The more exercise, the more to eat to stay healthy.

Women who follow strict exercise and diet regimens may harm their body’s ability to form new bone, which can lead to osteoporosis later in life. Researchers recommend that the more women exercise, the more they need to eat to stay healthy.

“Thousands of women severely restrict their diet and practice rigorous exercise programs for fitness and weight control,” said Anne Loucks, professor of biological sciences at Ohio University and lead author on the new study. “Because some don’t see obvious signs of undernutrition, such as a disrupted menstrual cycle, they may think they’re eating enough. If their diet does not supply enough energy to fuel their exercise level, though, they may be harming themselves. They need to replenish those calories.”

Earlier studies showed that too few calories (low energy availability) disrupts the reproductive system and impairs bone formation in teens and college-age females. If a young woman’s menstrual cycle stopped, it was considered a warning sign of bone loss.

Although the reproductive system is much less dependent on energy availability in slightly older women after they stop growing, these women remain at risk for bone loss, according to new findings presented today by Loucks and Ohio University undergraduate student Aiden Shearer at the Endocrine Society annual meeting in Toronto.

The researchers restricted the caloric intake of two groups of women with regular menstrual cycles and normal body fat over five days. Participants exercised for nearly two hours each day. One group ranged in age from 18 to 23 years, and the other was aged 26 to 32. Scientists know that bone formation continues in adulthood, as old bone is continually being replaced with new bone.

In both age groups, two bone formation markers were suppressed, suggesting that low energy availability continues to impair bone formation in adults as well as adolescents.

“Appetite is not a good indicator of how much female athletes should eat, and neither is a regular menstrual cycle,” warned Loucks, pointing out that low bone density puts women at higher risk for stress fractures and can lead to osteoporosis.