Another strong correlation was found with the amount of trans fats women consumed. The study found that eating a modest amount of trans fat -- just 2% of calories -- in place of healthier fats and carbohydrates would raise the risk of infertility by 73% to 131%. Previous research has shown that trans fats, unhealthy fats that increase the risk for cardiovascular disease, can affect how quickly insulin is cleared from the body.
But monounsaturated fats, such as those found in olive oil and canola oil, do the opposite, helping to increase insulin sensitivity and improve fertility.
One of the more surprising findings of the study was that whole milk and ice cream appear to be among the most potent fertility foods. That discovery does not have an obvious explanation, Chavarro says. It could be that removing fat from milk alters its hormonal composition, deleting estrogen and progesterone and leaving an excess of male hormones that can affect ovulation.
In the study, the researchers found that one serving a day of a low- or nonfat dairy product increased the risk of ovulatory infertility by 11%. But adding one serving per day of a full-fat dairy product, particularly whole milk, decreased it by 22%.
"The Fertility Diet" recommendation is to get one or two servings a day of whole milk or other full-fat dairy foods. But, Chavarro cautions, don't go crazy with the Haagen-Dazs. The idea is to balance the ratio of male to female hormones -- not pack on pounds, which in itself can disrupt the balance of hormones needed for reproduction.
"Having one glass of whole milk every day is different than having a gallon of whole milk and a bowl of ice cream," he says. "We know one of the best ways to improve insulin sensitivity is to lose weight. So weight control is among the top priorities of the diet" even while including some high-fat dairy in the diet.
Other recommendations include taking a multivitamin and iron. Though American women of childbearing age are advised to consume 400 micrograms of folic acid per day to prevent neural tube birth defects, the nurses study found that ovulation and conception improve with consumption of at least 700 micrograms per day.
The study also found that women who regularly took an iron supplement of at least 40 milligrams a day were 40% less likely to have trouble getting pregnant. But this was only true of women who got the iron from fruits, vegetables, beans and supplements. Those who consumed iron from mostly meat, which contains a different form of iron than that found in other food sources, weren't protected from ovulatory infertility. Previous research suggests that a woman's eggs and the embryo need iron to function properly.
Overall, Chavarro says, the more of these dietary factors a woman adheres to, the greater her chance of improving ovulatory function. The diet has no effect on other causes of infertility, such as blocked fallopian tubes or a low sperm count.
Plans are underway to conduct a randomized, controlled study to test the diet in a more scientifically rigorous manner, Chavarro says. In the meantime, achieving a healthy weight is a good first step for any woman planning a pregnancy, he says.
"We can't know how each individual person is going to respond to the diet," he says. "But in a group of women experiencing ovulatory problems, most of them are going to have improved ovulatory function."
Sharara says a healthy, low-carbohydrate diet is a must for women who wish to become pregnant and have a tendency to gain weight or have high blood pressure or diabetes. He routinely puts his patients with ovulatory problems on a low-carbohydrate diet such as the South Beach Diet.
"We see a number of people who can't get pregnant because of their diet," he says. "After changing their diet, many get pregnant on their own. They don't need special fertility medications that subject them to an increased risk of multiple birth."
Adamson also suggests that women attempting to become pregnant try diet changes first if they are in no great rush. A 40-year-old woman who has been trying to get pregnant for more than a year probably should see a fertility doctor to consider all her treatment options.
"It's incredible that some of the simplest interventions -- simple in theory at least -- may in fact be the most effective," says Adamson, a reproductive endocrinologist in San Jose. "It's really important for women to understand that there are a lot of things they do to ensure the best reproductive outcome for themselves."
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10 ways to enchance fertility
"The Fertility Diet," based on a Harvard study, offers 10 key recommendations to enhance fertility.
* Avoid trans fats.
* Use more unsaturated vegetable oils, such as olive or canola oil.
* Eat more vegetable protein, such as beans and nuts, and less animal protein.
* Choose whole-grain sources of carbohydrates instead of highly refined carbs.
* Drink a glass of whole milk or have a small dish of ice cream or full-fat yogurt each day.
* Take a multivitamin that contains folic acid and other B vitamins.
* Get plenty of iron, but not from red meat.
* Avoid sugared sodas. (Diet sodas are probably OK.) Coffee, tea and alcohol are OK in moderation.
* If overweight, lose 5% to 10% of your weight.
* Start an exercise program. If you already exercise, keep it up but don't overdo it if you are quite lean.
-- Shari Roan