Factors that influence peak bone mass formation: a study of calcium balance and the inheritance of bone mass in adolescent females

Sex differences in the acquisition of total bone mineral mass peak assessed through dual-energy X-ray absorptiometry

Dual energy X-ray absorptiometry evaluation of total body bone mineral content (TBBM), total bone mineral density (TBMD), and regional bone mineral content (BMC) (head, trunk, arms, and legs) was carried out in order to assess sex differences of bone in 120 women and 121 men aged 15-29 years. Subjects from both sexes were divided into 5-year groups (15 through 19, 20 through 24, and 25 through 29 years old, respectively). Significantly higher values for TBBM, TBMD, and regional BMC were observed in males compared with females in the 20 to 24 and 25 to 29-year-old groups (P less than 0.001), but not in the group aged 15-19. After adjusting TBBM for lean body mass (LBM), we observed significantly lower values of TBBM/LBM in the males compared with females in all the age groups. A positive and significant correlation was observed between TBBM and age in the males of all the groups (r = 0.624, P less than 0.001), but not in the females. These data suggest that total bone mass peak acquisition takes place earlier in women than in men, leading to more reduced bone mass value, which in turn may be an osteoporosis predisposing factor.

Calcium supplementation and increases in bone mineral density in children

BACKGROUND

Increased dietary intake of calcium during childhood, usually as calcium in milk, is associated with increased bone mass in adulthood; the increase in mass is important in modifying the later risk of fracture. Whether the increase is due to the calcium content of milk, however, is not certain.

METHODS

We conducted a three-year, double-blind, placebo-controlled trial of the effect of calcium supplementation (1000 mg of calcium citrate malate per day) on bone mineral density in 70 pairs of identical twins (mean [+/- SD] age, 10 +/- 2 years; range, 6 to 14). In each pair, one twin served as a control for the other; 45 pairs completed the study. Bone mineral density was measured by photon absorptiometry at two sites in the radius (at base line, six months, and one, two, and three years) and at three sites in the hip and in the spine (at base line and three years).

RESULTS

The mean daily calcium intake of the twins given placebo was 908 mg, and that of the twins given calcium supplements was 1612 mg (894 mg from the diet and 718 mg from the supplement). Among the 22 twin pairs who were prepubertal throughout the study, the twins given supplements had significantly greater increases in bone mineral density at both radial sites (mean difference in the increase in bone mineral density: midshaft radius, 5.1 percent [95 percent confidence interval, 1.5 to 8.7 percent]; distal radius, 3.8 percent [95 percent confidence interval, 1.4 to 6.2 percent]) and in the lumbar spine (increase, 2.8 percent [95 percent confidence interval, 1.1 to 4.5 percent]) after three years; the differences in the increases at two of three femoral sites approached significance (Ward's triangle in the femoral neck, 2.9 percent; greater trochanter, 3.5 percent). Among the 23 pairs who went through puberty or were postpubertal, the twins given supplements received no benefit.

CONCLUSIONS

In prepubertal children whose average dietary intake of calcium approximated the recommended dietary allowance, calcium supplementation increased the rate of increase in bone mineral density. If the gain persists, peak bone density should be increased and the risk of fracture reduced.

Assessing hip fracture risk in a population-based health survey: the NHANES III osteoporosis component

A unique study of osteoporotic hip fracture risk, currently being conducted as part of a national health survey of the United States population, is described. The osteoporosis component of the third National Health and Nutrition Examination Survey (NHANES III) will provide data on multiple risk factors for hip fracture, including bone density of the proximal femur, from a nationally representative sample of adults that includes the very old. The minimum age for inclusion in the component is 20 years, so risk factors can be examined across the adult age range. The component includes men as well as women, and blacks and Mexican Americans as well as non-Hispanic whites. Finally, a longitudinal follow-up of the cohort will allow risk factor data to be related to subsequent hip fracture occurrence.

The scientific basis of recommended dietary allowances for calcium

Dietary calcium is required for bone development during growth (attainment of peak bone mass), for maintenance of skeletal integrity during adult life, and thus for prevention of osteoporosis. The Recommended Dietary Allowance (RDA) of a nutrient for a particular group is considered to cover the needs of 98% of all individuals of that group, and thus takes into account a margin of safety to allow for interindividual variation in minimum requirements. It appears to be possible, on the basis of the available scientific literature, to calculate the daily amount of calcium that must be absorbed from the diet to compensate for the endogenous calcium losses (through urine, faeces and skin) and the calcium retention in bone. Similarly, it seems to be possible to obtain a reasonable estimate of calcium absorption for the different groups of the population. From these data, and taking into account a margin of safety, figures are obtained for calcium intake that are in reasonably close agreement with the authoritative 1989 RDAs of the USA Food and Nutrition Board, with the exception of the USA allowance for girls aged 19-25 years (probably too high) and older adults (possibly too low). With regard to optimal calcium intake, some important questions still remain unanswered. These bear upon the issue of calcium intake and peak bone mass development, and upon the effects of non-nutritional factors (e.g. genetics and physical activity) and nutritional factors (e.g. sodium, protein, alcohol and caffeine) on calcium requirements. Furthermore, it would appear that bone development and maintenance of bone health may not be the sole criteria for setting RDAs in the near future. These issues are briefly discussed.

Familiality and partitioning the variability of femoral bone mineral density in women of child-bearing age

The contributions of polygenic loci and environmental factors to femoral bone mineral density (BMD) in g/cm2) variability were estimated in modified family sets consisting of women of child-bearing age. Femoral BMDs were measured in 535 women who were members of 137 family sets consisting minimally of an index, her sister, and unrelated female control. The family set could also include multiple sisters and first cousins. Women included in these family sets were all between 20 and 40 years of age to minimize the cohort effects of maturation and menopause on measures of BMD. BMDs were measured at three femoral sites using dual photon densitometry. Values were regressed on age and Quetelet Index which explained 13-15% of the variability in BMD (dependent on site). Subsequent variance components analysis on the residuals indicated that unmeasured polygenic loci accounted for substantial additional variability: 67% for femoral neck, 58% for Wards triangle, and 45% for trochanter. These results suggest that polygenic loci account for approximately half of the variability in maximal femoral BMD.

Calcium and peak bone mass

On the basis of previous epidemiological, clinical and experimental studies, it was demonstrated that adequate calcium intake during growth may influence peak bone mass/density, and may be instrumental in preventing subsequent postmenopausal and senile osteoporosis. Calcium intake during adolescence appears to affect skeletal calcium retention directly, and a calcium intake of up to 1600 mg d-1 may be required. Therefore, adolescent females at the time of puberty probably represent the optimal population for early prevention of osteoporosis with calcium. Young individuals must be in positive calcium balance to provide the calcium necessary for skeletal modelling and consolidation, but the degree of positive balance required to achieve peak bone mass and density is unknown. To assess calcium requirements in young individuals, and also to evaluate the determinants of calcium metabolism during the period of acquisition of peak bone mass, 487 calcium balances from previously published reports have been collected and analysed according to developmental phase and calcium intake. The results of this analysis showed that calcium intake and skeletal modelling/turnover are the most important determinants of calcium balance during growth. The highest requirements for calcium are during infancy and adolescence, and then during childhood and young adulthood. Infants (adequate vitamin D supply) and adolescents have higher calcium absorption than children and young adults to meet their high calcium requirements. Calcium absorption during the periods of rapid bone modelling/turnover is probably mediated by Nicolaysen's endogenous factor. Urinary calcium increases with age, and reaches a maximum by the end of puberty. The results also show that calcium intake has little effect on urinary calcium excretion during the period of most rapid skeletal formation: a weak correlation is present in children and young adults. On the basis of the above studies it was suggested that the RDA for calcium should be higher than currently established for children, adolescents, and young adults, in order to ensure a level of skeletal retention of calcium sufficient for maximal peak bone mass. In addition to nutrition, heredity (both parents) and endocrine factors (sexual development) appear to have profound effects on peak bone mass formation. Most of the skeletal mass will be accumulated by late adolescence, indicating early timing of peak bone mass.

Effect of calcium intake vs. other life-style factors on bone mass

Several life-style factors are known to or have been suggested to interact with calcium metabolism and bone turnover. Immobilization or a sedentary life-style may result in substantial bone loss, and physical exercise may increase bone mass, to different extents in different parts of the skeleton. Excessive training and/or slimming may lead to amenorrhoea, which is in turn complicated by rapid bone loss. While calcium supplementation probably cannot override the negative calcium balance induced by immobilization or amenorrhoea, the calcium requirement may be enhanced during recovery from these states. A high body mass index may to some extent protect against bone loss, particularly in post-menopausal women. Tobacco smoking and high alcohol consumption are probably detrimental to bone mass. Insufficient exposure to daylight and/or insufficient vitamin D intake occur mainly in infants and elderly people, and may impair calcium balance and cause rickets, osteomalacia or osteoporosis. Whether high intake of caffeine, protein, phosphate or fibre is detrimental to the bone mass has not yet been clarified. In many populations smoking and consumption of alcohol or caffeine are negatively correlated with calcium intake, and this exemplifies a source of confounding factors. Increased attention would be paid to important life-style factors during investigations of calcium requirements in different sex and age categories.

Calcium in the prevention and treatment of osteoporosis

Osteoporotic fractures have many sources. Low bone mass is one such, and inadequate calcium intake, in turn, is one of the causes of low bone mass. Calcium intake may be inadequate because it is low in its own right or, even if 'normal', it may not be sufficient to compensate for exaggerated obligatory losses. Inadequate calcium intake may cause bone mass to be low either because calcium intake during growth limits achievement of genetically programmed skeletal mass, or because low intake later in life aggravates involutional loss, or both. Ensuring a generous calcium intake throughout life will prevent both of these consequences. However, it is important to stress that even a calcium surfeit will not prevent or reverse bone loss due to inactivity, gonadal hormone deficiency, alcohol abuse or, indeed, any other factor. Calcium is a nutrient, not a drug. The only disorder it can be expected to alleviate is calcium deficiency. However, the evidence suggests that calcium deficiency is prevalent among Western populations, particularly in North America, and that it thereby contributes substantially to their osteoporotic fracture burden. This component of that burden is therefore entirely preventable.

Effect of calcium on skeletal development, bone loss, and risk of fractures

In assessing the role of calcium, it must be stressed that calcium is not the cause of bone health but simply a necessary condition for it. It is mechanical usage that is of primary importance for bone. In just the same way iron is essential for hemoglobin synthesis and protein is essential for muscle mass, but neither is sufficient by itself. What, then, ought we to expect from a high calcium intake? Can we prevent estrogen-withdrawal bone loss? No. Calcium is not a substitute for estrogen, anymore than it is a substitute for exercise. Will calcium slow the remodeling loss that occurs with aging? Yes, to some extent; as calcium slows remodeling, it will inevitably slow remodeling-related loss. But most importantly, a high calcium intake will prevent calcium-deficiency bone loss. The only question, therefore, is the extent to which calcium deficiency loss may contribute significantly to bone fragility in various populations. The bone loss and fracture data reviewed briefly here indicate that an important portion of the osteoporotic fracture burden is calcium-related. What that portion is will be a function of the fraction of the population with inadequate intakes in any given country. Better than half of all adult American women have calcium intakes less than 500 mg/day, whereas only a small fraction of Dutch or Danish women, for example, would be under that level. Hence, a population-wide program to increase calcium intake in the United States would be likely to yield a greater benefit than in either the Netherlands or Denmark. That does not mean, of course, that there could not be substantial benefit to individuals with low intakes in all countries. Calcium intakes of greater than or equal to 1,500 mg are both safe and natural. While not all bone loss and low trauma fractures are due to low calcium intake, some almost certainly are. Adaptation to low intakes does occur, but it is seldom sufficient to compensate for the low intake. We cannot easily distinguish those who need more calcium from those who need less, and for that reason it makes good sense to ensure an adequate calcium intake for the entire adult population. What should that intake be? During adolescence, 1,500 mg will come close to ensuring the achievement of genetically programmed levels of peak bone mass.(ABSTRACT TRUNCATED AT 250 WORDS)

Lifelong calcium intake and prevention of bone fragility in the aged

Primary prevention of osteoporosis involves achieving the full genetic potential for bone mass. Secondary prevention is concerned with protecting what bone mass a woman may have at her current age. Calcium plays an important role in both. Calcium requirement varies with stage of growth, with physiological drains (e.g., pregnancy and lactation), and with factors that influence absorption and excretory loss (e.g., gonadal hormone status and sodium and protein intakes). The evidence is strong that prevailing calcium intakes contribute to the low bone mass component of osteoporotic fragility and that increases in intake would reduce the osteoporotic fracture burden. At the same time it needs to be emphasized that bone health is a multifactorial affair and that meeting calcium requirements alone will neither guarantee optimal bone growth nor protect against bone loss if other critical factors are missing. For example, calcium affords only minimal protection against either immobilization or estrogen withdrawal bone loss. Thus, while assuring an adequate calcium intake remains a sound strategy, it cannot be considered a total preventive for osteoporosis.