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1 From the Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston.
See corresponding article on page 257.
The importance of the intake during childhood and adolescence of dietary calcium, which in the United States is derived largely from milk and dairy products, to the risk of osteoporotic fractures in later life is generally assumed to be fact. Many studies have shown the contribution of calcium intake during development to the accretion of peak bone mass, which is assumed to be a critical factor in later-life osteoporosis. However, relatively few studies have shown a direct link between early diet and later risk of osteoporosis. Indeed, the effect of early calcium intake on later risk has been questioned, because follow-up observations after calcium supplementation trials usually showed that benefits regress when the supplements are no longer consumed (1). This finding raises the question of whether the contribution of high calcium intake in childhood persists into older age, when fracture risk accelerates.
Studies that examined milk or calcium intake in childhood in relation to later bone mineral density (BMD) or fracture relied on recalled data, and the results are inconsistent. Several studies reported positive associations between earlier reported milk or calcium intake and BMD in premenopausal women, but several other studies did not find this association. Very few studies examined early calcium intake and subsequent BMD or fractures in older women. A retrospective study of elderly patients with osteoporosis and of healthy control subjects found that recalled milk consumption during childhood and adolescence was significantly associated with later bone status, although knowledge of the prior diagnosis may have contributed to recall bias (2). Two studies of postmenopausal women (aged 60–79 y) also found significant associations between reported milk intake in childhood or adolescence (or both) and BMD, but no correction was made for current intake (3, 4). Fewer studies investigated childhood milk or calcium intake and later adult fractures, and the results among these studies are mixed. The largest of them, a 12-y prospective follow-up (5) using data from the Nurses’ Health Study, found no association between reported milk intake in adolescence and fracture.
In this issue of the Journal, Kalkwarf et al (6) present data from 3251 non-Hispanic, white female participants (aged 20 y) in the nationally representative third National Health and Nutrition Examination Survey (NHANES III), in which adult women were asked about the frequency of their milk consumption during childhood (ages 5–12 y) and adolescence (ages 13–17 y). Kalkwarf et al studied the association of BMD and bone mineral content (BMC) as well as reported histories of fracture after age 13 y (lifetime fracture) and after age 50 y (osteoporotic fracture) with these reported milk-intake frequencies in regression analyses. They found that earlier reported milk intake was associated with apparent protection against later fracture as well as with greater bone mass.
Despite limitations in data availability, the study results are compelling. Milk intake during childhood and adolescence was associated with BMC and BMD of the hip both in women aged 20–49 y (n = 1371) and in those aged 50 y (n = 1880). Younger adult women who consumed <1 serving of milk/wk during childhood had BMC 5.6% lower than that in younger adult women who consumed >1 serving/d. The results for hip BMC and BMD were similarly significant by earlier milk-intake category for women aged 50 y. Furthermore, low milk intake during childhood was associated with a 2-fold greater risk of fracture among older women, with significant associations between both childhood and adolescent milk intakes and greater incidence of lifetime fracture. The authors estimate that low milk intake during childhood was associated with 11% of osteoporotic fractures in women later in life.
One of the important considerations in the study by Kalkwarf et al was that current milk intake was adjusted, which partially removed the effect of the tendency of women who consumed more milk in childhood to do the same in adulthood. Earlier intake remained significant, despite a positive association between current adult dietary calcium intake and BMC and BMD. The statistical models were designed to control for important potential confounders. The adjustment for current calcium intake may have been limited by the fact that it was based on a single 24-h recall. However, calcium intake generally has a relatively low ratio of intra- to interindividual variance, because of regular patterns of milk usage.
The use of recall for data on milk consumption during childhood and adolescence has several limitations, particularly in women with osteoporotic fracture. There is the possibility of recall bias among those who know that they have been diagnosed with osteoporosis. However, this factor is less likely to affect BMD or BMC results. Furthermore, much of the error associated with the recall of milk consumption during childhood and adolescence is likely to be due to random error in memory, which may weaken the ability to see associations. Another important limitation of these data is that they cannot describe intakes greater than 1 glass per day. More than 84% of women reported consumption of at least one glass of milk per day during childhood and 70% reported similar consumption during adolescence. The fact that associations between childhood and adolescent milk intakes and adult fracture risk were detected despite this truncation is noteworthy, and the true associations may have been even stronger.
It is interesting that current dietary calcium intake from food was more consistently associated with bone measures than was dietary calcium intake plus calcium from supplements or antacids. Possibly, some current supplement use is stimulated by a diagnosis of osteoporosis or a previous fracture. This observation also raises an important question related to the mechanism of the effect of greater milk intake, given the frequent observation that improvements with calcium supplement interventions tended to regress once the supplement was removed. As noted by the authors, it is significant that milk contains not only calcium, but also vitamin D, phosphorous, protein, zinc, and magnesium. This natural complex of nutrients may have a greater effect on enduring skeletal integrity than does calcium given in a short-term supplementation program.
Whereas further investigation into the effect of lifelong dietary patterns on bone formation and protection against bone loss in later life is needed, this study presents evidence that milk consumption during childhood and adolescence is important. Most of the adult NHANES III participants reported drinking at least one glass of milk per day during these formative years. In light of both the current tendency in schools to allow beverage choices such as fruit punch rather than milk and of the growing pressure on schools to stock vending machines with soda and other caloric beverages that lack calcium, attention should be given to the potential for future effects of diminished milk intake on bone status and risk of fracture.
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