Too Little Fat Can Lead to Infertility

Women with too little body fat can be infertile and not even realize it, according to a Harvard School of Public Health expert.

Rose Frisch, associate professor of population sciences, published a study included in her recent book, Female Fertility and the Body Fat Connection, that found combining a low-fat diet with constant exercise can affect a woman’s ability to conceive, even if she appears perfectly healthy and is still menstruating.

“Losing as little as 10-15 percent of body fat even with normal weight and height can make women infertile,” said Frisch, who has studied the relationship between body weight and fertility for 20 years.

It is widely recognized that being excessively over or underweight can affect a woman’s chance of becoming pregnant.

But Frisch said the problem affects women who are even just slightly underweight, perhaps aiming to copy the body image of thin models and celebrities.

While men may have only 12 percent body fat, an average woman should have 25 percent, she said.

“Women need body fat in order to produce a normal weight baby and to be able to properly nurse it,” Frisch said.

Frisch’s finding has a biological basis. Body fat in women converts the male hormone androgen into the female hormone oestrogen.

It also controls the flow of a hormone called leptin, which affects appetite, energy metabolism and reproduction.

Frisch says a successful pregnancy takes about 50,000 calories more than normal metabolic requirements. If the body does not have the necessary calories, the brain restricts the flow of leptin and switches off the ability to reproduce.

Frisch said her findings should be a warning for women intent on staying as thin as possible.

“These women ate, but only enough to be slim and lean, and their diet consisted mostly of nonfat yogurt, pasta and diet drinks,” she said. “But women who want to have children won’t until they have the proper weight.”

—Staff writer Anat Maytal can be reached at maytal@fas.harvard.edu.