Medical Examiner

Is Fasting Good for Your Health?

Every year during Ramadan, I wonder how my fasting affects my body. This year, I dug into the research.

Rows of empty plates.
Photo illustration by Slate. Photo by Thinkstock.

Thursday night, at 8:28 p.m. on the dot, I took my first sip of water of the day. It tasted sweet. The hunger, thirst, and discomfort immediately disappeared, replaced by the quiet excitement of knowing it was time. This is a joy I have had every night for a month now—a month of hunger and thirst punctuated with late, festive nights of food and community. But Thursday night was extra special because it was the last one. Friday I woke up and experienced the euphoria of eating breakfast at breakfast time. Ramadan is over; Eid has begun.

For the quarter of the globe that is Muslim, fasting during the month of Ramadan is an obligation. Some Muslims love Ramadan; others struggle through. But for any believing Muslim, there is at least one big and unavoidable incentive to fast: to tip the scales favorably on the day of judgment. For the rest of the world, the motivation to fast might be aimed at tipping a different type of scale—the one on the bathroom floor. But even though health gurus of different sorts offer up fasting as a way to lose weight, evidence suggests that the main benefit is more long term. According to the robust body of scientific research conducted in animals, fasting may increase life span in a large number of species. Researchers are cautiously optimistic that the same finding could apply to us, too.

The counterintuitive observation that underfeeding animals can lengthen their lives is an old one. In 1935, scientist Clive McCay conducted a simple experiment: One group of lab rats was allowed to eat as much as they desired while the other was given a calorie-restricted diet. The result was dramatic and undeniable. Rats from the second group lived significantly longer than rats from the first group.

Since this pioneering work, the basic thrust of McCay’s finding has been repeated again and again all over the tree of life. In yeast, flies, worms, fish, mice, rats and monkeys, hunger appears to be the fountain of youth. The findings have been replicated with fasting diets as well. Ever since researchers in the 1990s found that mice fed every other day outlived mice that fed every day, evidence has poured in that fasting can increase the life spans of animals. The message seemed clear: Give one group of creatures more, give another group less, and the latter outlives the former.

Less is more, it seems—up until it isn’t. “Of course, it has to be optimized,” says Rozalyn Anderson, a professor at the University of Wisconsin. “The less you eat the longer you live, up until the point when you start doing damage. If you continue to eat less, well then you are going to die.”

As a recent fasting study in flies illustrates, striking this optimal balance is a difficult task. James Catterson of the Institute of Healthy Ageing at University College London walked me through the process by which he and his colleagues zeroed in on a successful fasting regimen. First, he tried depriving the flies of food for three to six hours a day and found no measurable effect. Next, he tried fasting for two consecutive days each week. Nothing. Three days? Nothing. Four days? Nothing. Upping the ante to five days seemed ridiculous, but Catterson tried it anyway. Finally, there was an effect, but it was a frustrating one: The fasting flies had shorter life spans than the controls. At this point, Catterson says he was ready to quit. In a last-ditch effort, he tried a slight tweak: a five-days-a-week fast during the first 30 days, followed by constant food for the remainder of their lives. This seemed to be the secret sauce—at last, the fasting flies lived significantly longer.

It took Catterson several years and many generations of fruit flies to find a successful fasting regimen. This “guess and check” strategy isn’t a viable approach for research on humans, though, for a plethora of practical and ethical reasons. A fruit fly can now thank the countless martyrs of Catterson’s research to live a longer, healthier life through fasting. But what about us? “We definitely wouldn’t recommend a five-day water-only fast in humans!” says Catterson, stating the obvious. “One of the main things I would stress is that due to the infancy of research into intermittent fasting … there is no method that anyone should necessarily be recommending, for the moment.” When I asked Anderson whether she practiced calorie restriction or would feel comfortable recommending it to someone else, her response was unequivocal: “No and no.”

Not all researchers are as cautious. Mark Mattson, chief of the laboratory of neurosciences at the National Institute on Aging, says that he practices a fasting regimen known as time-restricted feeding. “I don’t eat breakfast, work out midday, and then I’ll eat over a period of 6 hours. … That’s been my normal routine for quite a while.” At first glance, this routine resembles what Muslims do during the month of Ramadan: eat during a narrow window of the day, fast for the remainder. However, Mattson’s feelings about Ramadan fasting were mixed—refraining from water during the day concerned him. He also noted that many Muslims actually gain weight during Ramadan due to overeating at night. “That’s not good!” he exclaims. “During the month of Ramadan, one would almost surely accrue health benefits if one did not overeat.”

When I mention that the Prophet Mohammad was believed to have fasted two days a week, Mattson jokes: “Clearly, Mohammad was ahead of his time!” Mattson and his colleagues showed in 2011 that two days of fasting per week resulted in weight loss, decreased cholesterol, lower blood pressure, and other health benefits in young, overweight women. This original “5:2” regimen remains a widely popular diet today, followed by the likes of Benedict Cumberbatch, Jennifer Metcalfe, and Jimmy Kimmel.

Mattson seems optimistic that fasting will one day be generally embraced. “I think it’s going to take quite a while to get actual clinical practice and recommendations caught up with the times, but I think it will.” There has always been a heavy burden of proof on dietary research. As a point of comparison, the  “Mediterranean diet”  has long been recommended by physicians (and yes, there was a recent controversy around a retraction on this diet, but its generally efficacy is still widely agreed-upon). The PREDIMED trial, the seminal study supporting the Mediterranean diet, followed some 7,500 patients over the course of nearly five years. Over 250 studies citing the PREDIMED trial have found the Mediterranean diet to effectively curb obesity, diabetes, cardiac disease, and more. By contrast, the cleverly acronymed CALERIE clinical trial, the largest calorie-restriction study in humans to date, followed just 200 patients over the course of two years. Similarly, the largest fasting study followed 100 women for six months. The vast majority of calorie restriction and fasting studies only involve a dozen or so individuals. While many of these studies suggest that calorie restriction and fasting promote general health and curb most age-related diseases in humans, the sample sizes are simply too small to garner widespread support from physicians.

But to me, the study that matters the most is much smaller than any of these: my own experiences (n = 1). I’ll never get to know whether my own life was lengthened or shortened by virtue of fasting. Although I’ve been fortunate not to have any serious health problems, I’ll never be able to pin that precisely on Ramadan: maybe diet, exercise, or plain luck deserve the credit. I’ll never get to probe that counterfactual world of “What if?” What I can say is that I am different during the month of Ramadan. In this month, I feel my mind and heart at peace. Nearly every night I enjoy the blessing of eating dinner with other people. As a result, my friends and my family feel close during these 30 days. Every night when I break fast, I am reminded of my good fortune. Aware of the luxury to choose when and when not to eat, my gratitude grows. And yes, for those who care, I dependably lose 8 pounds.