Large-scale study finds no mortality benefit from vegetarian diets in U.S. adults, challenges previous beliefs

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Benedette Cuffari, M.Sc.Nov 26 2023

A brief report published in the Journal of Health, Population, and Nutrition describes the effect of vegetarian diets on all-cause mortality in adults in the United States.

Study: Vegetarian diets and risk of all-cause mortality in a population-based prospective study in the United States. Image Credit: Yulia Furman / Shutterstock.com

Background

Chronic diseases, including cardiovascular and metabolic diseases and cancer, account for most deaths in the U.S. Between 1990 and 2010, about 26% and 22% of all-cause and cardiovascular disease-related mortality in the U.S., respectively, occurred due to an unhealthy diet.

In recent decades, the vegetarian diet has gained significant popularity, mainly because of its health and environmental benefits. Plant-based vegetarian diets are known to reduce the risk of cardiovascular disease and diabetes by lowering blood cholesterol and improving insulin sensitivity.

Plant-based foods contain higher amounts of fibers, vitamins, minerals, and bioactive compounds, as well as lower amounts of total and saturated fats, as compared to animal-based foods. Despite these health benefits, some vegetarian diets that exclude all types of animal products, such as a vegan diet, may cause vitamin B12 deficiency, as animal food products are the only natural source of this essential micronutrient.   

In the current study, scientists investigate the association between vegetarian or vegan diets and all-cause mortality among U.S. residents who have registered for the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial.     

Study design

A total of 117,673 participants from the PLCO Cancer Screening Trial were included in the study. All participants completed the Diet History Questionnaire that was provided to them during the trial.

Data on all-cause mortality were obtained from follow-up questionnaires and the National Death Index database. The average follow-up period was 18 years.

Based on self-reported diet data, the participants were categorized into four groups. These groups included vegan, which excluded all animal products, lacto- and ovo-vegetarian, which included dairy products and eggs, pesco-vegetarian, which included fish and seafood, and omnivore, which included all animal products.     

Important observations

The analysis of self-reported diet history questionnaires revealed that about 99.3% of study participants were omnivores, 0.3% were lacto- or ovo-vegetarians, 0.3% were pesco-vegetarians, and 0.1% were vegans.

As compared to vegetarians or vegans, omnivorous participants were more likely to smoke and drink alcohol and had a lower likelihood of completing a college degree. Moreover, omnivorous participants had the highest body mass index (BMI) values, followed by vegans, lacto- and ovo-vegetarians, and pesco-vegetarians.

After the study follow-up period of 18 years, death occurred in 39,763 participants, which included 39,547 omnivores, 88 lacto- and ovo-vegetarians, 67 pesco-vegetarians, and 61 vegans. No statistically significant difference in the risk of all-cause mortality was observed between the study groups.

After adjusting for age, sex, study center location, and trial arm, pesco-vegetarians were associated with a significantly lower risk of all-cause mortality than omnivores. However, after adjusting for all covariates, including smoking status, pack-years of smoking, alcohol consumption, race/ethnicity, education, BMI, and comorbidities, no significant difference in mortality risk was observed between the omnivorous and vegetarian groups.

Likewise, the comparison between vegetarian groups, which comprised vegan and lacto- and ovo-vegetarian diets, and non-vegetarian groups, which included omnivorous and pesco-vegetarian diets, also showed no statistically significant difference in mortality risk after full adjustment.

Study significance

No significant impact of vegetarian diets on all-cause mortality risk was observed in a U.S. population of middle-aged and older adults. Over 90% of the study population identified as omnivores, whereas the remaining study participants self-identified as lacto- and ovo-vegetarians, pesco-vegetarians, or vegans.  

After adjusting for socioeconomic and lifestyle factors and comorbidities, no significant differences in all-cause mortality risk were observed between omnivores and vegetarians.

Notably, the study lacks information on the specific duration of following a vegetarian or vegan diet. Thus, it is possible that the duration of following a vegetarian or vegan diet might not be sufficient for the study participants to have the typical health benefits. Moreover, there could be a possibility of misclassification for each diet group, as the participants reported their vegetarian diet status based on only two questions regarding foods they excluded from their diets.

Given the study limitations, scientists advise that future studies investigating the association between vegetarian diets and mortality risk consider the duration of following vegetarian diets.