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. Given the recognized role of the gut microbiota in the pathogenesis of osteoporosis (OP), increasing attention is being directed toward novel strategies aimed at modulating its composition and activity. In this context, probiotics may play a significant role. Probiotics are defined as live microorganisms, most commonly bacteria, which, when administered in adequate amounts, confer beneficial effects on the physiological functions of the host, particularly through modulation of the gut microbiota.

Recently published clinical studies have demonstrated that oral supplementation of postmenopausal women with a combination of three Lactobacillus plantarum and Lactobacillus paracasei strains resulted in a statistically significant reduction in bone mineral density loss, as reflected by changes in T-scores, compared with the control group.