After observational studies, doctors can reasonably suggest a few key tips for diets geared towards fertility
Infertility, defined as the inability to conceive after a year of unprotected, frequent sexual intercourse, affects an estimated 15 percent of all couples during their reproductive years. The probability of pregnancy drops in women who are on the extremes of body mass, underweight or obese, defined as body mass index (BMI) of 30 k/m2 or higher.
The good news is that losing weight improves fertility in morbidly obese women. In particular, weight loss appears to significantly improve menstrual regularity and ovulation. However, the specific diet and micronutrients that would contribute to better pregnancy outcomes is only recently being rigorously investigated.
This is especially timely as one out of four reproductive-aged women in the United States is overweight or obese. I would like to review some recent scientific studies addressing diet as it relates to fertility.
Excess weight and anovulation
Let’s begin by zeroing in on a subset of patients who are unable to ovulate. For an estimated one third of all infertile couples, anovulation is the primary reason for infertility. Furthermore, the most common reason for anovulation is polycystic ovarian syndrome (PCOS), a condition characterized by oligomenorrhea, anovulation, hyperandrogenism, and frequently associated obesity.
We’ve long known that the pathogenesis of PCOS involves insulin resistance and exaggerated ovarian androgen production, but the precise relationship of these processes is still incompletely understood. While obesity is a frequent feature of PCOS, it is not a part of the accepted diagnostic criteria. Moreover, non-PCOS obesity is a well-recognized cause of reproductive impairment on its own, with study after study describing a detrimental influence on reproduction even when a woman has a normal menstrual cycle.
Obesity prolongs the time to pregnancy and decreases the likelihood of pregnancy in women with regular menstrual cycles. Obesity as a potential cause of infertility was described as long ago as 1328 in a Bible commentary by a medieval scholar. Yet the understanding of the mechanisms underlying this association is still in its infancy.
Altering food intake to impact fertility is also not new. In the early 20th century, married women in Chesire, England were reportedly known to eat a local “fertility loaf” of whole wheat and wheat germ credited to promote fertility in farm animals. In some cultures, ethnobiologists have noted links between eating fish eggs and “fertility and effective childbearing.” From the “fertility breads” in agricultural societies to a root vegetable known as maca in Peruvian communities, many cultures still retain seemingly peculiar food traditions to encourage successful conception.
Although intriguing, none of these studies rigorously examine populations large enough to confirm or refute the effects of dietary alterations to promote fertility.
Weight loss and exercise
Weight loss to improve fertility, mainly through diet and lifestyle changes, has been studied in patients with anovulatory disorders. In a small study, researchers reported that over a 6-month period, weight loss averaging 6.3 kg with exercise and lifestyle management led to 12 out of 13 women studied resuming ovulation. Bear in mind, however, that these results were accomplished in an aggressive intervention program and don’t always apply to general clinical practice.
Weight loss brought about by bariatric surgery in morbidly obese women has been reported to improve a variety of reproductive markers such as ovulation and pregnancy rates. A major drawback of most currently available bariatric surgery studies that report improved fertility is their small sample size and the lack of appropriate control group of subjects who are managed medically. However, even as larger bariatric surgery data become available, we need more information to understand how less drastic interventions such as modifying one’s diet fits in with efforts to improve fertility.
Creating a ‘fertility diet’
In a series of recent studies, investigators used the prospectively followed cohort from the Nurses’ Health Study II (NHS II). The analytic samples included more than 17,000 married American nurses who self-reported their inability to become pregnant after attempting for at least a year.
The main hypothesis tested in these studies was whether a certain diet composition could promote fertility in anovulatory females. The researchers used the prospectively collected dietary information to devise a ranking system for the creation of a “fertility diet.”
The first report of this work in 2007 described follow-up of 17,544 married women with no history of infertility and an average age of 32 years at the start of the study. During eight years of follow-up, 416 women reported ovulatory disorder as the cause of their infertility. The entire study sample was divided into five subgroups based on adherence to a diet that was deemed to promote fertility.
Specifically, the factors thought to be related to low risk of ovulatory disorders included lower intake of monosaturated trans fat, a lower intake of animal protein, greater vegetable protein intake, a higher intake of high-fiber, low-glycemic carbohydrates.
Women in the top 20 percent of this fertility diet score had a 66 percent decreased risk of developing ovulatory infertility as compared to the their counterparts in the bottom 20 percent. This relationship held true after accounting for the women’s age, parity, and body weight. Similarly, combining physical activity, diet, and weight control resulted in a 9 percent decreased risk of anovulatory infertility.
In a later report, the same group gave a more detailed evaluation of protein consumption as it relates to anovulatory infertility. High total protein intake was associated with a 40 percent greater risk for ovulatory infertility after controlling for age, BMI, physical activity, and other related parameters. Specifically, eating animal protein (mainly chicken and turkey) was thought to have contributed to the increased risk of anovulation and raised the risk of developing anovulatory infertility by 39 percent.
On the other hand, eating vegetable protein, such as derived from peanut butter and peas or lima beans has yielded a corresponding 22 percent decrease of the same risk. Moreover, intake of 5 percent of total energy from a vegetable rather than an animal protein source dropped the risk of developing anovulatory infertility by a factor of two.
Another report from the same group looked at the intake of dietary fatty acids and its association with de novo development of anovulatory infertility. The scientific rationale for this approach was the link between the dietary fatty acids and peroxisome proliferator-activated receptors γ (PPAR-γ).
PPAR are a family of nuclear receptors that are activated by binding of natural fatty acid ligands. Insulin sensitizers that are thought to work through PPAR- γ, such as pioglitazone, have been found effective in inducing ovulation in PCOS patients.
In an NHS II cohort, preference for transunsaturated fats in the diet appeared to raise the risk for anovulatory infertility by as much as 70% after adjusting for other related factors and potential confounders. This trend was true whether this diet preference was contrasted with carbohydrates or unsaturated fats commonly found in vegetables.
Finally, the same investigators analyzed in detail the link between vitamins and anovulatory infertility. They found an inverse relationship between the risk of ovulatory infertility risk and the use of multivitamin supplements. Specifically, there were fewer women with ovulatory infertility among those taking vitamin B complex supplements for three or more days a week.
Folic acid had an independent significant role in lowering relative risk of ovulatory infertility, thus providing an added benefit to the recommendation of ensuring adequate folic acid intake prior to conception. Additionally, iron derived from plants and vegetables, non-heme iron, lowered the risk of ovulatory infertility.
While our focus here has been on the relationship between excess weight and reproduction, underweight women also experience diminished ovulation and fertility. The Nurses’ Health Study found a U-shaped association between adult BMI and the relative risk of ovulatory disorder infertility in that women at both extremes of body mass had a higher risk of becoming infertile.
Although obesity is often the cause of ovulatory infertility, 12 percent of ovulatory infertility in that study could be attributed to low body mass. However, there’s a paucity of interventional trials that directly address this question. Reported studies generally have a relatively small sample size, such as a cohort of 26 infertile women with a mean BMI of 19.2 kg/m2 who had undergone counseling to increase BMI.22
Of note, these women gained almost 4 kg during the study period, and 73 percent conceived after these interventions. This might suggest that underweight infertile women may benefit from weight gain, which may improve both the probability of ovulation and the likelihood of conception.
In conclusion, with the ever-growing number of reproductive-aged women with obesity and its frequently associated anovulatory disorders, the need for information and guidance about fertility and diet is urgent.
Assisted reproduction technologies, such as in vitro fertilization, are easily available but are associated with inherent risks and expense. Following a “fertility diet” may result in a more cost-effective and less medicalized method of improving reproductive health in women with anovulatory disorders.
According to available data, doctors can reasonably suggest that anovulatory patients eat less animal protein, cut down on trans-unsaturated fats, and take multivitamins. We are cautious about making strong recommendations, though, as most of the available data come from observational studies.
Nonetheless, we can congratulate ourselves on getting closer to defining the “fertility loaf” of the 21st century, at least for women suffering from anovulatory infertility.