Diet has a significant impact on fecal microbiota, which in turn plays an important role in human health. To evaluate the impact of dietary habits on fecal microbiota, we investigated the fecal microbial composition in vegetarians and omnivores using 16S rRNA gene sequencing, and estimated the correlation between fecal microbiota, body mass and diet. The dietary data showed that vegetarians consumed more plant-based foods rich in dietary fiber, omnivores consumed more animal-based foods rich in fat and overweight and obese people consumed more high-energy foods. Compared to omnivores, vegetarians had greater richness and diversity in their fecal microbiota. The Firmicutes/Bacteroidetes ratio was lower and the Prevotella/Bacteroides ratio was higher in vegetarians. The meat intake correlated positively with the proportion of Bacteroides and negatively with the proportion of Prevotella. The composition and diversity in fecal microbiota in the normal weight group, overweight group and obesity group were similar to that of vegetarians and omnivores, respectively. This paper revealed the distinctive characteristics of fecal microbiota in vegetarians and omnivores. The omnivorous diet contained more fat, which reduced the fecal microbial diversity, and was more likely to lead to being overweight or obese.

Background: Data on the nutritional adequacy of unsupplemented vegetarians in Northeast China are limited. Methods: We compared dietary intake, body composition, and serum biomarkers among vegans, lacto-ovo-vegetarians, and omnivores. This cross-sectional study included 356 adults (all diet ≥ 2 years, no supplements). Dietary intake was assessed using a validated semi-quantitative FFQ, body composition by BIA, and serum biomarkers. Kruskal–Wallis tests with Bonferroni correction were used. Results: Vegans had lower BMI (22.0 vs. 24.6 kg/m²), body fat (24.5% vs. 28.0%), and visceral fat (0.65 vs. 1.05 L) than omnivores (all p < 0.002). Vegans consumed more fiber (38.5 vs. 18.0 g/d) and vitamin C (180 vs. 85 mg/d), but less vitamin B₁₂ (0.3 vs. 4.2 μg/d), vitamin D (0.5 vs. 3.2 μg/d), calcium (520 vs. 720 mg/d), iodine (65 vs. 130 μg/d), and selenium (45 vs. 85 μg/d). Serum vitamin B₁₂, 25-(OH)D, ferritin, and selenium were significantly lower in vegans, while homocysteine was higher. The proportion of vegans with dietary intake below the recommendation reached 100% for vitamin B₁₂ and 97% for vitamin D, whereas omnivores showed excessive sodium (75%) and SFA (70%) intake. Conclusions: In this Northeast China cohort, unsupplemented vegetarian diets offered favorable macronutrient profiles and body composition but were associated with a high prevalence of dietary intakes below recommendations for vitamin B₁₂, vitamin D, iodine, selenium, zinc, and calcium. These findings underscore the need for targeted supplementation and food fortification strategies for individuals adhering to plant-based diets without supplement use in this region.