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several studies from the United States and the United Kingdom, as summarized in Atkinson et al. (1995). Variations in milk calcium content have been found between population groups. For example, in comparison with the above data from the United States, milk calcium concentrations have been found to be lower (by approximately 20 mg/100 mL at 5 months of lactation) in mothers from the Gambia, but this difference appears to be genetic and not due to differences in total intake of calcium (Prentice et al., 1995; IOM, 1997).

Relative to the average amount of milk consumed by infants, there are three key studies based on weighing full-term infants before and after feeding (Butte et al., 1984; Allen et al., 1991; Heinig et al., 1993). While it has been noted that the volume of intake is somewhat lower during the first month of life than in subsequent months (Widdowson, 1965; Southgate et al., 1969; Lonnerdal, 1997) and that a number of factors contribute to variability in intake, an estimate of 780 mL/day is reasonable based on the data from the three test weighing studies. Therefore, given an intake of milk estimated to be 780 mL/day from the infant weighing studies and the average content of 259 ± 59 mg of calcium per liter, the intake of calcium for infants fed exclusively human milk is estimated to be 202 mg/day.

Childhood and Adolescence

Calcium deposition into bone is an ongoing process throughout childhood and into adolescence, reaching maximal accretion during the pubertal growth spurt. Measures of bone density in adolescent girls indicate that about 37 percent of total skeletal bone mass is achieved between pubertal stages 2 (mean age 11 years) and 4 (mean age 15 years), with an average daily calcium accretion rate of 300 to 400 mg/day (Matkovic et al., 1994). For growing children, bone modeling (i.e., formation over resorption) is the predominant skeletal process promoting longitudinal extension of the growth plate and periosteal expansion. Modeling requires mineralization; hence, calcium requirements are increased, particularly during neonatal and pubertal growth spurts. Approximately 40 percent of total skeletal bone mass is acquired within a relatively short window of 3 to 5 years, when gonadal steroids and growth hormone secretion are maximal (Weaver and Heaney, 2006b). During this time, bone formation far outpaces resorption and longitudinal growth, and consolidation of bone occurs. The most recent estimate of average calcium accretion is 92 to 210 mg/day calcium in 9- to 18-year-old boys and girls (Vatanparast et al., 2010), and bone calcium accretion can peak at 300 to 400 mg/day (Bailey et al., 2000).

During this developmental period, calcium absorption is maximal and variation in calcium intake accounts for 12 to 15 percent of the variance in calcium retention for both boys and girls. Increases in total calcium

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