Attainment of Peak Bone Mass

Abstract

Most physicians are aware of the inexorable loss of bone that accompanies aging. Osteoporosis is one consequence of this loss. The bone density of elderly women, who manifest osteoporotic fractures, depends on both the rate of loss and the initial bone mass. Attention to the factors that lead to the development of peak bone mass, which is the highest value that an individual attains during her lifetime, is warranted. The large variance of normal bone density is often not appreciated. A normal 30-yr-old woman in the bottom decile for bone density could have the same density as an 80-yr-old woman in the top decile. The variance does not appear to increase with aging; and because the normal range in young women is already so broad, inequities must be present during growth. Not surprisingly, the list of factors relating to peak bone mass is similar to the list of risk factors for postmenopausal osteoporosis, including race, sex, heredity, diet, hormonal factors, activity, weight, and drugs or diseases that lead to bone loss. Unfortunately, these factors are not understood, not well studied, and not at all independent. There is no consensus on the age at which peak bone mass occurs. Longitudinal measurements of bone density through adolescence and early adult years are not available. Some cross-sectional studies have found no increase in bone density after age 20, whereas others, with larger numbers of subjects, find the bone density increases until about age 35, then starts a downward trend (1). Thus, it appears that the density of the bone continues to increase for at least a decade after the maximum height has been attained. Hereditary and racial factors are probably the most important determinants of peak bone density. Black children have higher bone density than white children (2). The bone density of teenage girls is correlated to the densities of the fathers and mothers (3). Twin studies have given the best data about the influence of heredity on bone mass. Pocock et al. (4) showed that bone density was significantly better correlated in monozygotic than in dizygotic twins. How the genetic message governs the bone density is unknown. Bone density is related to body size, muscle mass, and hormone levels, all of which are also under genetic control, but these known factors do