 The role of the gut microbes for human health has gained considerable interest in recent years. Lactobacillus reuteri L.reuteri is a naturally occurring bacterial species in the human gut. L. reuteri has been widely studied in clinical trials of adults and children and treatment with L. reuteri is safe and is associated with health-promoting effects in humans. In animal models, L. reuteri has anti-inflammatory effects and in ovariectomized mice supplementation with L. reuteri partly prevented the bone loss induced by estrogen deficiency. Furthermore, both bone density and bone formation increased in male mice in another mice model. In mice with medically induced diabetes mellitus, L. reuteri reduced blood glucose and in a mouse model mimicking the metabolic syndrome, L. reuteri prevented diet-induced obesity. The present study is a double-blind, placebo-controlled, randomized, study in 90 elderly women with osteopenia recruited from the population. These women will be treated with L. reuteri or placebo orally twice daily for 12 months. The effects on bone will be investigated with dual energy x-ray absorptiometry, high-resolution peripheral quantitative computed tomography, and bone turnover markers. Hormones and markers of inflammation and metabolism will be followed as well as changes in the gut microbiota composition. 