The gut microbiota (GM) (mainly bacteria but also other microorganisms) has been suggested to regulate health risks and morbidity. However, previous human studies have included few participants, and the results have often been difficult to reproduce. In addition to the low number of participants, the low reproducibility may be due to the fact that the studies were cross-sectional and that they did not adjust for important confounding factors such as diet, bowel movements, medications, physical activity, smoking, and comorbidities. Furthermore, most previous studies have assessed the gut microbiota composition with a technique called 16S sequencing that do not provide detailed information on the gut microbiota composition on the species level or information on the functionality. In addition, it has not previously been possible to study whether the GM exerts effects on morbidity (= causality) or if the gut microbiota only statistically correlates with morbidity. The overall objective of the proposed project is to determine whether the GM affects the musculoskeletal health and thus the risk of suffering bone fractures. A large emphasis will be placed on determining causality, i.e. if the GM affects health and morbidity or if the morbidity affects the GM.