Effect of dietary fiber on mammary tumorigenesis, estrogen metabolism, and lipid excretion in female rats.

We previously reported that supplementary dietary wheat bran significantly reduced the incidence of N-nitrosomethylurea (NMU) induced mammary tumors in rats fed a high fat (HF) diet and reduced to a lesser extent the incidence in rats fed a low fat (LF) diet compared to their unsupplemented controls. Using the same cohort of experimental animals, we here report the results of biochemical analyses designed to investigate the effect of supplemental dietary fiber on estrogen metabolism and lipid excretion. Serum, hepatic tissue, urine, and feces were obtained from rats which had been fed a HF (23.5% corn oil) diet, a HF plus fiber (HF + F, 10% soft white wheat bran, SWWB) diet, a LF (5.0% corn oil) diet, or a LF plus fiber (LF + F) diet for 15 weeks beginning 3 days after a single dose of NMU. Our working hypotheses to explain how dietary fiber protects against mammary tumorigenesis were that fiber may: 1) act as an antiestrogen; 2) decrease circulating estrogens; 3) alter the enterohepatic recirculation of estrogens; and 4) physically bind to lipid and remove it in the feces. Therefore, various indices of estrogen metabolism were assessed including: 1) circulating estradiol (E2) and progesterone; 2) hepatic estrogen receptor (ER) protein; 3) excretion of estrogen in the urine and feces; 4) the in vitro estrogen binding properties of SWWB; 5) fecal lipid content; and 6) the in vitro lipid binding capacity of SWWB. Serum 17 beta-estradiol and progesterone remained unchanged as did hepatic cytosolic (cER) and nuclear (nER) estrogen receptor protein content, in the fiber supplemented groups compared to their respective controls.(ABSTRACT TRUNCATED AT 250 WORDS)