Consuming an adequate intake of dairy foods and/or calcium during childhood and particularly adolescence is important for optimizing bone health, which may help reduce the risk of fractures in childhood and adolescence and osteoporosis in later adult years (1,3,6,28-32). In young children, increasing calcium intake has been shown to increase bone mineral density (1,3,6,24,28-35). In a recent six-year prospective study of 151 Caucasian girls followed from age 5 to 11, higher calcium intakes (provided mainly from dairy products) at ages 7 and 9 years were positively associated with total body bone mineral content at age 11 (34). In another longitudinal study of young children aged 2 to 8 years, multiple nutrients (energy, calcium, phosphorus, protein, magnesium, and zinc) showed a positive and significant correlation with bone mineral content (35). This finding led the researchers to suggest that children should consume a variety of nutrient-rich foods to protect their bone health (35).
Meeting calcium needs during adolescence is particularly important for bone health given that 40% of adult bone mass is accrued during the few years of peak skeletal growth (1,8). The benefits of increased calcium on adolescents’ bone health are demonstrated in a number of studies (1,3,6,28-32,36-38). A one-year double-blind, placebo-controlled calcium intervention study (1,000 mg calcium carbonate/day) in 100 post-menarcheal girls with habitual low calcium intakes (<800 mg/day) found that calcium supplementation (1,000 mg/day) enhanced bone mineral acquisition, especially in girls who were two years past the onset of menarche (37).
To optimize bone health, the American Academy of Pediatrics recommends 3 servings of dairy foods (e.g., low-fat milk, flavored milk, cheese, or yogurt) a day for children 4 to 8 years of age and 4 servings a day for children and adolescents 9 to 18 years, along with weight-bearing exercise.
Because milk and other dairy products are the major dietary source of calcium and provide other bone-building nutrients, several studies have examined their effect on bone health in children and adolescents (30,38-45). Researchers found that bone mineral content and bone mineral density were significantly increased in a study of 80 girls aged 12 years who consumed additional calcium (i.e., 1,125 mg/day compared to baseline calcium intake of 746 mg/day) in the form of whole or low-fat milk (i.e., two cups) for 18 months (39). A clinical study in 28 boys between 13 and 17 years of age found that increasing milk intake (3 servings of 1% fluid milk/day) favorably affected bone mineral responses to resistance training (40). A recent two-year placebo-controlled intervention trial in Finland found that consuming cheese to increase calcium intake was more beneficial for bone mass accrual in girls aged 10 to 12 years than a similar amount of calcium from calcium supplements (41). Other investigations in U.S. and Asian children and adolescents indicate that intake of dairy products has a positive influence on bone mineral density (42-45). A retrospective study showed that women aged 20 to 49 years who consumed more milk during childhood had higher bone mineral content than those whose intake of milk intake during childhood was low (46).
Overall, controlled trials of calcium or dairy intake have reported positive short-term effects on bone measures during growth, particularly when habitual calcium intake is low (28,29,32). But whether or not there is a long-term benefit in attaining and maintaining maximum peak bone mass after the dairy food or calcium intervention is stopped has yet to be conclusively established (1,8,28,47). Several follow-up investigations indicate that the effects of dairy food or calcium intake on gain in bone mineral density are maintained (i.e., from 1 to 7.5 years, depending on the study) after the intervention is discontinued (33,48-52), whereas in other studies no sustained effect has been observed (53,54). Factors such as the timing of pubertal maturation, source of calcium (food or supplement), and habitual calcium intake may influence the lasting response to calcium supplementation. Because it is unknown whether a short-term increase in calcium results in a long-term benefit on bone health, it is important that dietary practices that promote adequate calcium intake be established in childhood and maintained throughout life (8).
Emerging evidence indicates that fractures in childhood and adolescence are related to low bone mass or skeletal fragility, which in turn is influenced by diet and physical activity (30,55-58). A recent meta-analysis of eight case-control studies found a positive association between low bone density and fractures in children (55). New Zealand children and adolescents aged 5 to 19 years with repeated forearm fractures had lower bone mineral content and weighed more, two factors that increase fracture risk, compared to fracture-free children of the same age and gender (56). Previous research by these investigators showed an increased incidence of fractures in children who avoided milk (58).
In addition to adequate calcium intake, weight-bearing exercise (e.g., running, jumping) is important to optimize bone health during growth (1,8). The AAP states that “there is evidence that childhood and adolescence may represent an important period for achieving long-lasting skeletal benefits from regular exercise” (8). Some studies demonstrate that increasing calcium or dairy intake enhances the positive effects of high physical activity on bone mineral status during growth (59-63). However, additional research is necessary to determine the combined effects of calcium and exercise on bone health during childhood and adolescence (8,64).
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