NEW YORK (Reuters Health) – As total dietary fiber intake increases, the risk of breast cancer, primarily ER-/PR- tumors, decreases, according to study findings published in the American Journal of Clinical Nutrition for September.

“Although dietary fiber has been hypothesized to lower risk of breast cancer by modulating estrogen metabolism, the association between dietary fiber intake and risk of breast cancer by hormone receptor status is unclear,” lead author Dr. Yikyung Park, from the National Cancer Institute, Rockville, Maryland, and colleagues note.

“Our study,” the researchers point out, “is the first study to examine the association between dietary fiber and breast cancer by histological type.”

The investigators analyzed data on 185,598 postmenopausal women (average age 62 years) enrolled in the National Institutes of Health-American Association of Retired Persons Diet and Health Study. A 124-item food frequency questionnaire was used to assess fiber intake at baseline.

During an average follow-up of 7 years, 5461 breast cancer cases were reported, including 2391 for whom data on hormone receptor status were available: 1641 ER+/PR+, 336 ER+/PR-, 48 ER-/PR+, and 366 ER-/PR-.

Relative to the lowest quintile of total fiber intake, the highest quintile was associated with a 13% decreased risk of breast cancer (p for trend = 0.02), the investigators found.

Further analysis showed the association to be stronger for ER-/PR- tumors than for ER+/PR+ tumors. Comparing the highest quintile of total fiber intake with the lowest, the risk of ER-/PR- breast cancer fell by 44% (p for trend = 0.008), while the risk of ER+/PR+ tumors dropped by just 5% (p for trend = 0.47).

The researchers also noted that the effect of fiber intake on breast cancer risk varied by histological type. High fiber intake was linked to a 34% reduced risk of lobular tumors (p for trend = 0.04) compared with a 10% reduced risk for ductal tumors (p for trend = 0.10).

Although total fiber intake seemed to impact breast cancer risk, fiber intake from several food groups, including grains, vegetables, fruit, and beans, did not.

Soluble fiber intake was inversely linked to breast cancer risk, whereas insoluble fiber intake showed no association.

“It is possible that fiber type makes a difference in pathophysiologic processes related to breast cancer,” the authors speculate. “Soluble fiber has been shown to be more effective in controlling blood glucose, insulin, and insulin-like growth factors, which have been positively related to risk of breast cancer.”

The association between total fiber intake and breast cancer risk was not significantly modified by total fat intake, the researchers report.

“Our finding suggests that dietary fiber can play a role in preventing breast cancer through nonestrogen pathways among postmenopausal women,” the researchers conclude. “Nevertheless, the totality of evidence at this point is far from consistent, and additional research is needed before definitive public health recommendations for fiber and breast cancer can be made.”

Am J Clin Nutr 2009;90:664-671.