Experts Discuss How a Banana a Day Can Keep Cancer Away

Bananas may offer more than just a good source of fiber, Vitamin C, potassium and other vitamins and minerals.

A new study by researchers at the University of Leeds and Newcastle University in the U.K., published on July 26 in the peer-reviewed journal Cancer Prevention Research, suggests that a starch found in green bananas (as opposed to ripe ones) and some other foods may potentially help reduce the risk of some cancers.

A 20-year trial conducted among 900 patients with Lynch Syndrome (LS), the most common cause of hereditary colorectal (colon) cancer, found that a daily dose of 30 grams of resistant starch for up to four years produced a "protective effect against non–colorectal cancer LS cancers."

However, more research is needed to know whether resistant starch can offer the same benefits among the general population, experts told Newsweek.

Patients with LS are more likely to get colon cancer and other cancers, and at a younger age (before 50).

A bunch of unripened, green bananas.
A bunch of unripened, green bananas. iStock / Getty Images Plus

The latest study stated: "Our findings of no effect of RS on colorectal cancer risk in this long-term follow-up study," however, "those randomized to RS had significantly lower risk of diagnosis with non-colorectal LS cancers.

Professor John Mathers from Newcastle University, one of the co-authors of the study, said: "The dose used in the trial is equivalent to eating a daily banana: before they become too ripe and soft, the starch in bananas resists breakdown and reaches the bowel where it can change the type of bacteria that live there."

The protective effect of the resistant starch was "particularly evident" for upper gastrointestinal (GI) cancers, including the following:

Mathers stated: "This is important as cancers of the upper GI tract are difficult to diagnose and often are not caught early on.

"We think that resistant starch may reduce cancer development by changing the bacterial metabolism of bile acids and to reduce those types of bile acids that can damage our DNA and eventually cause cancer. However, this needs further research," he added.

Speaking to Newsweek, Corinne Joshu, a cancer researcher at the Johns Hopkins Kimmel Cancer Center and an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, explained that Lynch Syndrome patients have an increased risk of colorectal and several other cancers "due to germline pathogenic variants in a DNA mismatch repair gene."

The finding in the latest study that "resistant starch supplementation was inversely associated with non-colorectal LS cancers, in particular, upper GI cancers is interesting," she added.

What might be the minimum amount of time required for the protective effect of the resistant starch to develop in LS patients? Josh said: "This is difficult to answer" because "some cancers develop over many years and protective benefits may not be immediately apparent."

However, "it is a strength of this trial design that investigators were able to examine the impact of aspirin and resistant starch supplementation at several time points," she said.

Bunches of bananas seen on a tree.
Bunches of bananas seen on a tree at a farm. iStock/Getty Images Plus

Could Resistant Starch Help Reduce the Risk of Cancer in Other People?

Joshu from Johns Hopkins said to know whether there may be similar protective benefits for these cancers in the general population, resistant starch would need to be studied in those without LS.

"The biologic mechanisms underlying these specific cancers can be heterogeneous and therefore the impact of resistant starch supplements may or may not be the same in those without LS. This could be done in trial testing resistant starch supplements in people without LS," the researched explained.

She said it is "probably" best to be examined first as a dietary component, "where diets rich in fiber are assessed in relation to upper GI cancers in population-based studies of people without LS."

Speaking to Newsweek, Cody Watling, a researcher at the Cancer Epidemiology Unit at the University of Oxford in the U.K. who studies diet and the risk of common cancers (breast, prostate, and colorectal), said: "Although this is a randomized controlled trial, this study is limited to those with Lynch Syndrome, a genetic condition the predispositions someone to a higher risk of colorectal cancer and other cancers.

"As such, the findings may not be generalizable to the greater population. More importantly, this study is quite small with only 918 individuals analysed in this trial and only 75 non-colorectal Lynch Syndrome associated cancer cases observed, which can subject the findings to the play of chance," he said.

The participants in the latest study were also only "randomized to the intervention" for around 25 months before being followed for around 19 years.

"So whether the influence of resistant starch is apparent 10+ years after consuming resistant starch and what the mechanism would be remains unknown," Watling noted.

"Further research is needed assessing intake of resistant starch in large randomized controlled trials to see if an observed effect is present before conclusions on this link can be made," he added.

How Does Resistant Starch Work Exactly and What Are Its Benefits?

Resistant starch is a carbohydrate that "resists digestion in the small intestine and ferments in the large intestine," explains The Johns Hopkins Patient Guide to Diabetes.

Resistant starch "behaves physiologically like fiber," reducing glycemia (blood sugar levels that are too low or too high) and helping to prevent or treat type 2 diabetes as well as decreasing the risk of developing chronic diseases, noted a June 2019 study in the peer-reviewed journal Nutrients.

Starches are typically broken down into glucose (blood sugar) when they're digested. But since resistant starch is not digested, it does not raise your blood sugar levels.

"Gut health is improved as fermentation in the large intestine makes more good bacteria and less bad bacteria in the gut. Healthy gut bacteria can improve glycemic control," explains the Johns Hopkins website.

Other benefits of resistant starch include the following, according to Johns Hopkins:

  • An increased feeling of fullness
  • The treatment and prevention of constipation
  • A decrease in cholesterol
  • A lower risk of colon cancer
  • Causing less gas than other fibers, because resistant starch is fermented slowly
A selection of different whole grains.
A selection of different whole grains in white bowls. iStock/Getty Images Plus

What Foods Contain Resistant Starch?

Johsu advised: "To reduce cancer risk, it is recommended that people eat foods rich in dietary fiber and wholegrains (like whole wheat bread, quinoa, barley)." This also includes fruits, non-starchy vegetables (such as carrots, broccoli, spinach) and beans.

Limiting the consumption of red and processed meats (such as deli meats, bacon and hot dogs) as well as alcohol is also beneficial as these dietary factors increase the risk of certain cancers.

"Adhering to these dietary guidelines can reduce cancer risk and improve overall health," she said.

Resistant starch is found in several everyday foods, including the following, as outlined by the Johns Hopkins website.

  • Green bananas
  • Plantains
  • Rice
  • Peas
  • Beans and lentils (white beans and lentils have the highest amount of resistant starch)

When cooked, oats, green bananas and plantains lose some of their resistant starch because "the amount of resistant starch changes with heat," explains the Johns Hopkins website.

"Another type of resistant starch is made in the cooking and cooling process. Cooked rice that has been cooled is higher in resistant starch than rice that was cooked and not cooled," the university says.

Spoonfuls of different beans.
A close-up view of spoonfuls of different beans. iStock/Getty Images Plus