A diet high in total fiber appears to lower the risk of breast cancer for both pre- and postmenopausal women, researchers who pooled data from 20 observational studies reported.
This association was found valid for different sources of fiber — foods like cereals, fruits, vegetables, and legumes that make up total fiber. But links between diet and cancer risk were significant only for fibers attained from fruits and other soluble fiber sources.
The study, “Fiber consumption and breast cancer incidence: A systematic review and meta‐analysis of prospective studies,” was published in the journal CANCER.
Several reports suggesting linking fiber intake and breast cancer risk have been published over the years, and the American Cancer Society (ACS) recommends a fiber-rich diet as part of its dietary guidelines for cancer prevention.
Diets rich in fiber foods are thought to be beneficial due to their effects on insulin sensitivity and alterations in sex hormone levels, two factors believed to account for the risk of breast cancer.
But some reports provide conflicting results, and evidence linking fiber intake and breast cancer risk are “weak and inconsistent,” this study noted.
Researchers analyzed data from 20 carefully selected observational studies conducted before July 2019, which were the basis of 21 publications. Their analysis described the association between total fiber consumption, intake of soluble and insoluble fiber, and cereal fiber, fruit fiber, vegetable fiber, and legume fiber individually with breast cancer risk.
Studies chosen involved from 270 to 691,571 people, who were followed over a range spanning two to 20 years, to determine the long term effects of fiber intake.
These data were adjusted, accounting for a number of breast cancer risk factors that include genetic predisposition, smoking, alcohol consumption, medical history, and caloric intake.
Total fiber consumption was associated with an 8% reduced risk of breast cancer.
Among 10 publications looking at the association between different sources of fiber and breast cancer risk, fruit fiber correlated with a 7% reduction in risk, vegetable fiber with a 5% reduction, and cereal and legume fiber each with a 3% lesser risk. Statistical significance was only reached for fruit fiber.
Nut and seed fiber were not included due to a limited amount of data.
Among seven studies that compared soluble and insoluble fiber intake, researchers found soluble fiber was associated with a 10% reduction in breast cancer risk and insoluble fiber with a 7% reduction.
Only soluble fiber — fiber that attracts water — demonstrated a statistically significant association. (Soluble fibers, which slow digestion, are found in oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables; insoluble fibers, which are thought to help foods pass more quickly, include wheat bran, vegetables, and whole grains.)
Researchers then looked more closely at the correlation between total fiber intake and breast cancer risk, to determine if a person’s age or the type of breast cancer played a role.
Their analysis indicated that breast cancer risk in both premenopausal and postmenopausal women dropped with higher levels of total fiber intake, with an 18% reduction in premenopausal, and a 9% reduction in postmenopausal breast cancer.
Different types of breast cancer, based on the status of the hormone receptors estrogen (ER) and progesterone (PR), were also analyzed. Although not statistically significant, results showed a risk of developing both ER/PR positive and ER/PR negative breast cancers was inversely associated with the amount of total fiber intake. In other words, the more fiber in the diet, the lower a person’s risk.
“Our study contributes to the evidence that lifestyle factors, such as modifiable dietary practices, may affect breast cancer risk,” Maryam Farvid, PhD, with the Harvard T.H. Chan School of Public Health and the study’s lead author, said in a press release. “Our findings provide research evidence supporting the American Cancer Society dietary guidelines, emphasizing the importance of a diet rich in fiber, including fruits, vegetables, and whole grains.”
Further study, like a randomized clinical trial, is necessary to demonstrate cause and effect, the researchers said.