calcium retention was 130 mg (3.3 mmol)/day, a value somewhat lower than the 174 mg (4.3 mmol)/day obtained in the meta-analyses cited above (Matkovic, 1991; Matkovic and Heaney, 1992).
Although calcium balance with varying levels of intake is possibly a more useful method, few investigators have estimated it in children. Calcium retention averaged 76 mg (1.9 mmol)/day in five girls aged 3 to 5 years when the girls had an intake of 370 mg (9.3 mmol)/day for 8 weeks and increased to an average of 122 mg (3.0 mmol)/day when the girls consumed 615 mg (15.4 mmol)/day for the next 5 weeks (Outhouse et al., 1939). Calcium intakes of about 200 to 280 mg (5 to 7 mmol)/day in children from India resulted in absorption rates of approximately 50 percent of the calcium while maintaining a small positive balance of 50 to 60 mg (1.3 to 1.5 mmol)/day (Begum and Pereira, 1969).
Calcium Accretion. The few studies of usual calcium accretion rates in small children give generally comparable results to that derived from balance studies. Calcium accretion rate was estimated from body weight values during childhood and adolescence (Leitch and Aitken, 1959). Although the method was not independently validated, there is a very close correlation between BMC and body weight in small children (Ellis et al., 1996). Calcium accretion on the order of 60 to 100 mg (1.5 to 2.5 mmol)/day by boys and girls aged 2 to 5 years and 100 to 160 mg (2.5 to 4.0 mmol)/day by boys and girls aged 6 to 8 years was calculated.
Bone Mineral Content. Recently, total body BMC by DXA was used to calculate average mineral increments in a population of approximately 100 children aged 3 to 10 years (Ellis et al., 1997). The rate of accretion in Caucasian (n = 46) and Hispanic (n = 23) children increased from approximately 150 mg (3.8 mmol)/day of calcium at age 5 to approximately 200 mg (5 mmol)/day at age 8. Values for African Americans (n = 36) were approximately 20 to 30 mg (0.5 to 0.8 mmol)/day greater at each age.
Intervention trials in which children were randomized to different calcium intakes have resulted in short-term changes in BMC. In one of the few intervention trials conducted in young children, 22 prepubertal identical twin pairs averaging 7 years of age were randomized to receive either calcium supplements or placebo (Johnston et al., 1992). Those receiving supplements had an increase in their mean intake from approximately 900 to 1,600 mg (22.5 to 40 mmol)/day which resulted in a significant increase in BMD in the radius and lumbar spine after 36 months of treatment