For centuries, voluntary fasting has been practiced by the devout from many religions to get closer to enlightenment. Now, it’s a dieting trend.

Some studies have linked intermittent fasting to weight loss​, while another links it to living longer​.

Fasting can indeed be beneficial, in some cases decreasing blood pressure and insulin resistance​ and even boosting heart and brain health.

However, the effects of fasting aren’t always positive. Two studies suggest that fasting does have downsides. While the studies are not on humans and more research needs to be done, they nevertheless suggest that for fasting, not everything is clear cut. 

Increase in cancer risk?​

A recent study on mice shows that during the post-fast feast, rather than the fast itself, stem cells would surge to repair the intestines of the participating mice.

However, this came with a catch. The surge in stem cells meant that the mice were more likely to develop precancerous polyps (abnormal growth that forms on the inner lining of an organ) on their intestines if a cancer-causing gene was activated than if they’d abstained from fasting. In the study, intestinal stem cells multiplied the fastest in mice given food after a fast.

During fasting, stem cells start burning fat instead of carbohydrates. This leads to their heightened ability to perform tasks like repairing mouse intestinal damage. But because intestinal stem cells divide constantly, they can also be a source of precancerous cells.

According to biogerontologist Valter Longo, increased cancer risk in mice with altered genes does not extend to other animals. Does this also mean it won’t hold true for humans? The researchers plan to find out.