The human body contains over 10 times more microbial cells than human cells, although the entire micro biome only accounts for about for 1-3% total body mass. Hundreds of trillions of microbes live in the human gut, with 300 times the total DNA as humans. The products of this “forgotten organ” include large amounts of DNA that are critical to create necessary human nutrients, for essential metabolism and to develop the most effective immune system. Recent research shows dramatic effects of microbe products from the gut on mental function—depression, stress, autism, and degenerative illness. In humans, many studies show microbes affect anxiety, mood, depression and social behavior. Direct effects are through secreted products, stimulation of the enteric nervous system and travel of microbes into the brain, while indirect factors are microbes’ influence on immune function affecting behavior. Microbes produce molecules that transform into hormones and neurotransmitters or they produce neurotransmitters themselves. Microbes effect on the brain includes fetal development and neurotransmitter function.
Increasing evidence suggests that the composition of the human gut microbiome is important in the etiology of human diseases; however, the personal factors that influence the gut microbiome composition are poorly characterized. Animal models point to sex hormone-related differentials in microbiome composition. In this study, we investigated the relationship of sex, body mass index (BMI) and dietary fiber intake with the gut microbiome in 82 humans. We sequenced fecal 16S rRNA genes by 454 FLX technology, then clustered and classified the reads to microbial genomes using the QIIME pipeline. Relationships of sex, BMI, and fiber intake with overall gut microbiome composition and specific taxon abundances were assessed by permutational MANOVA and multivariate logistic regression, respectively. We found that sex was associated with the gut microbiome composition overall (p=0.001). The gut microbiome in women was characterized by a lower abundance of Bacteroidetes (p=0.03). BMI (>25 kg/m2 vs. <25 kg/m2) was associated with the gut microbiome composition overall (p=0.05), and this relationship was strong in women (p=0.03) but not in men (p=0.29). Fiber from beans and from fruits and vegetables were associated, respectively, with greater abundance of Actinobacteria (p=0.006 and false discovery rate adjusted q=0.05) and Clostridia (p=0.009 and false discovery rate adjusted q=0.09). Our findings suggest that sex, BMI, and dietary fiber contribute to shaping the gut microbiome in humans. Better understanding of these relationships may have significant implications for gastrointestinal health and disease prevention.