Fat, Carbs and the Science of Conception

A related issue is whether dietary fats influence ovulation and reproduction. We were shocked to discover that this was largely uncharted territory. Until now, only a few studies have explored this connection. They focused mainly on the relationship between fat intake and characteristics of the menstrual cycle, such as cycle length and the duration of different phases of the cycle. In general, these studies suggest that more fat in the diet, and in some cases more saturated fat, improves the menstrual cycle. Most of these studies were very small and didn't account for total calories, physical activity or other factors that also influence reproduction. None of them examined the effect of dietary fat on fertility.

The dearth of research in this area has been a gaping hole in nutrition research. If there is a link between fats in the diet and reproduction, then simple changes in food choices could offer delicious, easy and inexpensive ways to improve fertility. The Nurses' Health Study research team looked for connections between dietary fats and fertility from a number of different angles. Among the 18,555 women in the study, the total amount of fat in the diet wasn't connected with ovulatory infertility once weight, exercise, smoking and other factors that can influence reproduction had been accounted for. The same was true for cholesterol, saturated fat and monounsaturated fat—none were linked with fertility or infertility. A high intake of polyunsaturated fat appeared to provide some protection against ovulatory infertility in women who also had high intakes of iron, but the effect wasn't strong enough to be sure exactly what role this healthy fat plays in fertility and infertility.

Trans fats were a different story. Across the board, the more trans fat in the diet, the greater the likelihood of developing ovulatory infertility. We saw an effect even at daily trans fat intakes of about four grams a day. That's less than the amount the average American gets each day.

Eating more trans fat usually means eating less of another type of fat or carbohydrates. Computer models of the nurses' diet patterns indicated that eating a modest amount of trans fat (2 percent of calories) in place of other, more healthful nutrients like polyunsaturated fat, monounsaturated fat or carbohydrate would dramatically increase the risk of infertility. To put this into perspective, for someone who eats 2,000 calories a day, 2 percent of calories translates into about four grams of trans fat. That's the amount in two tablespoons of stick margarine, one medium order of fast-food french fries or one doughnut.

Fats aren't merely inert carriers of calories or building blocks for hormones or cellular machinery. They sometimes have powerful biological effects, such as turning genes on or off, revving up or calming inflammation and influencing cell function. Unsaturated fats do things to improve fertility—increase insulin sensitivity and cool inflammation—that are the opposite of what trans fats do. That is probably why the largest decline in fertility among the nurses was seen when trans fats were eaten instead of monounsaturated fats.

The Protein Factor
At the center of most dinner plates sits, to put it bluntly, a hunk of protein. Beef, chicken and pork are Americans' favorites, trailed by fish. Beans lag far, far behind. That's too bad. Beans are an excellent source of protein and other needed nutrients, like fiber and many minerals. And by promoting the lowly bean from side dish to center stage and becoming more inventive with protein-rich nuts, you might find yourself eating for two. Findings from the Nurses' Health Study indicate that getting more protein from plants and less from animals is another big step toward walking away from ovulatory infertility.