Hard training may reduce fertility in women

Roughly seven per cent of all Norwegian women are believed to have infertility problems, which means that they are unable to become pregnant during the first year of trying -- even if they might later become pregnant.

Infertility can have many causes, both medical and lifestyle-related. Known risk factors include smoking, stress, and alcohol. Being extremely under- or overweight can also play a role.

It is known, however, that elite sports women have more fertility problems than other women. But does extreme physical activity play a role in fertility among other women as well? NTNU researchers examined precisely this question in a study involving nearly 3,000 women. They found that overly frequent and hard physical exercise appears to reduce a young woman's fertility. But the decrease in fertility probably lasts only as long as the hard training.

Two vulnerable groups

The study was based on material from the Health Survey of Nord-Trøndelag from 1984-1986 and from a follow-up survey in 1995-1997. All of the women who participated were healthy and of childbearing age, and none had a history of fertility problems.

In the first survey, women responded to questions about the frequency, duration and intensity of their physical activity -- and ten years later were asked questions about pregnancy and childbirth.

The NTNU researchers also recorded other information that could have significance for the study.

"Among all these women, we found two groups who experienced an increased risk of infertility," says Sigridur Lara Gudmundsdottir, a PhD candidate in NTNU's Human Movement Science Programme. "There were those who trained almost every day. And there were those who trained until they were completely exhausted. Those who did both had the highest risk of infertility."

Age an important factor

If the women also were under 30 years old in the first study, the relationship became even more evident in both groups. Among those who reported training to exhaustion (regardless of frequency and duration), 24 per cent had fertility problems. In the group that had trained almost every day (regardless of the intensity and duration), 11 per cent reported the same.

Even when the data were adjusted for other possible contributing factors (such as body mass index, smoking, age, marital status and previous pregnancies), the researchers found that women who trained every day had a 3.5 times greater risk of impaired fertility as women who did not train at all.

"And when we compared those who trained to exhaustion to those who trained more moderately, we found that the first group had a three-fold greater risk of impaired fertility," says Gudmundsdottir.

In women who reported moderate or low activity levels, researchers found no evidence of impaired fertility.

A transient effect

But the negative effects of hard training do not appear to be permanent, the researcher says.

"The vast majority of women in the study had children in the end. And those who trained the hardest in the middle of the 1980s were actually among those who had the most children in the 1990s," she adds.

There may be various explanations for why the women who first were least fertile ended up with the most children. "We do not know if they changed their activity level during the period between the two surveys. Or if they just had trouble getting pregnant the first time, but afterwards had a hormonal profile that made it easier to get pregnant again," Gudmundsdottir said.

Too demanding?

Scientists have a theory that high levels of physical activity are so energy intensive that the body actually experiences short periods of energy deficiency, where there simply is not enough energy to maintain all the necessary hormonal mechanisms that enable fertilization.

On the other hand, previous research shows that moderate physical activity gives women better insulin function and an improved hormonal profile -- and thus better conditions for fertility -- than total inactivity, particularly in overweight people.

Forget the easy chair

But Gudmundsdottir says that women who want to become pregnant shouldn't give up all physical activity.

"We believe it is likely that physical activity at a very high or very low level has a negative effect on fertility, while moderate activity is beneficial," she says.

But as far as identifying how much is "just right," the researcher is careful. "An individual's energy metabolism is a very important factor in this context. The threshold can be very individual," Gudmundsdottir says.

She also recommends that physically active women be particularly aware of their menstrual cycles. "A long cycle or no menstruation at all are danger signals," she says.