Some women who exercise excessively experience a cessation of their menstrual cycle for several months, known as amenorrhea, which could result from a negative energy balance, according to new research conducted at Penn State.

Researchers have long speculated whether amenorrhea is the result of a calorie imbalance or the direct stress of exercise on the reproductive system.

But Nancy Williams, assistant professor of kinesiology, believes the condition is most likely due to a negative energy balance.

In response to exercise, the body does one of two things, Williams said.

It either triggers weight loss or tries to adjust, she said.

The body's tendency to adjust, as opposed to lose weight, might trigger amenorrhea, she added.

Judy Cameron, associate professor of psychiatry and cell biology and physiology at the University of Pittsburgh, worked with Williams on the study.

"Loss of the normal menstrual cycles is a concern because of the decrease in estrogen production," Cameron said.

Estrogen and progesterone, the primary female hormones, provide cardiovascular protection against heart attacks and protect against osteoperosis by building bone mass, Cameron said.

The production of these hormones is tied to the reproductive axis, which consists of the hypothalamus, the pituitary gland and the ovaries, Williams said.

Those who develop amenorrhea are not typical athletes, Cameron said.

These individuals exercise strenuously, she said. Cameron added that, for example, amenorrhea is common in marathoners.

Williams and Cameron worked with a team of researchers at the University of Pittsburgh that studied female monkeys, who show similar menstrual cycles to those of humans.

The researchers chose to study the animals because of the difficulty they might encounter in controlling eating and exercising habits in humans.

After Williams and Cameron monitored the menstrual cycles of eight monkeys for several months, they trained the monkeys to run on treadmills, slowly increasing their daily mileage to six miles.

Throughout their exercise regimen, the monkeys were fed the exact same amount of calories, Cameron said.

Eventually, they all reached the same level of reproductive dysfunction (amenorrhea), she said.

The researchers kept the monkeys on the same exercise schedule and then presented them with more food.

"It was hard to get them to eat more," Cameron said. "We had to give them treats to get them to eat (additional calories)."

Those monkeys who increased their caloric consumption quickly took only 12 to 16 days to resume a normal menstrual cycle.

Those who increased their consumption more gradually, however, did not regain a normal menstrual cycle for almost two months, according to a Penn State press release.

Williams is now studying females who agree to eat according to a certain plan, she said. In addition, the participants agreed to exercise five days a week for several months.

The goal of the study is to find the optimal balance between calorie intake and expenditure, Williams said.

"It's going well," Williams said. "The participants are very dedicated."