There has been much discussion about the effect of strenuous exercise on the health of the human skeleton system.

In female athletes the literature abounds with investigations linking disordered eating, vigorous sustained exercise, and menstrual disruption with lower-than-normal bone mineral density, bony overload consequences and disrupted reproductive hormones. The irregularity or cessation of the female menstrual cycle provides an easy to recognize clinical sign that the physiology is somewhat askew.

In male athletes there are no such markers.

Low bone mineral density (BMD) and, for that matter, lower than normal reproductive hormone concentrations, do not have specific symptomatology. The completed research fails to provide any consistent answers to the question of what level, type and intensity of exercise puts male athletes at risk.

For instance, one study using 704 male athletes -- including rowers, swimmers, rugby, soccer, biking, climbing, triathletes and other athletes -- reported that the rowers and swimmers had low total body BMD whereas team sports and fighting sports had high total body BMD. In contrast, another study reported rowers with increased BMD, while yet another study showed no alteration of BMD in cyclists, triathletes, or swimmers.

It appears that male runners training at 15-20 miles per week have increased BMD while those at 60-75 miles per week have decreased BMD and a bone turnover rate that is greater at the higher training levels. The highest BMD levels are found in strength and power training athletes -- endurance athletes having lower BMD.

Recent research suggests that with male distance runners there may be a so-called "volume threshold" responsible for putting men at risk for lower than normal BMD as well as altered reproductive hormone status. The theory here is that training over a certain amount might increase risk of lowering BMD. Threshold could be interpreted in a number of ways: a threshold of miles run per week, a threshold of miles run per month, a threshold of miles run at a certain intensity, or frequency of training, etc.

Another contributing factor in the bone health issue may be the possibility that male athletes, just as with some female athletes, are neglecting the intake side of the energy balance equation. Excess energy expenditure has been shown to affect reproductive physiology in males and female mammalian species. This may have concomitant effects on bone health.

It is known that long-distance runners and endurance athletes expend many calories per week. For instance, a male distance runner training at 60 miles per week probably expends between 5,000 and 7,000 calories in addition to the normal daily expenditure of energy. The compensation for this additional caloric expenditure (volume and/or content), consciously or even inadvertently, may fall short and hence have contributing ramifications on bone health and hormonal status.

In conclusion, questions are raised as to the detrimental effect of endurance training on men's BMD, but no definitive conclusions can be reached as yet. In order to produce more decisive answers, research must be carried out that carefully monitors and controls the training volume and intensity.

While the "volume hypothesis" is an intriguing concept and may provide coaches and athletes with some basic guidelines to consider, researchers long have known that factors such as genetics, individual variability, and responders vs. non-responders play a significant but unknown role in the final formula.

Gary Skrinar, Ph.D., is professor and program director of applied anatomy and physiology at Sargent College of Health and Rehabilitation Sciences, Boston University. Frank Pettrone, M.D., is at George Mason University. Both are members of the NCAA Committee on Competitive Safeguards and Medical Aspects of Sports.