A new US study suggests that eating low fat dairy food every day can reduce a woman's fertility by affecting ovulation.

The study is published in the journal Human Reproduction.

Researchers from Harvard School of Public Health and Brigham and Women's Hospital and Harvard Medical School, Boston, followed 18,555 married, premenopausal women aged between 24 and 42, with no history of infertility, who were either trying to become pregnant or became pregnant over an 8 year period from 1991 to 1999.

The study was led by Dr Jorge Chavarro, a research fellow in the Department of Nutrition at Harvard School of Public Health.

The women were part of a cohort of 116,000 women in The Nurses' Health Study II.

Their dietover the 8 years was assessed with questions about type and frequency of dairy food intake. They were also asked questions about the regularity of their menstrual cycle, whether they had been diagnosed with ovulation failure, and whether they had been trying to conceive and with what success.

438 women reported ovulatory disorders during the period of the study.

The study showed that women who ate more than two portions a day of low fat dairy foods were 85 per cent more likely to be infertile due to ovulatory disorders than those who only ate it less than once a week.

Conversely they found that women who ate full-fat dairy foods, including ice cream, more than once per day had a 25 per cent reduced risk of infertility due to ovulatory disorders compared to those who ate full-fat dairy foods only once a week.

Previous studies have suggested that dairy foods can interfere with ovulation, but few of them have been on humans and they are inconsistent. The researchers in this study wanted to assess to what extent the fat content of dairy foods in a woman's diet might be linked to infertility due to ovulation problems.

The researchers concluded that "High intake of low-fat dairy foods may increase the risk of anovulatory infertility whereas intake of high-fat dairy foods may decrease this risk".

The study found no links between normal intake of lactose, vitamin D, calcium and phosphorous and infertility due to ovulation disorders.

Dr Chavarro's advice to women trying to conceive is to change their diet for a while. He said "They should consider changing low-fat dairy foods for high-fat dairy foods; for instance, by swapping skimmed milk for whole milk and eating ice cream, not low fat yoghurt."

But it was important to do this without increasing their daily calorie intake or upsetting the balance of overall saturated fat consumption, he said.

"Once they have become pregnant, then they should probably switch back to low-fat dairy foods as it is easier to limit intake of saturated fat by consuming low-fat dairy foods," said Dr Chavarro.

The researchers suggest that a fat-soluble substance in the full-fat dairy foods could be responsible for improved ovarian function, and that this substance is removed when full-fat dairy produce is converted to low-fat.

There could be other reasons too, to do with hormone balance. For instance, when full-fat milk is processed to give skimmed milk, whey protein is usually added back in to improve taste and colour. In tests with mice, whey protein is suspected of increasing testosterone-like effects.

"A prospective study of dairy foods intake and anovulatory infertility."
J.E. Chavarro, J.W. Rich-Edwards, B. Rosner, and W.C. Willett.
Advance Access published online on February 28, 2007
Human Reproduction, doi:10.1093/humrep/dem019

Click here for Abstract.

Click here for Centre for Reproductive Medicine (UK).

Written by: Catharine Paddock
Writer: Medical News Today