women in Nurses’ Health Study (NHS) (1984 to 2012), 92,329 women in NHS2 (1991 to 2013), and 43,259 men in Health Professionals Follow-up Study (1986 to 2012), free of chronic diseases at baseline. We created an overall plant-based diet index (PDI) from repeated semi-quantitative food-frequency questionnaire data, by assigning positive scores to plant foods and reverse scores to animal foods. We also created a healthful plant-based diet index (hPDI) where healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) received positive scores, whereas less-healthy plant foods (juices/sweetened beverages, refined grains, potatoes/fries, sweets) and animal foods received reverse scores. To create an unhealthful PDI (uPDI), we gave positive scores to less-healthy plant foods and reverse scores to animal and healthy plant foods.
Results: Over 4,833,042 person-years of follow-up, we documented 8,631 incident CHD cases. In pooled multivariable analysis, higher adherence to PDI was independently inversely associated with CHD (hazard ratio [HR] comparing extreme deciles: 0.92; 95% confidence interval [CI]: 0.83 to 1.01; p trend = 0.003). This inverse association was stronger for hDPI (HR: 0.75; 95% CI: 0.68 to 0.83; p trend <0.001). Conversely, uPDI was positively associated with CHD (HR: 1.32; 95% CI: 1.20 to 1.46; p trend <0.001). [Hazard ratios less than 1 indicate a lower risk of disease among those with a high value for the diet index, whereas ratios above 1 indicate a higher risk of disease; ‘p’ values are a measure of statistical significance, the smaller the value, the more believable the result. – Editor]
Conclusions: Higher intake of a plant-based diet index rich in healthier plant foods is associated with substantially lower CHD risk, whereas a plant-based diet index that emphasises less-healthy plant foods is associated with higher CHD risk.
The first point to note is that this was not a study of vegetarian/vegan diets in relation to coronary heart disease: relatively few of the 200,000 participants were vegetarians. Nevertheless, a vegetarian or vegan would be expected to have a high plant-based diet index (PDI), irrespective of what sort of plant foods they ate (healthful or otherwise). A health-conscious vegetarian/vegan would be expected to have a high healthy PDI (hPDI) score, but a ‘junk food’ vegetarian/vegan would have a high unhealthy PDI (uPDI) score. Another important point to note is that whereas all plant foods are scored in opposition to (all) animal foods under the PDI, healthy plant foods are scored in opposition to both unhealthy plant foods and animal foods under the hPDI, and unhealthy plant foods are scored in opposition to both healthy plant foods and animal foods under the uPDI.
Confusing, isn’t it! Imagine three people: Mrs L is a lacto-vegetarian who eats a lot of dairy products, whole grains, fruit and vegetables, but shuns ‘junk food’; Mr F is a health-conscious ‘flexitarian’ who follows a largely whole food vegan diet but occasionally eats some low-fat meat and fish; Ms J is a vegan who, for whatever reason, eats a lot of fast food such as chips and refined grains but relatively few fruits and vegetables. Mrs L would have a mid-range PDI, a high hPDI, and a low uPDI; Mr F would score rather higher for PDI and hPDI (by virtue of eating fewer animal foods than Mrs L), and have a very low uPDI; Ms J would score top marks for PDI and uPDI, but low marks for hPDI (despite not eating any animal foods). Thus, Mrs L and Mr F in particular would be expected to have a lower coronary heart disease risk than Ms J.
So, what is the moral of the story? A vegetarian/vegan diet is not a panacea: a ‘junk-food’ vegetarian/vegan diet is no more healthy than a ‘junk-food’ omnivorous one, and may be less healthy than the diet of a health-conscious omnivore, at least as far as coronary heart disease is concerned. As an article commenting on the study on the NHS website pointed out, vegetarians and vegans should, like everyone else, aim to “eat a balanced diet with at least five portions of fruit and vegetables every day, eat less sugar, salt, and saturated fat, and choose wholegrain carbohydrates where possible” [accessed 14/11/2017]
References
1. Satija, A. et al (2017) Healthful and Unhealthful Plant-Based Diets and the Risk of Coronary Heart Disease in U.S. Adults. Journal of the American College of Cardiology, 70 (4) 411-422; DOI: 10.1016/j.jacc.2017.05.047 [accessed 14/11/2017]
Review by RAC member Paul Appleby. This review originally appeared in in the September/October 2017 issue of OxVeg News.
The views expressed by our Research News contributors are not necessarily the views of The Vegan Society.
The views expressed by our Research News contributors are not necessarily the views of The Vegan Society.
