This article about bananas prevent cancer is a rewrite of one I published almost four years ago, and that was subsequently written into a new article. This is the 11th out of 13 most popular articles on Skeptical Raptor for the 2016 Year in Review.
I wrote an article about how to critically analyze pseudoscience and misinformation so that you might skeptically analyze evidence supporting a claim, even if it appeared to be accurate. On Facebook, Twitter and many internet sites (including Wikipedia), there is an amazing tendency of individuals to accept what is written as “the truth” without spending the effort to determine if what is written is based on accurate science.
Twitter, of course, limits itself to 140 characters, which means you either have to click on a link to get more information, or just accept that what is stated within the 140 characters is factual. And if you can make a complex scientific argument in 140 characters, that would be impressive.
Bananas prevent cancer memes
Facebook is filled with false memes on just about everything from vaccines to Monsanto. The anti-vaccination crowd fills Facebook with their amusing and highly inaccurate memes, which only leads to people thinking they’ve done their research, and not vaccinating their kids. For more than a year, there have been dozens of photos of bananas with a few words that some Japanese scientists claim that ripe bananas have high levels of “tumor necrosis factor“, so eat ripe bananas to boost your immune system, thereby preventing or curing cancer. Facebook is famous for these things, little pictures with a few words, no sources of the information, and broad conclusions.
Eat bananas. Cure cancer. And people share them with a click of the button and move on to the next cute cat picture. It’s really the lazy person’s way of learning. Although who doesn’t enjoy the cute cat pictures?
But what are the facts? What can bananas do nor not do? Let’s start at the beginning.
The mysterious Japanese Study
The actual study was published here (pdf) (or here, also pdf) in Food Science and Technology Research in 2009. I have several issues with the article with respect to the banana meme, especially in trying to make any conclusion that bananas have anything to do with “curing cancer” in humans:
- The authors do not make one single claim (as best as I can tell) that there is tumor necrosis factor (TNF) in a banana. No, this article does not say anything about bananas having TNF. Really, nothing else I write about this article really matters, because they just don’t make any claim whatsoever about TNF and bananas.
- The study is in a mouse model, and many animal model experiments just don’t transfer to human clinical research. The percentage of animal studies being applicable to humans is small, that’s why there’s the old joke that “we’ve cured cancer in mice thousands of times.” So, even if the authors claimed that bananas had TNF (and that would be Nobel Prize winning research), we have no clue as to whether it has any clinical impact. But let’s not forget that the authors make not a single claim that TNF is in bananas.
- The article is published in a low impact journal. This journal has an impact factor of less than 1.0, which indicates a very low quality journal and it’s not even indexed in PubMed, which further indicates its low value.
- The bananas are not fed to the mice, and without getting into details about the study, they try to stimulate production of TNF by essentially placing slices of banana in the mouse–of course, there will be an inflammatory response. Shocking. I am not sure why the authors did that, and I am unclear what it is supposed to prove. That it induces TNF activity might be expected since the body would react to any foreign substance (an apple, viruses, bacteria, anything) injected into the peritoneum, and production of TNF might be expected. But the TNF does not flow out of the banana, it is just the immune system’s reaction to a banana injected into the body.
- What is the clinical significance of what was induced? In other words, is there actually TNF isolated from the bananas in a form that actually can do something? Or is there some other effect just because there’s a blob of intraperitoneal banana extract which induces some other immune response.
- Bananas produce small amounts of serotonin (5-hydroxy tryptamine) and dopamine, depending on their stage of ripeness. These can have a stimulatory effect on neutrophils and macrophages in a living organism, and these can in turn produce the touted TNF-a, Interleukin-12 (IL-12) and other cytokines (signal molecules for part of the immune system reaction to antigens). In this role serotonin and dopamine are said to serve as a “biological response modifier” (BRM). In plain English, the stuff in ripe bananas can (but not necessarily will) stimulate a subset of white blood cells to produce chemical signals to deal with a variety of threats. However, this is a small effect, and it will not help you fight off cancer. But let’s clear about this. Bananas cannot stimulate or boost the immune system unless there’s some chronic malnutrition or disorder where you need to get more nutrition that bananas are adequate in providing. But, in healthy individuals, only vaccines can boost your immune system.
- No one has repeated this experiment, and certainly no one has shown this effect in humans. There is not a single double-blind clinical trial to show that bananas induce a TNF-alpha response in humans. And I can’t stress this point enough – bananas do not produce TNF. It is simply an evolutionary impossibility, unless through some amazing instance of evolutionary convergence, the banana plant evolved the ability to produce the TNF molecule for a completely different purpose for the banana plant. This would violate several principles of evolution, since there are reasons why the TNF molecule evolved in mammals and not in plants.
Just for review–I dug up the original “Japanese scientific paper”, and what I found was essentially simple–the authors did not claim bananas produce TNF, but that wasn’t even the point of the article. The article actually didn’t show much, but it did not provide any convincing evidence that in humans, bananas had any effect on the immune system. Again, unless that human is chronically malnourished.
Critical review of these banana claims
We need to examine this pseudoscience even more carefully:
- The “Japanese scientists” made no claim that there’s TNF in a banana, but the junk medicine pushers continue to make the claim, facts be damned. The problem is that anyone with a basic comprehension of biochemistry would understand that tumor necrosis factor (TNF) is a fairly complex protein, with a highly specific role in the human immune system (something notably lacking in a plant), so the chances that a banana would have some substance that exactly mimics or copies TNF is so tiny as to be close to impossible. A banana has no need for TNF, since it lacks an immune system of a vertebrate, so evolving a complex protein like TNF would be crazy; in fact, if it did, we’d have to rewrite our understanding of evolution. Let’s make this clear–we don’t have to rewrite evolution, because there is no TNF in bananas.
- Even if we could assume that a banana makes TNF, the digestive tract would break down the complex proteins and substances, such as TNF, into its constituent components, such as amino acids, simple sugars, and fats, before being absorbed into the bloodstream. The TNF probably wouldn’t survive intact within the digestive tract. The true scientific skeptic would, even if they thought that maybe a banana evolved a TNF molecule by some strange mechanism, know that it could not enter the body. It is just not physiologically plausible.
- But let’s pretend that TNF is produced in bananas (it doesn’t) and it can pass intact into the bloodstream from the digestive tract (it can’t), it would require eating hundreds of bananas to get a blood level of TNF that would actually have a biological effect. And even if TNF killed cancer cells (it doesn’t), how would your body “know” how to move the TNF to the cancer (it can’t). And which one of the 200 or so cancers would it effect? Moreover, the body would start down-regulating TNF in response to excess TNF in the blood, because TNF doesn’t do what you think it does.
- The name, tumor necrosis factor, seems to imply that it kills cancers (it really doesn’t). I would have to write a 20 page paper just to describe how TNF is up and down regulated within the immune system both locally and generally in response to a wide variety of immune challenges, including cancers. It is incredibly complex, and the name is simply one given without consideration to future alternative medicine pushers who jump on it as the the “cure” to all cancers. It isn’t.
- Of course, all of this leads us to what TNF actually does. It’s part of the inflammatory response, so TNF is associated with autoimmune disorders such as rheumatoid arthritis, ankylosing spondylitis, inflammatory bowel disease, psoriasis, hidradenitis suppurativa and refractory asthma.
- And this “boost the immune system” myth? The immune system is an incredibly complex system, with an almost infinite number of interactions between various proteins, compounds, organs, factors, immunoglobulins, receptors and cells. As long as you’re healthy, so is your immune system, there is nothing you can do to make it stronger, better, or “boostier.” As I’ve said, vaccinations actually do boost the immune system, and they rely upon an appropriate immune response.
So here we go again. Someone reads that some scientists discovered bananas cure cancer. They didn’t dig up the actual paper to find out that the scientists didn’t actually say that it did. They didn’t think through the problem that bananas wouldn’t actually contain tumor necrosis factor or that the digestive tract wouldn’t actually absorb it. Or that you couldn’t possibly eat enough bananas to get enough TNF to do any good. Or that if you could that it would have some very bad effects.
The TL;DR version
- The bananas prevent cancer meme relies upon a misinterpretation of an unimportant “Japanese scientist study.”
- The study was bad anyways.
- There is no TNF in bananas.
- Even if bananas had TNF, your digestive tract would destroy the TNF, breaking it into constituent molecules.
- But even if it could pass through the digestive tract and enter the bloodstream, you couldn’t consume enough bananas to raise your blood level of TNF to actually have a biological effect.
- And even you could raise your blood level of TNF, it’s name is a misnomer, since TNF doesn’t cause tumors to necrose. TNF, in fact, causes the inflammatory response, and numerous autoimmune disorders.
- And if you downed that much TNF, you’d die. So sure, the cancer is dead. But so are you.
- There are a few ways to actually prevent cancer, like not smoking. Consuming bananas is not one of those ways.
If you want to eat a banana, go ahead. It does have some benefits I’m sure. But rest assured that as much as you want to believe that bananas prevent cancer, the evidence does not support it. It’s not ever going to cure cancer. It’s not going to do make you live longer. It’s just going to provide nutrition.
Editor’s note: This article was originally published in June 2014. It has been completely revised and updated to include more comprehensive information, to improve readability and to add current research.
- Iwasawa, Haruyo, and Masatoshi Yamazaki. “Differences in biological response modifier-like activities according to the strain and maturity of bananas.” Food science and technology research 15.3 (2009): 275-282. doi:10.3136/fstr.15.275
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