A study suggests that eating too much fiber could interfere with your chances of getting pregnant. What does this mean for women and couples trying to conceive?
A Nutrition Diva listener recently wrote to me worried that she might be eating too much fiber.
I have a really high fiber diet, as I’ve found it difficult to eat less than 30g of fiber a day when eating whole, non-processed foods. I have not gotten my period in several months. (No I'm not pregnant, I take birth control pills.) My dietitian sent me a study about the effects of a high fiber diet on menstruation. I would love for you to do a podcast on how fiber intake may influence menstruation in child-bearing-age women.
The study she forwarded was actually somewhat surprising. It found that women eating even the recommended amount of fiber each day had an increased risk of anovulation. This means that they experienced a menstrual cycle in which the egg did not mature. The failure to ovulate may or may not lead to a missed period but it definitely means that you cannot get pregnant. So this study could be really important for women who are trying to conceive.
If you're regularly missing your period for any reason, it's a good idea to at least check in with your gynecologist.
However, these findings wouldn't apply to the listener who sent them to me. Birth control pills, by design, prevent ovulation no matter how much fiber you eat. If you're taking birth control pills, then your menstrual cycle, such as it is, is being controlled by external hormones, not your diet. And it is somewhat common for women taking birth control pills to stop menstruating. But, again, this usually has nothing to do with diet. If you're regularly missing your period for any reason, it's a good idea to at least check in with your gynecologist.
But what might this study for women who are trying to get pregnant? Should they be cutting down on fiber to increase their chances?
How does fiber affect ovulation?
The study, which was published in 2009 in the American Journal of Clinical Nutrition, followed 250 women of child-bearing age through two consecutive menstrual cycles. They found that about 8 percent of the time, the women failed to ovulate. They also collected a lot of dietary information and analyzed it to see if they could spot any connections between what the women ate and their risk of anovulation. They found that the women eating the recommended amount of fiber (at least 22 grams per day) were about three times as likely to miss ovulating as all the rest.
That may surprise you but it didn't particularly surprise the researchers. We already know that dietary fiber tends to lower circulating estrogen levels in the body. We generally think of this as a good thing. It's the reason that women who eat more fiber have a lower risk of breast cancer, for example. But for women trying to conceive, could lower levels of reproductive hormones be suppressing their fertility?
A 2009 study found that women eating the recommended amount of fiber (at least 22 grams per day) were about three times as likely to miss ovulating as all the rest, but the results are questionable.
The thing that surprised me most about this study was how few women were actually getting the recommended amount of fiber—fewer than one in 10! The majority weren't even getting half the recommended amount. (Ladies, we can do better!)
But this detail is also the thing that throws these results into question. The total number of women eating 22 grams of fiber or more per day was so low that the number of anovulatory cycles in that group was also quite low—we're talking single digits. When the numbers are that small, even one more or less would have a big impact on the percentages. And that makes it much more likely that much of the statistical difference seen here was actually due to random chance. The other tip-off that this finding might be distorted by chance error is that the risk of anovulation stays more or less the same across all fiber intake levels and then suddenly triples at the top tier. (Which, as I say, was not even all that high!)
The authors acknowledge as much in their remarks. And they also point out that following women for just 2 cycles might not give an accurate view of what the effects might be over a longer time frame. It's the old "more research is needed" situation.
Evidence that eating more fiber supports fertility
But there is other research to consider, and it doesn't confirm the 2009 study's finding.
A much larger, longer study (the Nurse's Health Study) also analyzed the relationship between fiber intake and ovulatory infertility. This project followed over 18,000 women of child-bearing age for almost ten years. But when they analyzed the dietary records, they found no relationship between fiber intake and risk of ovulatory infertility. The nurses, by the way, did a slightly better job meeting the recommended fiber intake, but not much: still only about one in five met the recommendation. (Ladies, we must do better!)
The preponderance of evidence suggests that getting the recommended amount of fiber is going to help your chances of getting pregnant, not hurt it.
Looking at the larger body of science on how diet and nutrition impact fertility, we see more evidence that higher fiber diets promote reproductive health:
- Being overweight can make it a lot harder to get pregnant. Those who eat more fiber are less likely to be overweight.
- The insulin resistance that often goes with being overweight and obesity can also impair fertility. Diets higher in fiber can reduce the risk of insulin resistance.
- Several studies have found that a Mediterranean Diet pattern seems to be conducive to fertility as well. The Mediterranean diet tends to be higher in fiber than the standard American diet, due to the emphasis on fruits, vegetables, legumes, whole grains, and nuts. It's also linked to reduced risk of insulin resistance and being overweight.
When you look at the whole picture, the preponderance of evidence suggests that getting the recommended amount of fiber is going to help your chances of getting pregnant, not hurt it. That doesn't mean that eating two or three times the recommended amount will help even more! It's always possible to overdo a good thing.
But most of us need to step up our game just to get to the recommended amount, which is 25 grams per day for women.