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Clients work out on machines at the Bally Total Fitness facility in Arvada, Colorado June 15, 2009.[Agencies]

NEW YORK - Exercise is good for you, but watch it: A new study found that young men engaging in strenuous physical activity are "an often overlooked" group that's at risk for low blood counts and iron deficiency.

Dr. Drorit Merkel from the Chaim Sheba Medical Center, Tel-Hashomer and colleagues studied 153 males, all 18 years old, who were training to join an elite combat unit in the Israel Defense Force.

In the September Journal of Adolescent Health, the investigators report that before the start of training, about 18 percent of the recruits had low blood count, or anemia. That rate almost tripled after six months of intensive military training, to just over 50 percent.

The rate of iron deficiency nearly doubled, from about 15 percent to 27 percent. The researchers do not report whether the recruits had any symptoms of anemia.

The researchers point out that the males in the study had a higher-than-average rate of anemia to begin with, which could explain some of the results. That's because recruits who intend to compete for membership in elite units often participate in intense pre-recruitment preparatory training.

"Iron deficiency and anemia," the investigators point out, "are generally uncommon findings in healthy male adolescents. However, athletes who engage in strenuous physical activity are known to be at increased risk for so-called 'sports anemia.'"

So why the higher risk? Potential reasons include drinking lots of water, which temporarily dilutes the blood and damage to blood cells that can result from high levels of physical activity.

"The prevalence of iron deficiency in new recruits indicates a military public health issue, and the preventive and therapeutic implications of these findings require further evaluation," Merkel and colleagues conclude.