A diet low in fat and high in fiber reduces levels of circulating ovarian hormones (such as estrogen and progesterone), but not enough to have significant effect in reducing a woman's risk of breast cancer, according to a new study. The finding, published in the journal Cancer, comes from the first long-term randomized trial to evaluate the effect of this dietary pattern (without caloric changes or weight reduction) on hormone levels in premenopausal women.

Some of the most significant risk factors for breast cancer include early menarche, later menopause, pregnancy later in life and no pregnancy. Those factors point to the important role increased cumulative circulating ovarian sex hormones, such as estrogen and progesterone, have on the risk of breast cancer.

Diet is thought to play a role in breast cancer risk due to its affect on levels of sex hormones, independent of a woman's body mass index. Some studies have shown associations between high levels of estrogen and high intake of fat or low intake of fiber. However, these studies have lacked significant numbers of subjects and long-term follow up.

Researchers led by Peter Gann, MD of the Feinberg School of Medicine of Northwestern University investigated the effects of diet (20% calories as fat, 25 grams/day of fiber and at least eight fruit or vegetable servings per day) on ovarian hormone levels and menstrual cycle among premenopausal women between 20-40 years who were randomly assigned to the low fat, high fiber (LFHF) diet or regular diet and followed for 12 menstrual cycles.

Serum estradiol decreased 7.5 percent in subjects on the LFHF diet, compared to only 0.9 percent in the control subjects, which approached but did not reach statistical significance after one year. Progesterone levels were also no different between the two groups. Moreover, menstrual cycle length and frequency of anovulatory cycles were similar between the diet and control groups.

"The results of this randomized trial indicate that an isocaloric diet, characterized by low fat, high fiber, and high fruit and vegetable content, does not produce substantial changes in midluteal serum ovarian hormone levels in premenopausal women, compared to women following their usual diet, over a one-year period," conclude the authors.

Future studies of the isocaloric low fat, high fiber diet pattern could focus on more drastic diet alterations, or adoption of the eating pattern at younger ages. Meanwhile, achieving and maintaining optimal body weight during midlife remains a potentially important strategy for reducing estrogen levels, and thereby breast cancer risk, during the postmenopausal period.

SOURCE:
Cancer, November 1, 2003