Fiber from whole grain products such as cereals was inversely associated with reduced risk of breast cancer.   While fiber in other foods was associated with a reduced risk of breast, but the association was not statistically significant.

Earlier studies are inconsistent with regard to the effect of dietary fiber on risk of breast cancer. The inconsistency may be due to the fact that the range of fiber intake was too narrow in the study subjects, according to researchers of the current study.

In the current study, Janet Elizabeth Cade at University of Leeds and colleagues followed 35,792 women including 17,781 postmenopausal women and 15,951 pre-menopausal women aged 45 and 59 on average respectively, who entered the UK Women's Cohort Study (UKWCS) between 1995 and 1998.

A food frequency questionnaire (FFQ) was collected as the baseline data at the start of the study. The second FFQ was taken from a sample of 1918 or 5 percent of the cohort. Fiber and other nutrients were calculated and cancer cases were checked out from cancer registries.

During the 6.7-year study, 350 post-menopausal women and 257 pre-menopausal women were diagnosed with invasive breast cancer.   The non-breast cancer subjects were followed for 7.5 years on average.

The participants with 99 percent white women had a mean body mass index 24.5 kg/m2.   18 percent of the women were vegetarian, 12 percent fish-eaters and 70 percent meat eaters.

The mean energy intake was 2360 kcal with 32 percent from fat, 53 percent from carbohydrates, and 15 percent from protein.

The mean fiber intake was 26 g/day with 29 g/day found in fish eaters, 28g/day in vegetarians and 24 g/day in meat eaters.

"Pre-menopausal women with breast cancer had a higher percentage of energy derived from protein and also lower total carbohydrate, sugar, dietary fibre (Englyst) and vitamin C compared with cancer-free women," the researchers write in their report. "Only the difference in percentage energy from protein was statistically significant in this univariable analysis."

Those pre-menopausal women whose intake of total dietary fiber was in the highest quintile (30 grams or more per day) were 50 percent less likely to have breast cancer compared to those whose intake of total fiber was in the lowest quintile (20 grams or less per day), the researchers found.

When other factors such as folate were considered as confounders, the inverse association between total dietary fiber and breast cancer risk in the pre-menopausal women became even stronger.   That is, those who consumed the highest total fiber were 67 percent less likely to develop breast cancer compared to those who ate the lowest quintile of total fiber.

According to the researchers, a few mechanisms have been proposed earlier to explain the protective effect of dietary fiber on the risk of breast cancer. Fiber may inhibit reabsorption of estrogen and androgens and activity of human estrogen synthetase, which in turn reduces production of estrogen.   Both estrogen and androgens have been linked to higher risk of breast cancer.  

Other researchers say that fiber may also reduce absorption of toxic chemicals, which may be another mechanism by which dietary fiber reduces the breast cancer risk.

An interesting observation the researchers did not elaborate is that compared to the pre-menopausal women without cancer, the pre-menopausal cancer patients ate higher amounts of protein, which may be a potential risk, according to a scientist affiliated with foodconsumer.org.

Early studies have at least indirectly linked high consumption of meat (high fat/high protein) with high cancer risk, according to Dr. T. Colin Campbell, a prominent nutrition professor from Cornell University.

Early studies have found dietary IGF can influence the serum levels of IGF-1 in humans, which has been linked with increased risk of breast cancer, according to the researchers of the current study, who did not elaborate the effect of meat on the breast cancer risk.

The current study did not find an association between dietary fiber and breast cancer risk in post-menopausal women.   “In post-menopausal women, nutrient intakes did not differ greatly between those with and without breast cancer,” the researchers write. This observation was in consistency with previous studies, according to the researchers.

The researchers explain that "the estrogen metabolism pathway differs between pre-and post-menopausal women."

In one early study, women supplemented with 10 or 20 grams of fiber-rich wheat bran each day for 2 months had significantly lower ostradiol concentrations. The study was published in 1997 by Rose DP, Lubin M, and Connolly J.M. in Nutrition 13:535–39.

Another study, published in 1998 by Stark AH, Switzer BR, and Atwood JR, et al. in Nutr Cancer 31:138–42 found that in postmenopausal women, wheat bran supplementation did not reduce estradiol, androstenedione or sex hormone-binding globulin.  

In yet another study, reported in 1999 by Barr S. I. in American Journal of Clinical Nutrition, 70:549s–54s, a high fiber diet or vegetarian diet was found to affect cycle length in pre-menopausal women, which is linked to estrogen exposure, but this effect was not found among postmenopausal women.

The intake of dietary fiber in an average American is 10 to 12 grams per day.  The study suggests that to have a protective effect against breast cancer, a woman needs to eat 30 grams or more per day.

In the U.S., 178,480 women will be diagnosed with invasive breast cancer this year and 40,910 are expected to die of the disease, according to the latest report released by the American Cancer Society.

Source;

Janet Elizabeth Cade, Victoria Jane Burley, Darren Charles Greenwood, and the UK Women's Cohort Study Steering Group

Dietary fibre and risk of breast cancer in the UK Women's Cohort Study

IJE Advance Access published on January 24, 2007.

doi:10.1093/ije/dyl295