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Diet high in fiber linked to lower breast cancer risk

Most Americans don't get enough fiber in their diets. Here's another compelling reason for women to eat more.
/ Source: TODAY

Women who eat a diet rich in fiber may have a lower risk of breast cancer, a review of studies has found.

Harvard University researchers analyzed 20 published papers about fiber consumption and the incidence of the disease, and then identified study participants who ate the most and the least fiber.

They found women with the highest total fiber consumption had an 8% lower risk of breast cancer compared to the group with the lowest intake. The findings were recently published in the journal Cancer.

"Our study contributes to the evidence that lifestyle factors, such as modifiable dietary practices, may affect breast cancer risk," said Maryam Farvid, lead author and a research scientist at the Harvard T.H. Chan School of Public Health in Boston, in a statement.

The findings suggest “a high intake of total and soluble fiber was significantly associated with a reduced overall breast cancer incidence,” she and her colleagues wrote.

Soluble fiber turns to gel during digestion and can be found in foods like oat bran, barley, nuts, seeds, beans, lentils, peas and some fruits and vegetables. (Insoluble fiber — which is found in wheat bran and whole grains, and passes through the intestines mostly intact — didn’t have as strong of an association in this analysis.)

A high intake of total fiber also was found to be “significantly associated” with a decreased risk in both premenopausal and postmenopausal breast cancers, the study noted.

The 8% lower risk is a decrease in relative risk, said Nigel Brockton, vice president of research at the American Institute for Cancer Research in Washington, D.C.

An American woman has a 1 in 8 chance of developing breast cancer in her lifetime, so this lowered the risk to approximately 1 in 8.6, he noted.

“Although that might not seem very consequential, it is expected that over 275,000 American women will be diagnosed with breast cancer in 2020, so even small changes in risk can have a substantial impact at the population level,” Brockton told TODAY.

Women may be puzzled why breast cancer and fiber intake would be connected, but there are several possible reasons.

First, fiber can help control blood sugar and improve insulin sensitivity. “There is a link between high levels of insulin and the pathogenesis of breast cancer,” Dr. Andrew Dannenberg, a professor of medicine and associate director of cancer prevention at the Meyer Cancer Center at Weill Cornell Medicine in New York City, told TODAY about a separate study.

In addition, sex hormones play a role in breast cancer risk and fiber can reduce the circulating levels of estrogen in the body, the authors wrote.

People who eat more fiber also tend to have a healthier diet overall and may consume more natural compounds that are protective against breast cancer.

The impact of diet on breast cancer risk is still a “very unresolved topic,” Brockton noted. It may have an effect through weight gain, and there is strong evidence foods that contain fiber reduce the risk of being overweight and obese, he added.

There’s also some evidence — but not strong enough to call for a recommendation — that eating more non-starchy vegetables and foods containing carotenoids may decrease breast cancer risk, Brockton said.

The findings of the new analysis support the American Cancer Society’s dietary guidelines to consume foods rich in total fiber, including fruits, vegetables and whole grains, the authors wrote.

The U.S. government recommends Americans consume 28 grams of dietary fiber a day, while the American Institute for Cancer Research advises even more — at least 30 grams. Most people only get about 20 grams a day.

The key is to eat a range of foods including whole grains, fruits, vegetables and beans, Brockton said. A banana contains about 3 grams of fiber, a slice of whole grain bread has about 2 grams, a medium sized baked potato with skin contains about 4 grams.

The Academy of Nutrition and Dietetics also listed these foods as naturally high in fiber:

  • 1 large pear with skin (7 grams)
  • 1 cup fresh raspberries (8 grams)
  • ½ medium avocado (5 grams)
  • 1 ounce almonds (3.5 grams)
  • ½ cup cooked black beans (7.5 grams)
  • 3 cups air-popped popcorn (3.6 grams)
  • 1 cup cooked pearled barley (6 grams)

To see if you are getting enough fiber and how the rest of your lifestyle stacks up, try the Cancer Health Check created by the American Institute for Cancer Research.