Women who eat low-fat dairy foods may have a higher risk of infertility than those who treat themselves to full-fat ice cream or cheese, surprised U.S. researchers said on Tuesday.

They found that women who ate two or more servings of low-fat dairy foods a day had an 85 percent higher risk of a certain type of infertility than women who ate less than one serving of low-fat dairy food a week.

Women who ate one serving of high-fat dairy food a day were 27 percent less likely to be infertile than women who avoided full-fat dairy foods.

It was not the finding that nutritionist Dr. Jorge Chavarro of the Harvard School of Public Health in Boston had expected.

"We wanted to know whether dairy foods in general affected fertility," Chavarro said in a telephone interview. "There was pretty strong evidence in animal studies suggesting that a specific sugar in dairy, lactose, could be deleterious."

But little work had been done in humans, so Chavarro and colleagues looked at data from the Nurses Health Study, an ongoing survey of tens of thousands of women who fill out regular questionnaires about their diet, activity and health.

They chose the records of 18,555 women aged 24 to 42 who had tried to become pregnant or had became pregnant between 1991 and 1999.

Chavarro said most were of European origin -- possibly an important factor because people of northern European descent are less likely to be lactose intolerant.

The women who ate the most low-fat dairy foods were the most likely to report they suffered from anovulatory infertility, when the body fails to produce enough egg cells.

Writing in the journal Human Reproduction, Chavarro said he found no difference between women who ate the most dairy and those who ate little or none. He found the differences only when he broke down the types of dairy products.

"It was a bit of a surprise to us that high-fat dairy foods were positively related to fertility," he said. "There is really not a very clear explanation. It is possible that dairy fat or something along with dairy fat such as the hormones in pregnant cows may be affecting ovulation in women."

He said more study was needed before conclusions could be drawn.

Chavarro's team had earlier found that women who ate more iron from supplements and from plant foods were less likely to be infertile, and found no link between fertility and various types of fats.

But he does not recommend that women trying to conceive use this as an excuse to eat "buckets and buckets of ice cream."

"The benefit is at as low as one serving a day," he said.