Background: The importance of the gut microbiome for bone metabolism in mice has recently been demonstrated, but no studies are available in humans. Lactobacillus reuteri ATCCPTA 6475 (L. reuteri 6475) has been reported to increase bone mineral density (BMD) in mice but its effect on the human skeleton is unknown. The objective of this trial was to investigate if L. reuteri 6475 affects bone loss in older women with low BMD.

Methods: In this double-blind, placebo-controlled study, women from the population who were 75 to 80 years old and had low BMD were randomized to orally receive 10¹⁰ colony-forming units of L. reuteri 6475 daily or placebo. The predefined primary end-point was relative change after 12 months in tibia total volumetric BMD (vBMD).

Results: Ninety women were included and 70 completed the study. L. reuteri 6475 reduced loss of total vBMD compared to placebo both in the intention-to-treat (ITT) analysis [-0.83% (95% confidence interval [CI], -1.47 to -0.19%) vs. -1.85% (95% CI, -2.64 to -1.07%); mean difference 1.02% (95% CI, 0.02-2.03)] and per protocol analysis [-0.93% (95% CI, -1.45 to -0.40) vs. -1.86% (95% CI, -2.35 to -1.36); mean difference 0.93% (95% CI, 0.21-1.65)]. In general, similar but smaller effects were observed in the secondary bone variable outcomes, but these differences did not reach statistical significance in the ITT population. Adverse events did not differ between groups.

Conclusions: Supplementation with L. reuteri 6475 should be further explored as a novel approach to prevent age-associated bone loss and osteoporosis.