BACKGROUND Mesenchymal stem cells（MSC） of bone marrow are the progenitor of osteoblasts and adipocytes.MSC tend to differentiate into adipocytes,instead of osteoblasts,with aging.This favors the loss of bone mass and development of osteoporosis.Hypoxia induces hypoxia inducible factor 1α gene encoding transcription factor,which regulates the expression of genes related to energy metabolism and angiogenesis.That allows a better adaptation to low O₂ conditions.Sustained hypoxia has negative effects on bone metabolism,favoring bone resorption.Yet,surprisingly,cyclic hypoxia（CH）,short times of hypoxia followed by long times in normoxia,can modulate MSC differentiation and improve bone health in aging.AIM To evaluate the CH effect on MSC differentiation,and whether it improves bone mineral density in elderly.METHODS MSC cultures were induced to differentiate into osteoblasts or adipocytes,in CH（3% O₂ for 1,2 or 4 h,4 d a week）.Extracellular-matrix mineralization and lipid droplet formation were studied in MSC induced to differentiate into osteoblast or adipocytes,respectively.In addition,gene expression of marker genes,for osteogenesis or adipogenesis,have been quantified by quantitative real time polymerase chain reaction.The in vivo studies with elderly（>75 years old;n = 10） were carried out in a hypoxia chamber,simulating an altitude of 2500 m above sea level,or in normoxia,for 18 wk（36 CH sessions of 16 min each）.Percentages of fat mass and bone mineral density from whole body,trunk and right proximal femur（femoral,femoral neck and trochanter） were assessed,using dual-energy X-ray absorptiometry.RESULTS CH（4 h of hypoxic exposure） inhibited extracellular matrix mineralization and lipid-droplet formation in MSC induced to differentiate into osteoblasts or adipocytes,respectively.However,both parameters were not significantly affected by the other shorter hypoxia times assessed.The longest periods of hypoxia downregulated the expression of genes related to extracellular matrix formation,in MSC induced to differentiate into osteoblasts.Interestingly,osteocalcin（associated to energy metabolism） was upregulated.Vascular endothelial growth factor an expression and low-density lipoprotein receptor related protein 5/6/dickkopf Wnt signaling pathway inhibitor 1（associated to Wnt/β-catenin pathway activation） increased in osteoblasts.Yet,they decreased in adipocytes after CH treatments,mainly with the longest hypoxia times.However,the same CH treatments increased the osteoprotegerin/receptor activator for nuclear factor kappa B ligand ratio in both cell types.An increase in total bone mineral density was observed in elderly people exposed to CH,but not in specific regions.The percentage of fat did not vary between groups.ConCLUSION CH may have positive effects on bone health in the elderly,due to its possible inhibitory effect on bone resorption,by increasing the osteoprotegerin/receptor activator for nuclear factor kappa B ligand ratio.