The risk of nutrient deficiencies increases with age in our modern Western society, and vitamin B 12 deficiency is especially prevalent in the elderly and causes increased homocysteine (Hcy) and methylmalonic acid (MMA) levels. These three factors have been recognized as risk factors for reduced bone mineral density and increased fracture risk, though mechanistic evidence is still lacking. In the present study, we investigated the influence of B 12 , Hcy, and MMA on differentiation and activity of bone cells. B 12 deficiency did not affect the onset of osteoblast differentiation, maturation, matrix mineralization, or adipocyte differentiation from human mesenchymal stem cells (hMSCs). B 12 deficiency caused an increase in the secretion of Hcy and MMA into the culture medium by osteoblasts, but Hcy and MMA appeared to have no effect on hMSC osteoblast differentiation. We further studied the effect of B 12 , Hcy, and MMA on the formation of multinucleated tartrate-resistant acid phosphatase-positive osteoclasts from mouse bone marrow. We observed that B 12 did not show an effect on osteoclastogenesis. However, Hcy as well as MMA were found to induce osteoclastogenesis in a dose-dependent manner. On the basis of these results, we conclude that B 12 deficiency may lead to decreased bone mass by increased osteoclast formation due to increased MMA and Hcy levels.Osteoporosis is a complex multifactorial disease with a strong genetic component, but lifestyle factors are also important for maintenance of bone mass. Nutrition is one of the factors that contributes to bone health. During the first decades of life, adequate intake of calcium and vitamin D is essential to establish peak bone mass (see [1,2] and references therein). It has also been observed that nutritional deficiencies can cause impaired growth and skeletal abnormalities during childhood [3], while in adults inadequate intake of nutrients such as proteins, vitamins, and minerals has been associated with loss of bone quality [2,4]. Nutritional deficiency is very common in the elderly, emphasizing that adequate nutrient intake may slow down the age-related loss of bone mass.A challenge in nutrition research is to identify food components that contribute to the maintenance of bone health. During the last years, vitamin B 12 has gained particular attention since a number of epidemiological studies have reported a correlation between B 12 status and bone quality and fracture risk [5][6][7][8][9]. The micronutrient B 12 is involved as a cofactor in two enzymatic reactions [10]. The enzyme methionine synthase requires B 12 for the remethylation of homocysteine (Hcy) into methionine. The other enzyme dependent on B 12 is methylmalonyl-CoA mutase, which catalyzes the conversion of methylmalonyl-CoA into succinyl-CoA. As a consequence of B 12 deficiency, these