Eating these unhealthy fats has been strongly linked to an increased risk for heart disease. Now, new research suggests they also increase a woman's risk of infertility.
In their study, nutrition researchers at the Harvard School of Public Health found that women with ovulation-related fertility problems tended to eat more trans fats than fertile women.
Obtaining just 2 percent of total calories from trans fats instead of healthier monounsaturated fats was associated with a doubled risk for this type of infertility.
In addition, each 2 percent increase in trans fat consumption as a replacement for carbohydrates brought a 73 percent greater risk of ovulation-related infertility, after adjusting for other known and suspected infertility risk factors, according to the study.
Lead author Jorge E. Chavarro, M.D., Sc.D., tells WebMD the findings must be confirmed.
But he says women planning pregnancies should be especially vigilant about replacing trans fats, just in case.
"We do know that trans fats have a very deleterious effect on heart disease and metabolism in general," Chavarro says.
"Avoiding trans fats is a good idea for many reasons, and one of them may be reducing infertility risk," he says.
Trans Fats in the News
Thanks to a greater public awareness of the health risks associated with trans fats, avoiding them may be much easier for Americans in the near future.
KFC, Taco Bell, and Starbucks all recently announced they would eliminate or greatly reduce the amount of trans fats in their products, following the lead of restaurant chains like Wendy's and Chili's.
And last month, New York City banned use of the fats in restaurants. Chicago officials are said to be considering a similar move.
Fewer trans fats now lurk in the processed foods on supermarket shelves, and the U.S. government has started making food manufacturers list them on package labels.
But, Chavarro says, such labeling may still be misleading.
"You still have to read the labels of the foods you buy," he says. "If you see 'hydrogenated' or 'partially hydrogenated,' that is a red flag."
Trans fats are liquid oils that are put through a chemical process called hydrogenation to make them more solid and extend shelf life.
Nutritionist Isn't Convinced
Chavarro's research team is among the first to examine the impact of specific foods and nutrients on infertility.
Last fall, they reported that iron supplements appear to help protect women from ovulation-related infertility.
Both that research and the newly published findings were based on data from the Nurses Health Study II, an ongoing health study involving female nurses.
More than 18,500 married, premenopausal nurses who either became or attempted to become pregnant between 1991 and 1999 were included in the latest assessment. A total of 438 were diagnosed with ovulation-related infertility during that eight-year period.
Neither total fat intake nor total cholesterol intake was found to be associated with ovulatory infertility.
Trans fats were the only fats found to negatively affect ovulation-related fertility.
But nutritionist Marion Nestle, PhD, MPH, tells WebMD the findings are unconvincing. Nestle is a professor in the department of nutrition, food studies and public health at New York University in Manhattan.
She points out that the researchers were only able to show an increased infertility risk when they adjusted for many other possible infertility risk factors.
"If you look at their raw data, it just didn't show an increase in risk," she says. "And even when the adjustments were made, the numbers were still very small."
Nestle says the only dietary factors proven to play a role in infertility are eating way too little and eating way too much. Infertility is common among womewho starve themselves for long periods or who are very obese.
There is little to suggest that the individual foods women eat play a significant role in fertility, Nestle says.
"I am always skeptical when I hear the claim that a particular food or food component has a very large impact on health," she says.
SOURCES: Chavarro, J. American Journal of Clinical Nutrition, January 2007; vol 85: pp 231-237. Jorge E. Chavarro, MD, ScD, department of nutrition, Harvard School of Public Health, Boston. Marion Nestle, PhD, MPH, professor, department of nutrition, food studies and public health, New York University.
By Salynn Boyles
Reviewed by Louise Chang