Magnesium is related to a number of biological enzymatic reactions such as catalytic role for the reaction of kinases in ATP production. On the other hand, magnesium is one of the essential minerals for bone formation. In the magnesium-deficient rats, apparent bone loss caused by increase in bone resorption and decrease in bone formation was observed. Although, epidemiological studies suggest that magnesium deficiency is one of the risk factor for osteoporosis, a relationship between magnesium intake and bone mineral density is not clear. This may be due to the differences in the population, decrease in sex hormone secretion, and the possibility that magnesium-deficiency is also accompanied with another nutrient insufficiency, e.g., calcium.