High intake of dietary fiber appears to be associated with reduced risk of breast cancer. This is not surprising since fiber is found primarily in fruits, vegetables, legumes, nuts, and seeds, most of which have chemopreventive components. However, fiber appears to have its own anti-cancer effects. Increasing fiber intake could be particularly beneficial for those who (1) have high levels of inflammation; (2) have type 2 diabetes; (3) have high cholesterol; (4) need to lose weight; or (5) are undergoing hormone replacement therapy (HRT).

Types of fiber

Dietary fiber comes from edible portions of plant-based foods that are resistant to digestion. Soluble fibers (pectins, gums, mucilages) dissolve in water and form a gel, which slows digestion. Insoluble fibers (cellulose, lignins) do not dissolve in water; they add bulk to the diet and speed up the passage of food and waste.

The following foods are good sources of soluble fiber while also having been found to be associated with reduced risk of breast cancer:

Apples
Broccoli
Brussels sprouts
Buckwheat
Dry beans
Kale
Oats
Seaweed
Turnips
The following foods are good sources of insoluble fiber while also having been found to be associated with reduced risk of breast cancer:

Apples
Bell peppers
Blackberries
Blueberries
Broccoli
Brussels sprouts
Carrots
Celery
Cranberries
Dry beans
Flaxseed

Greens
Kale
Oats
Raspberries
Rice, brown
Tomatoes
Turnips
Walnuts
Wheat bran
Zucchini

Fiber is also added to some processed foods in order to qualify the foods as "high fiber" or to improve texture. This "isolated fiber" is extracted or synthesized from plant sources. Examples include inulin (from chicory root or sugar beets), soluble corn fiber (corn), cellulose (wood pulp), maltodextrin (corn, rice, or potato starch) and polydextrose (corn starch). Such fiber-fortified foods typically lack the flavonoids and other biologically active components of fruits, vegetables, grains and nuts and may lack nutritional value, however there is no evidence that isolated fiber is harmful.

Fiber and risk of breast cancer

While not all studies have reported an association, numerous studies have found a reduction in risk of breast cancer among women with high dietary fiber intake:

  • A 2016 major prospective study reported that eating fiber-rich foods during adolescence was associated with reduced risk of breast cancer in adulthood.
  • A meta-analysis of 16 prospective studies reported a 7% reduced risk of breast cancer for women with the highest compared to the lowest intake of dietary fiber. When risk reduction was calculated according to the type of fiber consumed, the reduction in breast cancer risk was 9% for soluble fiber and 5% for insoluble fiber.
  • A U.S. study found that consumption of soluble fiber was associated with reduced risk of premenopausal ER- breast cancer.
  • A Chinese study reported that high fruit and vegetable fiber consumption was associated with approximately half the risk of breast cancer as low intake. The results for soy fiber were not significant and no association was found for cereal fiber intake. When the data was analyzed according to hormone receptor status, an inverse association between fiber intake and breast cancer risk was found for progesterone receptor positive tumors (ER+/PR+ and ER-/PR+), but not PR- tumors (ER+/PR-, ER-/PR-).
  • A major U.S. prospective population study that included 185,598 postmenopausal women found that overall consumption of dietary fiber was inversely associated with risk of breast cancer, with a 13% lower risk of breast cancer for the highest compared to the lowest quintiles (fifths) of intake. The inverse association was much stronger for hormone receptor negative (ER-/PR-) tumors (44% risk reduction) than for ER+/PR+ tumors (5%). High fiber consumption reduced the risk of lobular more than ductal tumors. When stratified by type of food, fiber intake from grains, fruit, vegetables, and beans was not found to be related to risk of breast cancer.
  • A large Swedish prospective study that examined the associations between breast cancer and fiber and various plant food groups reported that high-fiber bread was associated with a 25% lower breast cancer incidence (comparing the highest with the lowest quintile of intake).
  • Another Swedish study reported risk reductions for overall (34%) and for ER+/PR+ (38%) breast cancer for the highest versus lowest quintile of fruit fiber, and non-significant inverse associations for other subtypes of cancer and types of fiber. Among ever-users of hormone replacement therapy, total fiber intake and especially cereal fiber were found to be associated with a 50% reduction in risk for breast cancer overall and for ER+/PR+ tumors when comparing the highest to the lowest quartile.
  • A major U.S. prospective study of women in the Nursesí Health Study reported no overall associations between breast cancer risk and dietary carbohydrates or total dietary fiber intake.

Fiber and risk of breast cancer recurrence

There is very little data concerning fiber intake and risk of recurrence in breast cancer survivors. A 2011 U.S. study designed to investigate the associations between dietary fiber, carbohydrates, glycemic index and glycemic load and breast cancer prognosis reported an inverse association between fiber intake and overall likelihood of death (from any cause). However, no additional benefit was observed for fiber intakes above 9 g/day, indicating a threshold effect. Fiber intake also was found to be inversely associated with breast-cancer specific death and risk of non-fatal recurrence or second breast cancer, but the results were not statistically significant.

How fiber influences breast cancer

Most fiber-rich foods contain a variety of compounds that could influence breast cancer growth and development. Therefore, it is difficult to determine whether fiber, per se, has anticancer activities. However, researchers have attempted to solve this puzzle and provided the following explanations of fiber's role:

  • Fiber increases frequency of bowel movements (bowel motility), which in turn reduces breast cancer risk by increasing estrogen excretion. Circulating estrogen levels are an established risk factor for breast cancer.
  • High fiber intake reduces serum cholesterol levels. There is some evidence that high cholesterol promotes breast cancer.
  • Diets high in fiber reduce circulating C-reactive protein, a marker of inflammation. Inflammation is thought to increase breast cancer risk and worsen its subsequent prognosis.
  • Fiber intake reduces weight gain for similar levels of calorie intake. Overweight postmenopausal women are at increased risk of breast cancer and recurrence compared to normal weight women.
  • A high intake of dietary fiber, particularly soluble fiber, improves blood sugar control and reduces excess circulating insulin (hyperinsulinemia) in patients with type 2 diabetes. Hyperinsulinemia is associated with increased risk of breast cancer and recurrence.

Please see our article on how to optimize your breast cancer diet for information on what to eat during all stages of treatment and recovery.

Below are links to recent studies on this dietary fiber and breast cancer. For a more complete list of studies, please click on the tag fiber.