Adherence to the vegetarian diet may increase the risk of depression: a systematic review and meta-analysis of observational studies

Context: Several epidemiological studies have investigated the association between a vegetarian diet and risk of depression, but because of inconsistency between studies, the exact association remains unclear. Objective: In this systematic review and meta-analysis, the relationship between vegetarian diets and risk of depression in observational studies was evaluated. Data sources: The Medline, Embase, Scopus, ISI Web of Science, and Cochrane Library databases were searched from inception through September 1, 2020. Study selection: Observational studies were included that examined mean levels of depression and risk for depression in vegetarians compared with nonvegetarians. Data extraction: Pooled effect sizes were estimated using the random-effects model and were reported as standardized mean differences or odds ratios (ORs) with their corresponding 95%CIs. Heterogeneity was tested using the I-2 statistic. Results: Combining 9 effect sizes in this meta-analysis illustrated that adherence to a vegetarian diet was associated with a 53% greater risk of depression compared with that of omnivores (95%CI, 1.14-2.07; I-2 = 69.1%). Subgroup analysis of depression risk suggested that results depended on the type of vegetarian diet and country where the study was conducted. For studies that assessed a semivegetarian diet (OR, 1.86; 95%CI, 1.42-2.44; I-2 = 35.7%) and those conducted in Europe and the United States (OR, 1.45; 95%CI, 1.06-1.98; I-2 = 73.2%), there was a positive association between a vegetarian diet and depression, but in lacto-ovo vegetarians and Asian countries, a null association was found. Comparing mean depression scores showed no evidence of difference between vegetarians and nonvegetarians (n = 16; standardized mean difference, 0.10; 95%CI, -0.01 to 0.21; I-2 = 79.1%). Conclusion: Vegetarian diet significantly increased depression risk; however, the findings were not robust, and more studies are required to investigate the vegetarian diet and depression association.