Not eating enough calories seems to impair the ability of healthy young women to form new bone, according to a study being presented Tuesday, June 5, at The Endocrine Society’s 89th Annual Meeting in Toronto.

The results serve as a warning for teenagers and young women who undereat and rigorously exercise during their prime bone-building years, said Anne Loucks, PhD, principal investigator and a professor at Ohio University. “The more women exercise, the more they need to eat, to protect their bone health,” she said.

Until about age 30, our bodies form enough new bone to replace the bone that breaks down. But severe undernutrition causes a drop in estrogen that speeds the breakdown of bone. Young women who severely restrict their diets or fail to increase their energy intake sufficiently when they exercise often stop menstruating and have low bone density with resulting fractures.

Loucks’ research showed that formation of new bone also depends on energy availability, the difference between calories eaten and calories burned during physical activity. This held true not just for adolescents but also for adults in their late 20s and early 30s.

“We fear that these women, at a very early age, are inhibiting their ability to reach their normal peak bone density,” Loucks said. “They are at increased risk of osteoporosis later in life.”

When growing adolescents stop menstruating due to anorexia or to inadequate energy intake for their level of physical activity, doctors consider this a warning sign for bone loss. However, Loucks’ study, which included adults whose reproductive systems are less sensitive to low energy availability, showed that continuing to menstruate is not a reliable indicator of sufficient energy availability in adults.

“Our research suggests that physically active young adult women who show no menstrual symptoms of undernutrition may still not be eating enough to preserve their bone health,” she said.

Loucks and her co-workers deprived the energy availability in two groups of healthy women by restricting their calorie intake for five days and having them exercise for about 100 minutes each day. One group ranged in age from 18 to 23 years, and the other group had an age range of 26 to 32. All subjects had normal body fat and menstruated regularly. The researchers then measured blood levels of two substances that are markers of bone formation: osteocalcin and procollagen type 1 N-terminal propeptide. Low energy availability suppressed these two important substances similarly in both age groups.

The study, which is being presented as a scientific poster co-authored by student Aiden Shearer, was funded by the U.S. Army, Ross Laboratories and the Ohio University Provost’s Undergraduate Research Fund.

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