idence for the relationship between breast-milk consumption and mental development was provided by the observation of a positive dose-response relationship between the proportion of mother’s milk in the diet and subsequent IQ, with greatest effects observed on the verbal scale. Children whose mothers had chosen to provide milk but had failed to do so had IQ values nearly identical to those whose mothers had chosen not to provide breast milk, a finding which indicates that the observed effect on IQ was not a function of other parental attributes that influence the decision to breast-feed. Preterm breast milk has been shown to contain more protein (after correcting for nonprotein nitrogen) for a postdelivery interval than does term breast milk.4 Breast milk also contains various factors that might affect development of the nervous system, including long-chain fatty acids such as docosahexaenoic acid (22:6, n-3), which accumulates in the developing brain and retina.5 Numerous hormones and other trophic factors that may influence brain growth and maturation3 are also present in breast milk. These findings suggest that human milk contains beneficial substances that currently are not found in infant formulas, including those intended for the preterm infant. The conclusions of Lucas et a1.2 should be accepted with caution until the beneficial factors in

breast milk have been more thoroughly characterized and the effects on intelligence have been confirmed by other similar types of measurements. Moreover, the beneficial effects of breast milk on the subsequent intelligence of preterm infants suggested by this study cannot be extrapolated to term infants until specific evidence is available from this population as well. Despite these caveats, the findings of Lucas et a1.2 support the need to encourage breast-milk feeding of preterm infants and to identify and correct any nutrient or other deficiencies in dietary formulas designed for preterm infants. 1. Lucas A, Morley R, Cole TJ, et al. Early diet in preterm babies and developmental status at 18 months. Lancet 1990;335:1477-81. 2. Lucas A, Morley R, Cole TJ, Lister G, Leeson-Payne C. Breast milk and subsequent intelligence quotient in children born preterm. Lancet 1992;339: 261-4 3. Weschler Intelligence Scale for children, Anglicized revised edition. Sidcup, Kent: The Psychological Corporation Ltd., 1974 4. Moreira F, Kunz C, Lonnerdal B. Qilantitation and analysis of proteins in preterm milk from mothers delivering at different gestational ages. FASEB J 1992;6:A1115(abstract) 5. Connor WE, Neuringer M, Reisbick S. Essential fatty acids: the importance of 17-3 fatty acids in the retina and brain. Nutr Rev 1992:50:21-9

Maximizing Peak Bone Mass: Calcium Supplementation Increases Bone Mineral Density in Children Attaining peak skeletal bone mass during childhood may reduce the incidence of osteoporosis in later life. A recent study in six- to 14-year-old identical twins showed that calcium supplementation increased bone mineral density. The effects of supplementation were especially pronounced in prepubertal children.

Although osteoporosis generally manifests itself during old age, one predictor of future fracture risk, peak skeletal mass, is determined during childhood and adolescence. Factors that limit peak skeletal mass, such as genetics or inadequate nutrition, may predispose a person to fractures later in life following normal adult bone loss. Adult bone loss, which can be detected early in adulthood (between 20 and 40 years of age), occurs at a rate of 6 8 % per decade, resulting in a 3040% cumulative loss from the peak skeletal mass by the age of 70. Public health programs aimed at promoting even small in-

Nutrition Reviews, Vol. 50, No. 1 1

creases in peak skeletal mass could lead, therefore, to substantial reductions in fractures later in life. Researchers have known for some time that increased dietary intake of milk during childhood is associated with increased bone mass in adulthood. Recently, a three-year, double-blind, placebocontrolled trial was designed to determine whether this increase in bone mass is related to the calcium content of milk.2 Seventy pairs of identical twins aged six to 14 years were given either a placebo or 1000 mg of calcium per day (as calcium citrate malate), with one twin serving as the control for the other. Children were examined and anthropometric measurements taken at both the beginning and end of the study. Dietary calcium intake was determined initially from a three-day food record, and monthly calcium intake was assessed from one-day records taken each month. Compliance to the treatment regimen was determined by a monthly pill count. Bone mineral density was measured by photon absorptiometry in the spine and at three sites in

335

the hip at baseline and at three years; similar measurements were made at two sites in the radius at baseline, six months, and at one, two, and three years. Baseline and three-year dietary and anthropometric characteristics of the pairs of twins were nearly identical, regardless of their assigned treatment group. Mean calcium intake over the threeyear period was 908 mg/day in the placebo group and 1612 mg/day in the twins given the calcium supplement (894 mg/day from the diet and 718 mg/day from the supplement). As a result of supplementation, the mean increase in bone mineral density for the six sites examined was 1.4% greater in the 45 twin pairs who completed the study. However, if one splits the study population into twin pairs who were prepubertal throughout the study and those who were either pubertal or postpubertal, the increases are even more pronounced in the prepubertal group (Figure 1). Bone mineral density at all sites was significantly greater in the treatment group than in the placebo group, with a mean increase of 2.9%. In contrast, no significant differences in bone mineral density were detected between the pubertal and postpubertal twins who received calcium supplements and their controls. In one site (midshaft radius), as the difference in calcium intake between twins increased, the difference in the bone mineral density also increased. Johnston et aL2 speculate that this increase in bone density could be related to a decrease in bone turnover in subjects receiving calcium supplements. This hypothesis is supported by the finding that serum levels of osteocalcin-a protein used as an indicator of bone turnover-were lower in subjects who received the calcium supple-

=

=

Prepubertal

Pubertal

h

s\o v h

61

c,

.I

m

I

5.1

5 ’

4

-

3.9

3.4

3.3

E

u

-1

-0.1

’ 6 mo

1 Y‘

2 Y‘

I

3 Y’

Time in Study Figure 1. Mean differences within twin pairs in the change in the bone mineral density of the midshaft radius among prepubertal and older children, according to their time in the study. The differences were significant among the prepubertal children at six months and thereafter. (Figure was redrawn from Johnson et a1.2) 336

ments. However, reductions in serum osteocalcin levels may simply reflect reduced serum 1,25 dihydroxyvitamin D, levels-an expected consequence of high calcium intake. The authors2 conclude that in prepubertal children consuming approximately the RDA for calcium (800 mg/day), calcium supplementation could increase bone mineral density. If the increase persisted, peak bone density would likely be increased, thereby reducing the risk of fractures later in life. The suggestion that increasing calcium intake in childhood can improve peak mineral density is not new. Matkovic, and Matkovic and Heaney4 reanalyzed a large number of calcium balance studies and concluded that the threshold at which calcium balance no longer increases with intake (and, presumably, where bone accretion no longer occurs) exceeds the RDA for calcium in persons ranging in age from childhood to 30 years. For example, in children two to eight years of age the threshold level of calcium intake was estimated at 1390 mg/daynearly twice the RDA of 800 mg/day for that age group. In a study examining the link between reported milk consumption during various stages of life and postmenopausal bone density in women, Sandler and coworkers’ found a positive association between milk consumption early in life and bone density following menopause. This finding suggests that early exposure to good eating habits may augment peak bone mass, influencing the extent of bone loss later in adulthood. The present study demonstrates that improving early calcium nutriture, even in children who meet the RDA, can result in significant increases in bone density. The use of pairs of twins in the study by Johnston et a1.2 offers a significant advantage over past experiments by increasing the statistical power. For example, although Matkovic et al.’ showed a pronounced increase in bone mass over time in calcium-supplemented adolescent females, the differences were not statistically significant because the sample size was small (n = 8). If the positive effects of calcium supplementation on bone density could be maintained, they might help to reduce the incidence of fractures in elderly persons following a lifetime of normal bone loss. These results raise two important questions. First, if increased dietary calcium results in more bone accretion in prepubertal children, should the RDA for calcium be raised in this age group? In light of the medical cost of osteoporosis,6 it may no longer be acceptable to set the RDA at a level that results in “adequate” calcium retention for skeletal growth. Rather, given the suggestion that calcium balance in children will continue to improve up to a threshold of almost twice the current RDA for calNutrition Reviews, Vol. 50, No. 11

c i ~ m , it~ may ? ~ be more prudent to increase the RDA to a level that promotes optimal peak bone mass, thereby helping to maintain skeletal mass despite normal adult bone loss. Second, would it be practical to recommend a higher calcium intake for children? Although dietary surveys show that preadolescent children generally consume enough calcium to meet the current RDA, consumption of calcium-rich foods frequently declines as children enter adolescence, resulting in inadequate calcium intake. Given the improbability that merely increasing the RDA would trigger an increase in intake of calcium-rich foods, should the nutrition community recommend calcium fortification or supplementation? While this might help some children attain optimal peak bone mass, it would do little to promote the good eating habits essential to long-term health. As an alternative, one might target efforts to improve dietary calcium intake at those children with the genetic predisposition for osteoporosis, as determined by family history, rather than attempting to raise the calcium intake of all children. A discussion of rela-

Nutrition Reviews, Vol. 50, No. 11

tive costs and benefits of these alternative approaches should be a high priority. 1. Sandler RB, Slemenda CW, LaPorte RE, et al. Postmenopausal bone density and milk consumption in childhood and adolescence. Am J Clin Nutr 1985;42:270-4 2. Johnston CC, Miller JZ, Slemenda CW, et al. Calcium supplementation and increases in bone mineral density in children. N Engl J Med 1992;327: 82-7 3. Matkovic V. Calcium metabolism and calcium requirements during skeletal modeling and consolidation of bone mass. Am J Clin Nutr 1991;54: 245s-60s 4. Matkovic V, Heaney RP. Calcium balance during human growth: evidence for threshold behavior. Am J Clin Nutr 1992;55:992-6 5. Matkovic V, Fontana D, Tominac C, Goel P, Chestnut CH. Factors that influence peak bone mass formation: a study of calcium balance and the inheritance of bone mass in adolescent females. Am J Clin Nutr 1990;52:87&88 6. Norris RJ. Medical costs of osteoporosis. Bone 1992; 13:Sl l -6

337