Abstract
Bone βdensityβ (bone mass/bone volume) declines with age from the menopause in women and from about age 55 in men. This fall in bone density (osteoporosis) weakens the bones and leads to a progressive rise in fracture rates, particularly in women. Many risk factors contribute to the boneβlosing process, but one which attracts increasing attention is calcium absorption.
The main physiological regulator of calcium absorption is vitamin D. This is manufactured in the skin under the influence of UVβlight and then converted to more potent metabolites in the liver and kidney. Although the serum levels of the most potent metabolite 1,25(OH)~2~D~3~ (calcitriol) are generally normal in osteoporotic women, treatment with small doses of calcitriol (about 0.25 ΞΌg daily) has a remarkable effect on absorptive performance and slows down the rate of bone loss. Improved synthetic metabolites are under development.
There is likely also to be greatly increased scope for the use of vitamin D itself in osteoporosis. With advancing age, there is a tendency for men and women to be exposed to less and less sunlight, which is the main natural source of vitamin D. Vitamin D levels, therefore, decline with age, particulaly in those who are housebound, and are found to be low in most reported series of hip fractures. It is likely that this form of vitamin D βinsufficiencyβ has an adverse effect on calcium absorption in the elderly which accelerates bone loss and increases the risk of hip fracture and can be treated with small doses of vitamin D or its 25βhydroxy derivative. This offers a second important role for vitamin D in the prevention and management of osteoporosis.