By studying changing levels of leptin, a hormone reported to reflect energy availability in the body, researchers found that leptin levels decreased when the women's energy stores were low, regardless of whether the cause for the low energy was dieting or exercise. When the leptin levels went down, so did levels of a hormone that controls ovarian function.
The research, presented June 11 in Minneapolis at the 79th Annual Meeting of the Endocrine Society, is the first to suggest that energy availability -- not exercise -- may control reproductive hormones, said Anne Loucks, associate professor of physiology at Ohio University and principal investigator on the project.
"Previously researchers thought that exercise itself may have an impact on metabolic hormones that may control reproductive function," Loucks said. "However, by controlling the way in which women expend energy, we have found that it's the energy availability that's responsible for these changes."
When faced with an insufficient amount of energy, or calories, the body may be forced to ration its supply, doling out energy to some systems, but not others. Researchers suspect that the reproductive system may be affected, and that certain hormones, such as those that control menstruation, are decreased.
This could be why some female athletes who burn more calories than they consume suffer from amenorrhea -- a cessation of the menstrual cycle.
"If there's not enough energy available to meet the body's needs, it may prioritize how it uses its energy," Loucks said. "The reproductive system may be low on that priority list."
For the study, 22 women age 18-27 were divided into three groups: One group consumed 400 to 500 calories a day and did not exercise; another consumed 1,100 to 1,200 calories a day and performed a moderate exercise routine; and the third consumed about 1,800 calories a day and participated in a more vigorous exercise program.
All participants were on a liquid diet for four days, consuming only a liquid dietary supplement and water.
Researchers controlled energy availability in the three groups by monitoring how many calories the women burned during exercise, and subtracting those burned calories from the amount of calories they consumed from their specific diets. They then measured changes in leptin levels and a hormone that controls ovarian function.
"As the amount of energy available decreased, the leptin levels dropped by as much as 50 to 70 percent," said Loralyn Hilton, a doctoral student in exercise physiology and co-author of this new study. "We also saw a drop in the reproductive hormone that controls ovarian function. This adds more evidence to the theory that leptin may play a role in reproductive hormone release."
Leptin was first discovered by researchers at Rockefeller University in 1994. Studies of the hormone in genetically obese mice found that mice who were given doses of leptin lost weight. Although studies of leptin in humans are under way, it doesn't appear that obesity in humans is caused by a defect in leptin production as it is in obese mice, Hilton said.
"However, it is possible leptin may be controlling reproductive hormones suspected to cause menstrual problems in some female athletes," said Loucks, who currently is working on a U.S. Army project to study the energy needs of physically active women.
The research is part of an ongoing project sponsored by the U.S. Army, National Institutes of Health, Ohio University's College of Osteopathic Medicine and the American Heart Association.