No evidence that bananas contain cancer-fighting protein

Do the brown patches of a banana contain “tumor necrosis factor”, a protein that can lower the risk of cancer? A meme shared on Facebook in South Africa claims so.

It reads: “Why should you eat ripe bananas? Fully ripe bananas with brown on their skin have a substance called Tumor Necrosis Factor (TNF) which can eliminate abnormal cells. The darker the patches, the higher the banana’s ability to boost your immunity and lower the risk of cancer.” 

We checked to see if this was true.

What is tumor necrosis factor?

According to the US National Cancer Institute, TNF  is “a protein that is made by white blood cells in response to an antigen (a substance that causes the immune system to make a specific immune response), or infection”.

TNF “may boost a person’s immune response, and also may cause necrosis (cell death) of some types of tumor cells”.  

Bananas do not contain the tumor necrosis factor

The NCI told Africa Check that if you look at the nutritional and elemental breakdown of a banana it is clear that “bananas do not contain the tumor necrosis factor”.

“With few exceptions, studies of people have not yet shown definitively that any dietary component causes or protects against cancer”, the NCI added.

Claim possibly originated with Japanese study on mice

Researching the claim in the meme leads to a Japanese study, published in 2009 by the journal Food Science and Technology Research. 

This study, which was done on mice – not humans – studied the differences in biological response modifier (BRM) activities according to the strain and maturity of bananas.

BRMs are substances that stimulate the body's response to infection and disease, and either enhance or suppress an immune response.

Other academics have referred to this study when making the claim that the brown patches on a banana are helpful in fighting cancer. But this is misleading.

Study limited

The mice in the study did not eat bananas, but were given pulverized slices of bananas in water through an injection.

The immune systems of the mice produced tumour necrosis factor as part of the normal immune response to dealing with something that was forcefully injected, rather than consumed.

And the Japanese researchers did not claim that the factor was produced in bananas, or that they'd found a cancer cure through this study. 

False claims harmful

Medical experts agree that false rumours about cancer cures are potentially harmful. 

Tanya Ha, a member of the executive committee and board of Science and Technology Australia, has explained that spreading claims like these “adds to the noise that vulnerable people with serious illnesses are dealing with”.

Dr Skyler Johnson, former chief resident of therapeutic radiology at Yale School of Medicine in the US, says that “every day patients are bombarded with false and exaggerated information via the internet or from well-meaning friends and family, promoting unproven treatments”.

Because of this, many people who are overwhelmed by a cancer diagnosis do not recognise good sources of cancer research. This can be dangerous. 

There is no evidence that the brown spots on bananas contain anything cancer-fighting.