Fat, Carbs and the Science of Conception

Evaluating total carbohydrate intake can hide some important differences. So we looked at something called the glycemic load. This relatively new measure conveys information about both the amount of carbohydrate in the diet and how quickly it is turned to blood sugar. The more fast carbs in the diet, the higher the glycemic load. (For more on glycemic load, go to health.harvard.edu/newsweek.) Women in the highest glycemic-load category were 92 percent more likely to have had ovulatory infertility than women in the lowest category, after accounting for age, smoking, how much animal and vegetable protein they ate, and other factors that can also influence fertility. In other words, eating a lot of easily digested carbohydrates increases the odds of ovulatory infertility, while eating more slow carbs decreases the odds.

Because the participants of the Nurses' Health Study complete reports every few years detailing their average daily diets, we were able to see if certain foods contributed to ovulatory infertility more than others. In general, cold breakfast cereals, white rice and potatoes were linked with a higher risk of ovulatory infertility. Slow carbs, such as brown rice, pasta and dark bread, were linked with greater success getting pregnant.

Computer models of the nurses' diets were also revealing. We electronically replaced different nutrients with carbohydrates. Most of these substitutions didn't make a difference. One, though, did. Adding more carbohydrates at the expense of naturally occurring fats predicted a decrease in fertility. This could very well mean that natural fats, especially unsaturated fats, improve ovulation when they replace easily digested carbohydrates.

In a nutshell, results from the Nurses' Health Study indicate that the amount of carbohydrates in the diet doesn't affect fertility, but the quality of those carbohydrates does. Eating a lot of rapidly digested carbohydrates that continually boost your blood-sugar and insulin levels higher can lower your chances of getting pregnant. This is especially true if you are eating carbohydrates in place of healthful unsaturated fats. On the other hand, eating whole grains, beans, vegetables and whole fruits—all of which are good sources of slowly digested carbohydrates—can improve ovulation and your chances of getting pregnant.

Balancing Fats
In 2003, the government of Denmark made a bold decision that is helping protect its citizens from heart disease: it essentially banned trans fats in fast food, baked goods and other commercially prepared foods. That move may have an unexpected effect—more little Danes. Exciting findings from the Nurses' Health Study indicate that trans fats are a powerful deterrent to ovulation and conception. Eating less of this artificial fat can improve fertility, and simultaneously adding in healthful unsaturated fats whenever possible can boost it even further.

Women, their midwives and doctors, and fertility researchers have known for ages that body fat and energy stores affect reproduction. Women who don't have enough stored energy to sustain a pregnancy often have trouble ovulating or stop menstruating altogether. Women who have too much stored energy often have difficulty conceiving for other reasons, many of which affect ovulation. These include insensitivity to the hormone insulin, an excess of male sex hormones and overproduction of leptin, a hormone that helps the body keep tabs on body fat.