A comprehensive study from the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS), in collaboration with University Hospital Leipzig, analyzed how vegetarian diets relate to body composition and psychological traits in nearly 9,000 participants—independent of age, gender, and education. The results show that lower animal product intake correlates with reduced body mass index (BMI) and lower extroversion. Unlike prior research, no connection to depressive moods was found.
Participants with fewer animal foods in their diet had lower BMI and body weight. This likely stems from reduced intake of heavily processed, high-fat, and sugary foods common in animal-based diets. “Fat- and sugar-rich products drive weight gain by boosting appetite and delaying satiety,” says Evelyn Medawar, lead author. Plant foods, rich in fiber, support a healthy gut microbiome and promote quicker fullness. “Plant-based eaters often consume fewer calories,” Medawar notes. Factors like more exercise and health awareness may also contribute.
BMI differences hinge on animal product types. Diets heavy in primary products—meat, sausage, fish—link to higher BMI than those emphasizing secondary ones like eggs, milk, dairy, cheese, and butter. The association was statistically significant.
Medawar explains: “A 1.2-point lower average BMI occurs in those fully avoiding primary animal products on a vegetarian diet, or simply eating meat and fish less often.” Causation is unclear; a follow-up study with University Hospital Leipzig will clarify.
Nutrition and Personality
Vegetarian and vegan diets also tie to personality, especially lower extroversion in plant-based eaters. “The reasons are unclear,” says Veronica Witte, study leader. “Introverts may prefer restricted diets, or dietary choices could foster social withdrawal.” More research is underway.
Contrary to some studies, no neuroticism link appeared. “Past work tied neuroticism to broad food restrictions, but animal product avoidance showed no correlation here,” Witte explains.
Prior research hinted at depression risks with plant-based diets, but none was detected. “Confounders like BMI and depression-linked traits may have skewed earlier results; we controlled for them,” Witte notes. Plant-based eating is now more accepted and widespread.
Conducted via the LIFE project with University Hospital Leipzig, the study used questionnaires on past-year animal product frequency (from 'several times daily' to 'never'). Personality was measured with the NEO-FFI inventory; depression via the CES-D scale.
