Athletes who participate in sports that involve weight-supported (e.g., non–weight-bearing) exercise, such as cyclists and swimmers, have commonly been found to have low BMD levels. Because these activities generate relatively high levels of muscle forces but no impact forces, this may be viewed as evidence for the importance of gravitational loading. Nikander et al. (31
) evaluated femoral neck BMD and parameters of the hip structure in female athletes who participated in impact (i.e., volleyball, hurdling, squash, soccer, speed skating), no/low-impact weight-bearing (i.e., weight lifting, orienteering, skiing), or no-impact weight-supported (i.e., cycling, swimming) sports. As expected, cyclists and swimmers had the lowest BMD levels. However, only the athletes who participated in impact sports had high levels of both femoral neck BMD and section modulus; the latter is an index of the strength of bone against bending. Athletes who participated in low/noimpact weight-bearing activities had increased BMD levels but not section modulus levels. A follow-up study by this research group suggests that impact exercises may specifically generate an increase in cortical thickness (30
). Thus, impact activities, through either ground- or joint-reaction forces they generate, seem to confer a unique benefit on bone strength.
It is important to acknowledge that the relatively low BMD levels observed in some athletes, such as cyclists, swimmers, and even long-distance runners (12
), may result from factors other than the loading characteristics of the activity. A prospective study of competitive male road cyclists through 1 yr of training and competition found significant decreases in BMD of the total hip and its subregions and a trend (P
= 0.08) for a decrease
in lumbar spine BMD; average changes in BMD were −0.7% to −1.5%. Although the study did not include a control group, young healthy men would not be expected to have a significant decrease in BMD for 1 yr. This raises the possibility that, under certain conditions, exercise may have deleterious, rather than beneficial, effects on the skeleton.
It has been estimated that dermal calcium loss (i.e., sweating) during moderate to vigorous exercise is ~70 mg·h−1
). Thus, competitive cyclists and swimmers, who typically train more hours per week than athletes in other sports, may have increased calcium requirements. However, other metabolic factors related to dermal calcium loss during exercise may trigger bone loss. The authors postulate that the dermal loss of calcium during exercise results in a decrease in serum ionized calcium. Because serum calcium level is vigorously defended, such decreases would be expected to trigger an increase in parathyroid hormone (PTH) to mobilize calcium from the skeleton and prevent a further decline in serum calcium. In support of this hypothesis, we (1
) and others (11
) have observed a significant increase in PTH during exercise that was attenuated by the consumption of calcium-enriched water (11
). Thus, it is possible that repeated prolonged bouts of relatively vigorous exercise trigger increases in bone resorption that offset the beneficial mechanical loading effects of exercise. Under this paradigm, the deleterious effects of exercise on bone would be expected to be more prominent if the types of loading forces generated during exercise are not optimal, which may be the case for cycling and swimming.
In addition to dermal calcium loss during exercise, other factors could stimulate excess bone resorption. A detailed discussion of this topic is outside the scope of this article. However, some potential factors include chronic low energy availability, disruptions in gonadal function, nutrient deficiency, and exercise-induced increases in proresorptive cytokines.