Intermittent fasting may increase the risk of cancer, according to a new study. Researchers from the Massachusetts Institute of Technology (MIT) published their findings on August 21, 2024 in Nature. While previous studies have shown that fasting or fasting-mimicking diets can reduce cancer risk, these studies primarily focused on the fasting state, neglecting the effects of refeeding after fasting. This new study examined both the fasting and refeeding phases to provide a more comprehensive understanding of how intermittent fasting impacts cancer risk.
Using mice, the researchers found that fasting can be a double-edged sword. On one hand, fasting helps the gut recover from damage or inflammation by boosting the regenerative capacity of intestinal stem cells. However, this same regenerative process, if disrupted by mutations, may also elevate the risk of cancer.
The researchers divided the mice into three groups: one group was allowed to eat ad libitum for 72 hours after a 24-hour fast, another group ate ad libitum for 24 hours after a 24-hour fast, and the control group, by contrast, ate freely from the beginning without fasting.
Their comparison showed that while stem cell regeneration was suppressed during fasting, it surged dramatically once the mice began eating again. Stem cell proliferation peaked 24 hours after refeeding, and these cells regenerated more effectively than those of non-fasting mice. However, this enhanced regeneration carries a downside: it increases the risk of early-stage tumor development if mutations occur in the stem cells during their heightened activity.
This study does not negate the health benefits of fasting. The same researchers have found in previous studies that fasting has beneficial effects on gut health. The process of using fat as an energy source during a 24-hour fast improves the regeneration of stem cells. The researchers also cautioned against resuming eating after a fast. Your first meal after a fast doesn't have to be a charred steak.
The results of this study were conducted on animals, so they cannot be directly applied to humans. Rats have a higher metabolic rate than humans and need to eat more frequently. Therefore, a 24-hour fast in rats is unlikely to have the same effect in humans. For example, a March 2024 British study found that people need to fast for three days to achieve benefits beyond weight loss. However, you don't have to fast for that long. Another study, presented at the European Nutrition Conference in November 2023, found that fasting for 14 hours a day and eating within a 10-hour window was sufficient to boost energy, improve mood, and manage hunger.
Not everything in the world is universally beneficial. While longer fasting periods may offer greater benefits, they also come with increased risks. It's important not to overdo it, especially with refeeding after a fast.
Jeong Jae-hoon is a food writer and pharmacist. He covers a variety of subjects, including trends in food, wellness and medications. This column was originally published in Korean in Joongang Ilbo on Aug. 29, 2024. – Ed.