When John Kane was approaching 50, he noticed his weight had crept up to 275 pounds. At 6-foot-2 and medically obese, he turned to the gym to slim down, doing an aerobic workout five days per week.

But the regular gym sessions became tiresome after a few years, and when Kane stopped exercising, he gained back all the weight. That’s when he decided to experiment with something else: fasting.

Kane’s fasts have evolved over the past year and a half, but for a while, he was eating only one meal a day six days per week, and abstaining from food altogether every Friday. A couple of weeks ago, he decided to try “alternate day fasting” — eating whatever he wants every other day, and nothing in between.

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Within seven months of his fasting experiment, the Madison, Wisconsin, bus driver lost 30 pounds, or more than 10 percent of his bodyweight, reaching his lowest weight in 15 years.

Fasting, he says, makes him feel not just lighter but also brighter, sharper, and happier. “It wakes you up and perks you up,” Kane told me. It’s also easier for him than going to the gym or counting calories: “The most appealing thing about it is the fact I don’t have to do any kind of meal planning. I just don’t eat.”

I recently got in touch with Kane, a reader who’d emailed me in the past, to hear about his experience with fasting because I’ve been noticing the buzz growing louder lately. Famous enthusiasts include reality TV star Kourtney Kardashian, musician Moby, and model Molly Sims. Actor Chris Pratt has been Instagramming about his Bible-based fast. In the media world, New York Times economics columnist Paul Krugman and MSNBC host Chris Hayes have mentioned their fasting routines. Over the past couple of decades, as dozens of diets and weight management schemes have come in and out of fashion, fasting has steadily gained popularity.

Meanwhile, there’s a growing scientific literature on the subject — exploring whether different types of caloric restriction can burn fat, stave off and reverse disease, and even help you live longer.

As it turns out, the science behind these diets is still pretty nascent and exploratory — more than the acolytes might have you believe. While there’s lots of animal research, human studies on fasting are only just beginning to ramp up. And while we have learned that fasting helps people lose weight, it’s only if you can stick with it. But that doesn’t make fasting any less fascinating. Here’s what we know and don’t know.

Not all fasting diets are equal

First, let’s be clear on what we’re talking about when we talk about fasting. There are a number of different types of fasts, many of which are nicely outlined in this review article and this one.

Generally, fasting involves eating no or very little food and caloric beverages for periods ranging from 12 hours to three weeks. These fasts can take on a number of forms. Here are a few types that are popular now:

Intermittent fasts involve eating no food or massively cutting back on calorie intake (e.g., 500 calories per day) only intermittently (like the very popular 5:2 diet). Time-restricted feeding involves consuming calories only for a four- to six-hour window each day (for instance, skipping breakfast and only eating lunch and an early supper). Periodic fasts, the most extreme, typically last several days or longer. These diets involve drinking only calorie-free fluids or very few calories for long stretches to get the body into full fasting mode (instead of switching back and forth between fasts and feeding). Fasting-mimicking diet, a plant-based diet that involves eating very few calories — through light foods like soups, energy bars, and energy drinks — for several days each month. Researchers who study fasting developed this diet because it gets people into a fasting state but doesn’t compromise their body’s access to nutrients.

Often, the major argument for periodic caloric restriction is that we did not evolve to eat three meals a day, every day. Some fasting proponents argue that our bodies were designed to be able to run on little or no food for as long as several weeks or even months. After all, we didn’t have access to a steady food supply until the advent of agriculture, and it wasn’t until the neolithic revolution that humans adopted a more regular meal pattern.

“Cycles of fasting can reset and rejuvenate the human body,” said Valter Longo, director of the Longevity Institute at the University of Southern California. Periods with no food, he said, help the body eliminate and replace damaged cells with new ones through a process called autophagy (the study of which recently garnered a Nobel Prize). (Longo is a leading fasting researcher but he’s pretty dug in — with a book about fasting, and a company that sells fasting products. He says he donates all the proceeds from the company and the book to health charities.)

Fasting proponents will also note that there’s a long tradition of religious fasting, though the focus there tends to be more spiritual than health-oriented. “Many religious groups incorporate periods of fasting into their rituals,” this article points out, “including Muslims who fast from dawn until dusk during the month of Ramadan, and Christians, Jews, Buddhists, and Hindus who traditionally fast on designated days of the week or calendar year.”

Much of the science on fasting focuses on disease prevention and longevity, not weight loss

But here’s something important to note about what we know from science about fasting: Though a lot of the popular interest is in weight loss, many of the key researchers who study fasting aren’t focusing on that at all. In fact, many of the studies on fasting come from institutes of aging, like this one, and the researchers behind the studies actually focus on longevity and disease prevention.

Researchers have known for decades that when you restrict the food intake of lab rats — and many other species including mice, hamsters, and even yeast — you can extend their life span. In humans, fasting seems to enhance the ability to counteract the disease process.

”A lot of organs start shrinking [during a periodic fast],” Longo explained. “A lot of cells start dying and we have evidence a lot of the cells killed by this process are the bad cells. Then the stem cells get turned on, and we see the body starts regenerating itself.” What’s more, he added, certain inflammatory markers and biomarkers for cancer seem to be lowered when animals and humans fast.

In 2017, Longo was a co-author on the first human trial of whether fasting might reduce the risk factors for diseases like diabetes, cancer, and cardiovascular disease. The researchers randomized 100 people into one of two groups for three months: The first group ate anything they wanted, and the second fasted for five consecutive days each month. (By “fasted” here, I mean, they followed the fasting-mimicking diet.) After three months, the first group was crossed over into the fasting group, so the researchers could gather even more data on fasting.

The findings were clear: Fasting just five days per month improved people’s health outcomes. The group that fasted lost weight (about 7 pounds on average), lost some body fat, lowered their blood pressure, and decreased their IGF-1, a genetic marker for diseases such as cancer. (Their total cholesterol, blood glucose, and triglycerides didn’t budge.)

Now, here’s an important catch: The research was still pretty short-term, only looking at biomarkers in people during a few months. So it’s not clear what effects on disease risks longer-term fasting will have, even though the changes to biomarkers look promising.

What’s more, 25 percent of people who were assigned to the fasting group in the first part of this study dropped out, compared to 10 percent of those in the regular diet group. That suggests that fasting is hard and maybe not for everyone — even when you’re part of a study. So someone like John Kane, who lost a lot of weight through fasting and loves it, may be in the minority.

Still, researchers are exploring whether fasting might help fight cancer, or help cancer patients tolerate chemotherapy. And they’re putting serious thought into whether fasting has a role in treating and preventing diabetes, cardiovascular disease, and multiple sclerosis. But while the animal literature is rich, the literature in humans is promising — though far from conclusive. As this 2017 review of the science found, the studies on fasting to control Type 2 diabetes come to contradictory results, and there’s “minimal data” comparing the effects of fasting to plain old calorie restriction in overweight or obese people with the disease. There were also no studies on fasting and human cancer rates.

Fasting may help you lose weight — if you can stick to it

As for weight loss, Longo said, “We see this as a bonus.” There are a couple of reasons some people seem to slim down from periodic fasts. The first one is obvious: Any type of fasting involves restricting your calorie intake for prolonged periods of time, which may lead to a lower overall energy intake and weight reduction.

The second one is more subtle: The body shifts into fat-burning mode when it doesn’t get food for an extended period of time. That’s because the body’s first source of fuel is glycogen, and it only turns to burning body fat once that quickly available energy source is depleted. So when you fast for long enough, you drive down stores of glycogen and start burning fat tissue. “In people, we see a change in fasting glucose — it’s lower — and abdominal fat is affected without much of an effect on the muscles,” Longo explained.

But people don’t always lose weight on fasting diets — and they don’t necessarily lose more than on plain, old calorie-restriction diets.

Some of the best evidence on the impact of fasting on bodyweight and fat comes from a December 2018 systematic review. The researchers looked at randomized controlled trials of intermittent fasting and found that the people who fasted lost about 4 to 8 percent of their original bodyweight, on average. So fasting worked, but, interestingly, it didn’t outperform regular, continuous calorie restriction (“eat less every day” dieting), and it didn’t lead to dramatic weight loss.

Intermittent fasting also improved people’s blood sugars, lipid values, and blood pressure — but again, about as much as the traditional daily (non-intermittent) energy restriction.

There are a couple of other caveats here. Fasting diets require working through hunger, saying no to the bagels and muffins put out in your morning meeting or the food at your business lunch. So it’s no surprise that many people can’t stick to fasting diets long enough to keep the weight off. In another 2018 review of the literature on fasting’s impact on weight, the researchers note, “Dropout rates have been as high as 40 percent. Thus, despite the statistical significance of weight loss results, the clinical significance and practicality of sustaining an [intermittent fasting] regimen are questionable.”

There are some people who should never fast

Debra Safer, a professor of psychiatry and behavioral sciences at Stanford University, told me in 2016 that people with troubled relationships with food should think twice before fasting to lose weight.

”The research evidence generally shows that patients with eating disorders do best when they eat regular meals and snacks,” Safer said. “Intermittent restriction of intake is often one of the behaviors that people with eating disorders engage in as part of their eating disorder — and it often sets them up to binge and/or purge.”

So in people with eating disorders, fasting is “potentially risky in that it disrupts attempts to build and maintain hard-won normalized eating patterns.”

Longo warned that any fasting diet should start with a visit to the doctor. Fasting studies have not been done in children, very elderly people, or people who are underweight — so it’s possible fasting could be harmful in these cases. “If you’re a diabetic and taking insulin or any other drugs, or if you have metabolic disorders, you should not fast,” he added, noting that fasts done poorly can also increase the risk of gallstones.

Researchers are still very unsure about ideal eating frequency and timing

Having talked to researchers about whether to eat breakfast and the health impact of eating late at night, one thing is clear: The scientific community is still very unsure about the best timing and frequency of eating for health.

In one study, researchers called for a clinical trial tracking people who follow multiple eating patterns — from a regular three-meal model to complete fasts — since that kind of comparative study has never been done.

For some, fasting periodically may be a helpful weight control strategy. A couple of the intermittent fasting adherents I spoke to, including John Kane, told me they find it simpler to avoid food from time to time instead of worrying about everything they put in their mouths.

Fasting may even help you stave off disease and live longer. Maybe. Just know that there are lots of question marks about the long-term health consequences of fasting. And, as Longo said, there’s a reason fasting has been around for so long but very few people do it regularly: “It’s too difficult.”