The impact of vegetarian diet on sperm quality, sex hormone levels and fertility: a systematic review and meta-analysis

BackgroundAn increasing number of people have adhered to a vegetarian diet for several years. Nowadays, the favourable effect of this dietary pattern on metabolic diseases is well established, but its impact on fertility and reproductive health as a serious health concern is not clear yet.Therefore, we aim to summarise existing evidence regarding the possible association between a vegetarian diet and fertility as measured by key indicators such as sperm quality and sex hormone levels.BackgroundAn increasing number of people have adhered to a vegetarian diet for several years. Nowadays, the favourable effect of this dietary pattern on metabolic diseases is well established, but its impact on fertility and reproductive health as a serious health concern is not clear yet.Therefore, we aim to summarise existing evidence regarding the possible association between a vegetarian diet and fertility as measured by key indicators such as sperm quality and sex hormone levels.MethodsWe systematically searched online databases, including PubMed, ISI, Scopus and Google Scholar, up to 1 December 2022 using relevant keywords. We included observational studies that compared semen quality, sex hormone levels and infertility in people who adhered to a vegetarian diet versus an omnivore diet. Heterogeneity between studies was assessed using I2 and Q tests. Standardised mean differences (SMD) using a random/fixed model were calculated to assess outcomes between vegetarians and omnivores in included articles.ResultsFinally, out of 972 documents that were retrieved, 20 articles met our inclusion criteria, and 16 were eligible for quantitative synthesis. Results of meta-analyses showed that there were no significant differences between vegetarians and omnivores in terms of semen quality parameters, including total sperm count, total and progressive sperm motility, sperm morphology and sperm concentration. Seven studies on female sex hormone profiles were eligible for meta-analysis. The only significant difference was the lower level of plasma oestrone in vegetarians (pooled SMD: -0.56; 95% confidence interval [CI]: -1.08, -0.05; p-value = 0.03) compared to omnivores. Furthermore, our meta-analysis revealed significantly higher sex-hormone-binding globulin levels in vegetarian men than in omnivores (pooled SMD: 0.52; 95% CI: 0.18, 0.86; p-value = 0.002).ConclusionDespite the numerous health benefits of a vegetarian diet, our review suggested that there were no conclusive positive or negative associations between vegetarian diet and semen quality, sex hormone levels and infertility. Further studies are recommended to better understand vegetarian dietary pattern effect on infertility and reproductive health. Semen analysis parameters and sex hormone levels were compared between vegetarians and omnivores in our systematic review study. No significant differences were found in terms of semen analysis parameters between the two groups. Except for lower plasma oestrone levels in vegetarian females and higher sex-hormone-binding globulin in vegetarian males, no significant differences were found regarding sex hormones.Key pointsimage This systematic review and meta-analysis examined the association between vegetarian diet and fertility, focusing on sperm quality and sex hormone levels. The results indicate that there were no significant differences in semen quality parameters between vegetarians and omnivores; however, female vegetarians showed lower plasma oestrone levels, and male vegetarians exhibited higher sex-hormone-binding globulin levels compared to omnivores. Further research is warranted to gain a better understanding of the effect of vegetarian diet on reproductive health and fertility.