The exact age at which peak bone mass is attained is not certain. The generally accepted notion has been that peak bone mass is finally attained in the third or early fourth decade of life (Ott, 1990; Recker et al, 1992). However, it is possible that gains in bone mass beyond the end of the second decade are small. A cross-sectional study of 247 females aged between 11 and 32 years showed that 99% of peak total body BMD had been attained by age 22.1±2.5 years, and 99% of peak total body BMC by age 26.2±3.7 years (Teegarden et al, 1995). Additionally, these authors found that only a maximum of 4% of total body BMC was achievable after the age of 20 years. In agreement with this study, Matkovic et al (1994) studied 265 women aged 8-50 years, and suggested that approximately 95% of total body BMD and BMC are attained by the age of 18 years.

Perhaps more importantly, however, studies investigating the age of attainment of peak bone mass at specific sites of clinical interest highlight the fact that in adolescent girls, much of the potential for bone accrual at these sites is complete by the end of longitudinal growth. Theintz et al (1992) studied 198 healthy adolescents, aged 9 to 19 years, and found that the rates of gain in lumbar spine and femoral neck BMD and BMC in girls peaked over a 3-year period, from 11 to 14 years of age. This is corroborated by the work of Kroger et al (1993), who demonstrated that BMD in the spine increases by about 13% per year and BMD in the femoral neck increases by about 10% per year during puberty. The rate of bone growth falls sharply at about 16 years and/or two years after menarche (Theintz et al, 1992). In males, the period of bone mass accumulation occurs slightly later and over a longer period, between the ages of 13 and 17 years (Theintz et al, 1992; Kroger et al, 1993). Studies indicate that during this period, BMD increases by about 11% per year in the spine and by about 9% per year in the femoral neck (Kroger et al, 1993).

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It is therefore probable that both notions of the timing of peak bone mass are correct for different skeletal regions. By the end of the second decade, post-puberty, young women will have accrued most of their bone mass, with an early timing of peak bone mass for the hip and trabecular bone of the spine (Bonjour et al, 1991; Matkovic et al, 1994). At the level of the total body, it is probable that a few per cent gain in bone mass is still possible into the third decade and beyond, through consolidation of bone mineral and continued periosteal apposition. What is certain is that calcium needs are high when the skeleton is growing. During adolescence, the rate of calcium deposition triples and skeletal mass doubles – around 50% of peak bone mass is laid down at this time (Bonjour et al, 1991).

Several randomised controlled studies have demonstrated that adding calcium to the diets of pre-pubertal children increases BMD and BMC by 2-6% depending on the skeletal site measured (Johnston et al, 1992; Lee et al, 1994; Lee et al, 1995; Bonjour et al, 1997). For example, Bonjour and colleagues (1997) studied the effect of calcium supplementation in a group of eight-year-old girls using a double-blind, placebo-controlled design. Subjects were allocated to receive either two food products (cakes, biscuits, confectionery and yogurts) supplemented with a total of 850mg of extra calcium (n=55) or the two food products without the calcium supplement (n=53) for 48 weeks. At the end of the study, there was a 3.5-5% greater increase in BMD in the girls in the calcium-supplemented group compared with those in the non-supplemented group. Girls who had previously had the lowest calcium intake (below 880mg/day) benefited most from the supplementation.

Calcium or dairy intervention studies have demonstrated positive effects on bone mass in adolescent girls (Lloyd et al, 1993; Chan et al, 1995; Cadogan et al, 1997; Nowson et al, 1997). For example, Cadogan and colleagues (1997) monitored the effect of diet on the bone development of 80 twelve-year-old girls over an 18-month period. Half the girls consumed an extra 300ml milk every day and the remaining girls continued with their usual diet. Compared with the control group, the girls who consumed extra milk had significantly greater increases in total body BMD (9.6% vs 8.5%) and total body BMC (27% vs 24%). Although milk was used as a source of calcium, it also contains many other nutrients. As a result, girls in the intervention group also consumed significantly more protein, phosphorus, magnesium, zinc, riboflavin and thiamin than the control group. However, consuming extra milk was not associated with a difference in body weight or body fat gain between the groups.

Although a number of studies have demonstrated that calcium has a positive effect on bone accretion during growth, further research is needed to identify whether increases in BMD due to supplementation with calcium salts or dairy foods lead to enhanced peak bone mass. That is, whether the improvements in bone health remain after withdrawal of the supplement. It could be that a certain level of calcium intake needs to be maintained throughout childhood and adolescence in order to have a positive effect on peak bone mass, i.e. that taking supplements or eating extra dairy foods for one or two years is not enough.

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Bonjour and colleagues (1997) reported that significant differences in bone mass between supplemented and non-supplemented subjects persisted one year after the dietary intervention had finished. Others have found that differences in bone mineral density disappear 18 months to 2 years after withdrawal of the supplements (Slemenda et al, 1993; Lee et al, 1996). Fehily and colleagues (1992) found a non-significant trend for BMC and BMD of the forearm to be higher in a group of 20- to 23-years-olds who, at the age of 7 to 9 years, had participated in a two-year milk supplementation trial. As it is unknown whether or not there were significant differences in BMD and BMC between the milk and the control group at the end of the original trial (bone mass was not measured), it is not possible to determine whether the benefits were maintained.

The type of supplement used in such studies may be important. Some of these studies used calcium salts such as calcium carbonate (Lee et al, 1994; Lee et al, 1995) and calcium citrate-malate (Johnston et al, 1992; Lloyd et al, 1993) whereas others used calcium derived from milk extracts (Bonjour et al, 1997), milk (Cadogan et al, 1997) and dairy products (Chan et al, 1995). Calcium salts appear to act through a suppression of bone remodelling (Johnston et al, 1992), but milk supplements do not (Cadogan et al, 1997). Rather, calcium derived from milk appears to exert an anabolic effect on the growing skeleton (Bonjour et al, 1997; Cadogan et al, 1997).

Studies demonstrate that calcium or dairy supplementation has positive effects on bone acquisition during a critical time in bone development, childhood and adolescence. If the benefits are maintained into adulthood, they will reduce the risk of osteoporosis later in life.

References

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