Childhood and Adolescence.
Because much of genetically determined peak bone mass is accumulated during the first two decades of life, childhood and adolescence are critical times to optimize skeletal health (1,8,32,33). Maximal peak bone mineral content velocity occurs in boys at 14 years of age and in girls at12.5 years (33). During the two years of peak skeletal growth, over 25% of adult bone mass is accumulated (33), and 90 to 95% of an adult’s bone mineral is achieved by the end of adolescence (34).
According to USDA's Healthy Eating Index, which is a tool to assess and monitor the dietary status of Americans, blacks have significantly lower milk scores than their white counterparts (16,17). A recent analysis of data from USDA's Continuing Survey of Food Intakes by Individuals 1994-96,1998 revealed that African American children and adolescents ages 6 to 19 years drink less milk than do white children and adolescents (18).
Studies in children and adolescents demonstrate that increasing consumption of dairy foods has a beneficial effect on bone health or that low intake of dairy foods compromises bone health and increases the risk for bone fractures during growth. When Chinese children aged 9 to 10 years consumed milk powder equivalent to 1300mg of calcium for 18 months, bone mineral density of the hip and spine significantly increased (35). A study of children ages 3 to 10 years in New Zealand found that those with a history of chronically avoiding milk had a lower average dietary calcium intake, lower bone mineral density of the total skeleton and specific skeletal sites, and were shorter and heavier than children who drank milk (36). Also, the annual incidence of arm fractures in the milk avoiders was higher (3.5%) than the expected annual incidence (1%) (36).
A more recent investigation found that significantly more children aged 3 to 13 years who avoided milk for prolonged periods experienced bone fractures (especially forearm fractures) compared to a group of children born at the same time from the same city (37). Nearly one in three of the milk avoiders experienced a bone fracture before 8 years of age, most often from a minor trip or fall. Previously, these researchers showed that bone density was 3% to 5% lower at different skeletal sites in girls aged 3 to15 years with a recent forearm fracture compared to those who had never broken a bone (38). Interestingly, a new report documents a significant increase in the number of distal forearm (wrist) fractures in U.S. children and adolescents over the past 30 years (39). Although the specific cause of this increase was not explored, the researchers suggest that the decline in milk consumption and dramatic increase in soft drink intake in recent decades is likely a contributing factor (39).
A study found that the calcium intake of girls from age 5 to 9 predicted bone mineral status at 9 years of age and that girls who met the calcium recommendation consumed on average nearly twice as much milk as those who did not meet the calcium recommendation (40). Further, girls who met the calcium recommendation were served milk more often and had mothers who drank milk more frequently than did girls with low calcium intakes.
Studies demonstrate that increasing intake of milk and other dairy foods benefits bone health throughout life by helping to maximize genetically determined peak bone mass in the early years and reduce age-related bone loss and osteoporoticrelated fractures.
Several investigations support a beneficial effect of dairy foods on bone health during adolescence as well. A British study of 80 adolescent girls found that bone mineral density and bone mineral content increased in those who consumed an additional daily pint of milk for 18 months (41). Findings from recent studies of adolescent girls in China demonstrate that increasing milk intake increases bone mineral content and bone mineral density, particularly when milk is fortified with vitamin D (42,43). In a 12-week randomized clinical trial in the U.S., whole body bone mineral density was significantly higher in adolescent boys (13 to 17 years of age) who drank 3 servings/day of milk while participating in a strength training program compared with boys who drank juice (44). The researchers suggest that this beneficial effect on bone was most likely due to the additional calcium and/or vitamin D consumed by the milk drinkers (44).
Further support for a positive effect of calcium and dairy products on bone health in adolescents is provided by findings from a study of two groups of adolescent girls (15 to18 years) (45). One group was part of a randomized double-blind, placebo-controlled clinical trial with calcium supplements, and the other group was part of an observational study in which calcium was provided by dairy products. Both calcium and dairy products improved bone mass accrual, leading to a higher peak bone mass (45). While calcium influenced volumetric bone density, dairy products had an additional impact on bone growth and bone expansion, perhaps due to the calcium and protein content of dairy products (45).
Retrospective studies indicate that milk intake during childhood and adolescence is associated with greater bone mass and protection against fractures in later years (46,47). According to data from more than 3,200 women, reported frequency of milk intake during childhood and adolescence was positively associated with bone mineral content and bone mineral density of the hip in those aged 20 and older (46). Among women 50 years and older, those with low milk intake during childhood had a 2-fold greater risk of osteoporotic fractures than women with high milk intake during childhood (46). In an investigation of young women aged18 to 31 years, higher reported milk intakes during childhood and adolescence were associated with higher total bone mineral content in young adulthood (47). This finding indicates that early establishment of dietary habits that include milk may contribute to similar milk-drinking habits in later years (47). Health professional organizations such as the American Academy of Pediatrics (23) and the American Dietetic Association (48), as well as the U.S. Surgeon General (1) recognize the importance of calcium and calcium-rich foods such as milk, cheese, and yogurt for children’s bone health.
Adulthood. Numerous studies indicate that increasing calcium intake, either alone or in combination with other nutrients in dairy products, helps to reduce bone loss and risk of fractures in adulthood (4,7-12,25,49-53). The effectiveness of calcium varies according to skeletal site, menopausal age, and usual calcium intake (8). For example, the effect of calcium on bone health is relatively small in early menopause (i.e., the first 5 to 8 years of menopause) when bone loss is accelerated due to estrogen deficiency (8). However, calcium is an important adjunct to estrogen and other medications to reduce bone loss (54-57).
Consuming milk and other calcium-rich dairy foods has a positive effect on bone health in adults (58-63). In a cross-sectional study of healthy Norwegian women aged19 to 35 years, lack of milk intake was associated with low forearm bone mineral density, whereas milk consumption was a significant source of calcium, which was associated with higher bone mineral density of the wrist and forearm (58). In a two-year randomized controlled trial of postmenopausal Chinese women aged 55 to 65 years in Malaysia, intake of high calcium skimmed milk significantly reduced bone loss at clinically important lumbar spine and hip sites (59). Compared to the control group, which continued to consume their usual diet, the milk treatment group had significantly higher intakes of dietary calcium and improved vitamin D status at 24 months (59). The researchers note that the findings of this study support those of a previous investigation demonstrating that high calcium milk reduces age-related bone loss in postmenopausal Chinese women (60).
According to the first-ever Surgeon General's Report on Bone Health and Osteoporosis, it is never too late to take steps, such as increasing calcium intake and physical activity, to help prevent osteoporosis.
Increasing adults’ dietary calcium in the form of foods (e.g., yogurt, milk) has been demonstrated to decrease bone resorption (breakdown), the first step in reducing fragility (31), as well as in maintaining bone density (61,62). Consuming a yogurt snack three times a day significantly improved postmenopausal women’s intake of calcium and other nutrients (e.g., riboflavin, vitamin B12, potassium, protein) and decreased their rate of bone resorption, as indicated by urinary excretion of N-telopeptide, according to a randomized controlled clinical trial (61). In another study of healthy older adults with habitually low intakes of dairy foods, those who consumed 3 servings of fat free or lowfat milk as part of their daily diet for12 weeks experienced a significant decrease in bone resorption compared to those who maintained their usual diets (62). According to a recent cross-sectional study, higher consumption of dairy foods was associated with increased hip bone mineral density in older men (63).
Dairy foods’ beneficial effect on bone health in older adults is consistent with findings from studies of dairy food nutrients and bone health (27,51-53,63-66). A review of the calcium needs of adults over 65 years of age concludes that increasing daily calcium intake (e.g., from1300 to 1700mg/day) will reduce osteoporotic fracture risk by 30% to 50% (53). According to a meta-analysis of 25 randomized controlled trials in postmenopausal women, vitamin D significantly reduced spine fractures and showed a trend toward reduced incidence of non-spine fractures (64). Interventions that have used both calcium and vitamin D demonstrate a reduced risk of falls (65) and bone fractures (51) in older adults. Protein is another nutrient in dairy foods that favorably affects bone health in older adults, particularly when adequate calcium is consumed (27,66).
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