Increased dietary intake of calcium during childhood, usually as calcium in milk, is associated with increased bone mass in adulthood; the increase in mass is important in modifying the later risk of fracture. Whether the increase is due to the calcium content of milk, however, is not certain. We conducted a three-year, double-blind, placebo-controlled trial of the effect of calcium supplementation (1000 mg of calcium citrate malate per day) on bone mineral density in 70 pairs of identical twins (mean [+/- SD] age, 10 +/- 2 years; range, 6 to 14). In each pair, one twin served as a control for the other; 45 pairs completed the study. Bone mineral density was measured by photon absorptiometry at two sites in the radius (at base line, six months, and one, two, and three years) and at three sites in the hip and in the spine (at base line and three years). The mean daily calcium intake of the twins given placebo was 908 mg, and that of the twins given calcium supplements was 1612 mg (894 mg from the diet and 718 mg from the supplement). Among the 22 twin pairs who were prepubertal throughout the study, the twins given supplements had significantly greater increases in bone mineral density at both radial sites (mean difference in the increase in bone mineral density: midshaft radius, 5.1 percent [95 percent confidence interval, 1.5 to 8.7 percent]; distal radius, 3.8 percent [95 percent confidence interval, 1.4 to 6.2 percent]) and in the lumbar spine (increase, 2.8 percent [95 percent confidence interval, 1.1 to 4.5 percent]) after three years; the differences in the increases at two of three femoral sites approached significance (Ward's triangle in the femoral neck, 2.9 percent; greater trochanter, 3.5 percent). Among the 23 pairs who went through puberty or were postpubertal, the twins given supplements received no benefit. In prepubertal children whose average dietary intake of calcium approximated the recommended dietary allowance, calcium supplementation increased the rate of increase in bone mineral density. If the gain persists, peak bone density should be increased and the risk of fracture reduced.
