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DRI Dietary Reference Intakes Calcium Vitamin D
vention, Diagnosis, and Therapy, 2001). Osteoporosis is most commonly associated with women, but the condition also occurs in men.
Menopause can initiate osteoporosis through elevated bone remodeling, which occurs characteristically in postmenopausal women. Remodeling activity, although designed to repair weakened bone, actually makes it temporarily weaker when remodeling is excessive. It can lead to enhanced skeletal fragility (Heaney, 2003). Although it is unclear to what extent calcium intake can mitigate such bone loss, inadequate calcium intake can exacerbate the situation.
Men experience age-related bone loss as well, although not due to menopause. This, in turn, can result in osteoporosis. However, the incidence of fracture risk increases some 5 to 10 years later in men than it does in women (Tuck and Datta, 2007).
CALCIUM ACROSS THE LIFE CYCLE
The body’s need for calcium relative to skeletal growth and remodeling varies by life stage. The major physiological activities include bone accretion during skeletal growth and maintenance of bone mass after growth is completed. Later in adult life, net calcium is lost from the body when bone formation no longer keeps up with bone resorption. For all life stages highlighted below, specific studies and conclusions are detailed in Chapter 4.
At full-term birth, the human infant has accrued about 26 to 30 g of calcium, most of which is in the skeleton. When calcium transfer from the placenta ceases at birth, the newborn infant is dependent on dietary calcium. Calcium deposition into bone occurs at a proportionately higher rate during the first year of life than during other periods. Breast-fed infants absorb about 55 to 60 percent of the calcium in human milk (Abrams et al., 1997). Formula-fed infants receive more calcium than breast-fed infants because formula contains nearly double the calcium of breast milk. However, fractional calcium absorption is lower in formula-fed infants, averaging about 40 percent among different formula types (Abrams et al., 2002). Studies to establish the level of calcium provided by human milk are long-standing in nature, and little information has emerged to change the conclusions of earlier analyses. Although the composition of milk varies significantly from the start to the end of each feed, the average calcium concentration of milk produced in total for each feeding remains relatively constant over the months of lactation, with an estimated value of 259 ± 59 mg/L at 30 days, followed by a small decrease during the second 6 months. This estimate is based on the average concentrations found in