The number of gut-colonizing bacteria (1,014 cells) is comparable to that of all the cells in the human body and is 10-fold higher than the number of nucleated human cells. Currently, the gut microbiota is considered a ‘super organ,’ which plays many roles in the human body; namely, it helps in the development and functioning of the immune system (particularly the innate immune responses) and the intestinal epithelial barrier; it induces anti-inflammatory cytokines, produces vitamins (primarily group B vitamins and vitamin K), and facilitates digestion and absorption of nutrients, including the minerals necessary for bone metabolism. Experimental studies in germ-free (GF) mice have shown that the gut microbiota affects osteoclast activity. Absence of intestinal microbiota may lead to increased bone mass, with higher bone mass in GF mice being achieved mainly via inhibition of osteoclastogenesis.

The intestinal microbiota may also affect the absorption of calcium. This process involves mainly short-chain fatty acids, which are formed as a result of fermentation induced by intestinal bacteria and are responsible for the normal functioning of the intestinal epithelial barrier. Taking into account the role of the gut microbiota in osteoporosis development, new ways of modifying the microbiota composition and activity have been sought, and the potential role of probiotics has been considered. Recently published clinical studies have demonstrated that probiotics may facilitate osteoporosis treatment and prevention. A randomized double-blind placebo-controlled trial has shown that oral supplementation with a mixture of Lactobacillus plantarum and Lactobacillus paracasei species in postmenopausal women significantly improved the T-score in comparison with that in the control group.