Dr. Denis Burkitt, an Irish surgeon, went to Africa in the 1950s on a missionary trip to improve the health of people there.
At least one aspect of their health didn’t need any improvement, he found: Africans had an astonishingly low rate of colorectal cancer.
Americans, by contrast, had a lot of colon problems, including high rates of cancer.
Burkitt thought that was because of the difference in diets. Africans ate large amounts of corn, beans, and other high-fiber foods. Most Americans, by contrast, had been eating processed foods.
Almost 70 years later, scientists are still mulling the issue. Some studies have found a link between a fiber-rich diet and lower cancer risks. Others haven't.
"There's a lot of conflicting data," says Mary Daly, MD, chair of the department of clinical genetics at Fox Chase Cancer Center in Philadelphia.
Eating lots of fiber mayhelp stave off certain types of cancer -- it just hasn't been proven yet.
Research is clear that eating a high-fiber diet can help you stay at a healthy weight, which in turn, lowers your risk for many kinds of cancer.
Colon Cancer
Most of the research on fiber and cancer prevention has focused on this type of cancer, because fiber does most of its work in the colon.
The nutrient passes quickly through the colon, perhaps flushing cancer-causing compounds out of the body. It may even change these compounds, making them less harmful, Daly says.
A number of studies indicate that the more fiber you eat, the less likely you are to get colon cancer.
But plenty of other studies don’t spot a connection.
A large one published in 2007 found that people who ate a high-fiber, low-fat diet had the same amount of colorectal adenomas, small tumors that can sometimes turn into cancer, as those who didn’t eat that way.
Yet when the researchers zeroed in on study participants who really stuck to the diet, as opposed to those who were less consistent, they did find a link between eating lots of fiber and fewer tumors, says Electra Paskett, PhD, director of cancer prevention and control at The Ohio State University Comprehensive Cancer Center.
"Fiber is critical for colonic health," says Brett E. Ruffo, MD, a clinical assistant professor of surgery at Stony Brook University Hospital.
Still, no one can say for sure whether fiber helps prevent colorectal cancer.
Breast Cancer
A similar story emerges when you look at the research on this disease.
Some researchers think fiber might reduce the risk of breast cancer, perhaps by influencing the body’s hormone production. There were some studies to back this up, including one that found eating a lot of high-fiber foods, especially vegetables, might lower the risk of breast cancer slightly.
But other researchers found no such link, and again, as the number of studies grew, the theory became weaker.
Prostate Cancer
One theory is that fiber might prevent this disease by affecting hormone levels, like with breast cancer.
Other experts think fiber's effect on something called "insulin sensitivity" might play a role. Insulin is a hormone your body makes after you eat. It lets blood sugar enter your cells and be used for energy. When blood sugar levels rise, insulin levels tend to spike. But fiber, especially the soluble kind like oatmeal, can soften this effect by slowing the absorption of sugar in the bloodstream.
The few studies on fiber and prostate cancer are inconclusive, though.
Take-Home Advice
There's just not enough strong evidence to say for sure that fiber protects (or doesn't protect) against breast, prostate, or colon cancer.
But there's plenty of research on how it helps you in other ways. It's best known for keeping your bowel movements regular, and it can also stave off more serious conditions. That includes type 2 diabetes and heart disease.
Fiber helps with losing weight, too -- and staying at a healthy weight is one of the best things you can do to lower your risk for lots of diseases, including cancer.