Women who get the recommended amount of fiber in their diets may have lower estrogen levels and ovulate less often than women who eat less fiber, a new study suggests.
Researchers found that among 250 women ages 18 to 44, those who reported eating the recommended amounts of fiber had the lowest blood levels of estrogen and other reproductive hormones.
Higher fiber intake, particularly from fruit, was also linked to a higher risk of having anovulatory menstrual cycles — where the ovaries fail to release an egg.
The findings, reported in the American Journal of Clinical Nutrition, do not mean that eating fiber-rich foods is a bad thing.
High-fiber diets are associated with numerous health benefits, including lower risks of heart disease, diabetes, colon cancer and breast cancer. Experts generally recommend that adults get 20 to 35 grams of fiber each day, depending on their calorie intake.
However, the current results do "call into question" whether those recommendations are best for women who are trying to become pregnant, write the researchers, led by Audrey J. Gaskins of the National Institute of Child Health and Human Development in Rockville, Maryland.
Anovulation can have various causes, including excessive exercise, having either too little or too much body fat, thyroid gland dysfunction and polycystic ovarian syndrome — a hormone disorder that is a common cause of infertility.
Women who are not ovulating regularly often have irregular menstrual periods or none at all. However, some women do continue to have periods.
All of the women in the current study were healthy and having regular menstrual periods. Still, those who reported the highest fiber intake — 22 grams per day or more, in line with general recommendations — were more likely to have at least one anovulatory cycle over two months. The researchers gauged anovulation by measuring the women's reproductive-hormone levels over two menstrual periods.
Of the total menstrual cycles in this group, 22 percent were anovulatory, compared with 7 percent among women with lower fiber intakes.
When the researchers accounted for other factors that could affect ovulation — including body weight, race, exercise levels and calorie intake — high fiber intake itself was linked to a roughly 10-times higher risk of anovulation.
Looking at specific sources of fiber, the researchers also found that fiber from fruit, specifically, was most clearly associated with anovulation.
The results do not prove that fiber, per se, disrupts some women's ovulation. However, it is biologically plausible, Gaskins and her colleagues point out.
High-fiber diets, they explain, decrease activity in certain intestinal enzymes, leading to less estrogen reabsorption in the colon. Fiber can also cause more estrogen to be excreted from the body in feces.
In line with that, the researchers found that women with the highest fiber intakes generally had the lowest estrogen levels over the course of their menstrual periods. They also had lower levels of other reproductive hormones, including progesterone, luteinizing hormone and follicle-stimulating hormone.
The findings, according to Gaskins and her colleagues, raise the possibility that women who are trying to conceive should lower their fiber intake. However, they write, more studies are needed before any recommendations can be made.
SOURCE: American Journal of Clinical Nutrition, October 2009.