Not too long ago, I had a post (link here) about the fact that much of what is published in the medical literature are very poor quality studies, and the symbiotic relationship between academia and the media that hypes these studies up, thus sewing fear and confusion into the average person.
I really can’t stress this point enough. It’s not that most medical studies are good, with a few poor ones thrown in. It’s that a large percentage of what is published and reported on in the news sheds absolutely no light at all on how you can live a healthier life, and is often frankly misleading. So it’s a point worth revisiting over and over.
Of course, if you read the actual study and think it over for a few minutes, it becomes clear that it does not in any way prove that intermittent fasting causes cancer in humans.
In the study, they had mice fast for 24 hours. Then they fed them a large meal. When compared to a control group of mice who didn’t fast and were just allowed to eat normally, they found that the fasting mice developed pre-cancerous polyps. Which might sound frightening until you consider the following:
Humans are not rodents. So we have no idea if this same finding occurs in humans or not.
Mice have a shorter life span than humans. Much shorter. A 24 hour fast to a mouse is like a several week fast to a human. Virtually no humans who practice intermittent fasting fast this way.
The mice in the study are genetically bred to be excessively prone to colon cancer. So even for an average mouse, this study doesn’t tell us much.
One might wonder if what a mouse eats, or how much it eats, after breaking a fast plays a role here. Possibly the specific meal the mice were given wasn’t good for them, but another meal would be. But we can’t know that from the study. However, that didn’t stop the newspapers from commenting that it’s bad for people to eat “charred meats such as steak” after a fast, even though the paper in question didn’t look into this variable at all.
No long-term data on the mice was obtained. Colon polyps can go away on their own over time. Possibly – and I do stress that this is only a possibility – these polyps would have done so, and not gone on to cause cancer in these mice. Or maybe they would have caused cancer. We’ll never know, because the study didn’t assess for this.
Most of the time when humans fast, they do it as part of a strategy to lose or maintain weight, and we know that being normal weight is in and of itself protective against colon cancer (or to put it another way, obese people are at higher risk for developing colon cancer than lean people). Even if fasting did lead to the development of some polyps, we would need to consider the net impact of fasting on a person, because hypothetically if you lost 10 pounds via fasting, that would serve to decrease your risk of cancer. Would the results of this be a net benefit, or net risk, or a wash? This study didn’t account for this at all. In fact, they point out in the study that the mice did NOT lose weight on this eating plan, because they ate so much after their fasts that they quickly regained all of their weight back. So again, we see that the mice are following fasting protocols that are different to what humans would typically follow.
Many other studies have shown reduced risks of various cancers in people (and rodents) who fast, so this study might just be an outlier.
Bottom line: this study really tells us nothing about how fasting impacts colon cancer risk in humans. And I mean nothing. Is it interesting research that may someday lead to the discovery of something useful? Sure. But for now, it would be foolish to alter your own behavior based on these results.
In my own practice, I’ve had many patients do well with intermittent fasting. For others, it’s either too hard to stick to, or not that effective. Therefore, I don’t believe that fasting is a magic bullet, but I do believe it can be a very useful tool for some people. If you practice intermittent fasting (or are considering giving it a go) and it is otherwise working well for you, I would not advise you to change anything based on this paper.
