Loss of bone is an almost universal accompaniment of aging that
proceeds at an average rate of 0.5-1% per annum from midlife
onwards. There are at least four nutrients involved in this
process: calcium, salt, protein, and vitamin D, at least in
women. The pathogenesis of osteoporosis in men is more obscure.
Calcium is a positive risk factor because calcium requirement
rises at the menopause due to an increase in obligatory calcium
loss and a small reduction in calcium absorption that persist
to the end of life. A metaanalysis of 20 calcium trials shows
that this process can generally be arrested by calcium supplementation,
although there is some doubt about its effectiveness in the
first few years after menopause. Salt is a negative risk factor
because it increases obligatory calcium loss; every 100 mmol
of sodium takes 1 mmol of calcium out of the body. Restricting
salt intake lowers the rate of bone resorption in postmenopausal
women. Protein is another negative risk factor; increasing animal
protein intake from 40 to 80 g daily increases urine calcium
by about 1 mmol/day. Low protein intakes in third world countries
may partially protect against osteoporosis. Vitamin D (sometimes
called a nutrient and sometimes a hormone) is important because
age-related vitamin D deficiency leads to malabsorption of calcium,
accelerated bone loss, and increased risk of hip fracture. Vitamin
D supplementation has been shown to retard bone loss and reduce
hip fracture incidence in elderly women.
