Athletic young men may lose the extra bone mineral density (BMD) they've accumulated if they reduce their physical activity levels as they grow older, according to new research from Sweden.

Weight bearing exercise is known to build bone density, especially in youth. But whether or not bone strength is preserved if people's activity levels decline is less clear.

To investigate, Dr. Taru Tervo of Umea University and colleagues followed 92 young men for 12 years, measuring BMD at five different time points at the femoral neck (which connects the "ball" that joins with the hip socket to the shaft of the thigh bone), the total body, and the lumbar spine.

Sixty-seven men were active athletes training in ice hockey or badminton, and the rest served as a control group. The men's average age at the beginning of the study was 17. Fifty-one of the athletes stopped training during the study's follow-up period.

The athletes had begun training, on average, when they were about 7 years old.

Throughout the entire study, the researchers found, the active athletes had higher BMDs than the control group. The athletes who stopped training had higher BMDs than the control group as well, except at the final follow up visit.

And from the beginning to the end of the study, the ex-athletes lost more BMD in their femoral neck than either the active athletes or the control group. Within 8 years after active training ended, on average, the added BMD the former athletes had built had disappeared.

Increased BMD due to previous high levels of physical activity may not prevent the brittle bone disease osteoporosis in later years, the investigators warn in the Journal of Clinical Endocrinology and Metabolism.

"We found that BMD in young men is constantly changing with the current level of physical activity," Tervo and colleagues report. "This is especially evident at the clinically important femoral neck."

Based on the findings, they conclude, it appears that early athletic activity may not preserve bone strength later in life if it is not maintained.