The gastrointestinal tract houses a particularly diverse microbial community, so called "microbiota". Variability in the gut microbiota has been linked to many diseases. Because the microbiota is easily modifiable, this could have major therapeutic impact. In particular, it has been shown that serum vitamin D might influence disease risk by modifying the composition of the gut microbiota. This would be the first interventional study on gut microbiota analysis following vitamin D supplementation in vitamin D deficient postmenopausal women. Moreover, the secondary outcome will be to assess biochemical modification of the players of bone metabolism such as PTH, 25(OH)D, 1,25(OH)2D, calcium and phosphorus after cholecalciferol supplementation to investigate their possible correlation with the modification of the microbiota. We decided to utilize two commonly prescribed dosages of vitamin D (25.000 or 50.000 IU per week) as suggested by the guidelines and also to observe if there are any differences between them as regards microbiota modification.
