We have previously in animal studies shown that the gut microbiota regulates sex steroid metabolism. In addition, we and others have shown that both the gut microbiota and sex steroids are crucial regulators of bone mineral density. However, it is unknown if the effect of the gut microbiota on bone health in humans is mediated via effects on sex steroid levels. We will test our hypothesis that the effect of the gut microbiota on bone mineral density is mediated via effect on sex hormone levels. This will be studied using both traditional association studies and Mendelian randomization, giving information on causality.