Calcium and bone health

Although genetic factors are important in attainment of peak bone mass, there is some evidence that it can be enhanced by increased calcium consumption, particularly during puberty. There has however been considerable debate regarding the role of calcium throughout lifetime. After peak bone mass has been achieved (generally in the early twenties) there is a period of relative stability when higher calcium intake may not be necessary although low calcium intake may still be associated with low bone mass.

Calcium supplements have been widely used in managing established osteoporosis. However, there have been few prospective studies of their impact on either bone density or the risk of fracture.

Women within five years of the menopause lose bone at similar rates regardless of calcium supplementation. It seems that the benefits of calcium supplementation are maximum in older women (those who are more than five years beyond the menopause) and particularly in those with low calcium intake (less than 400 mg per day.) or if they have established osteoporosis. Calcium does not generally increase bone mass at the lumbar spine. However, although the effects of calcium intake on hip fracture risk are controversial, pharmacological doses of calcium have been shown to delay the rate of bone loss. In patients on a low-calcium diet calcium supplementation augments the effect of hormone replacement therapy at the femoral neck.

Few recent studies suggested the association of calcium supplementation and increased risk of cardiovascular events. In view of this the current recommendation is that all patients with low BMD or osteoporosis should maximize calcium intake in their diet rather than take calcium supplements. However, a proportion of osteoporotic patients with low serum calcium levels should still be prescribed calcium supplements.

An adequate calcium intake is essential at all ages. The recommended calcium intake is a minimum of 1000 mg daily for adults of either sex. Calcium intake should however be higher at specific periods of life: for adolescents during skeletal growth 1200-1500 mg/day, during pregnancy 1200-1500 mg/day and in postmenopausal women 1500 mg/day.

The greater requirement for postmenopausal women is because of the combined effects of less efficient calcium absorption from the gut and poorer calcium reabsorpiton by the kidneys. Treatment of postmenopausal women with oestrogen results in better absorption and retention and lowers daily calcium requirements. Only elemental calcium is available for absorption. Calcium is absorbed primarily in the small intestine. Absorption is equally effective from calcium supplements as from food rich in calcium. Absorption is complete within 4 hours of calcium intake and is more efficient with low amounts of calcium. Intestinal calcium absorption is improved if supplements are taken with meals. Calcium supplements should be taken at night as bone turnover is most active during the night. If more than 500 mg/day are needed, this should be taken in two doses so that there is less likelihood of intestinal absorptive mechanisms being saturated.

Renal stones generally form in individuals who lack the enzymes necessary to keep the calcium and urine in solution, not because of excessive calcium intake. Since calcium citrate inhibits calcium oxalate crystalization, the former salt is the preparation of choice for those at risk of kidney stone formation. The 24h urinary excretion of calcium should be monitored and kept below 300mg/day with a high fluid intake. High calcium intake may occasionally cause constipation and flatulence or possibly decrease iron availability.