What no one talks about when they extol the benefits of fasting

Welcome to Dear Julia, a column where readers submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? Julia Belluz sifts through the research and consults with experts in the field to figure out how science can help us live happier and healthier lives.

Dear Julia: Do fasting diets really work for weight loss?

Over the past couple of decades, many diets have come in and out of fashion, from eating very little fat to eating more protein and fewer carbs to avoiding meat all together.

Now some dieters are betting on avoiding food all together — well, at least for short periods of time. Fasting for health is, of course, an ancient tradition, practiced by Plato and Socrates, among others. But now, it seems, fasting is being embraced as yet another approach to the conundrum of how to lose weight. The New York Times noted recently that "fasting diets are gaining acceptance." The latest proponents claim that brief periods of near starvation will not only burn unwanted fat but also stave off disease and even make you live longer.

As it turns out, the science behind these diets is much, much less certain than fasting followers would have you believe. Yes, these diets will probably help you lose weight — but that's only if you can stick with them. The long-term health consequences of fasting are unknown, and there are many people for whom a fasting diet may be dangerous.

Not all fasting diets are equal

First things 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 consuming very little or no food or caloric beverages for periods ranging from 12 hours to three weeks. These fasts can take on a number of forms. Here are three types that are popular now:

1. Intermittent fasts involve eating no food or massively cutting back on calorie intake (e.g., 500 calories per day) only intermittently (such as two days per week).
2. Time-restricted feeding involves consuming calories only for a four- to six-hour window each day (e.g., skipping breakfast and eating lunch and an early supper).
3. Periodic fasts, the most extreme, typically last several days or longer. These diets involve drinking only calorie-free or very low-calorie fluids for long stretches to get the body into full fasting mode (instead of switching back and forth between fasts and feeding).

The major argument behind forgoing food from time to time comes from evolution. 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. 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.

Fasting acolytes will also note that there's a long tradition of religious fasting, though the focus there is spiritual benefits, not health. "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 longevity, not weight loss, and long-term health effects are unknown

But here's something important to note about what we know from science about fasting: Though a lot of the public interest in fasting centers on weight loss, many of the key researchers who study fasting aren't actually interested in weight loss. 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, such as Valter Longo, director of the Longevity Institute at the University of Southern California.

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 (like IGF-1) seem to be lowered when people fast.

But here's the thing: This research has been short term, only looking at health measures in people during a study period of a few weeks or months. "As far as I know no one has ever done a fasting and life span study in humans," Longo said. So while the short-term results of fasting have so far been promising, what they mean for a life is unknown.

Fasting may help you lose weight — but may not help you keep it off

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. (That said, people don't always lose weight on fasting diets.)

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 off 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.

He also noted, however, that you'd need to fast for more than 24 hours to reach this state, and that many of the intermittent fasts that promise this benefit are overselling.

But there are a couple of other caveats here too. Fasting diets require quite a bit of discipline, like working through hunger and saying no to the bagels and muffins put out in your morning meeting. So many people can't stick to fasting diets long enough to keep the weight off.

"Of the human [studies], they seem to be — perhaps not so surprisingly, given what’s being asked of the subjects — plagued with higher than normal attrition rates as compared with other diet studies," said Yoni Freedhoff, an obesity doctor and author of the book The Diet Fix. "So sustainability is definitely my biggest concern."

When asked about whether he's had any patients succeed on intermittent fasting diets, he said the majority have been "young male bodybuilder types" and noted that fasting regimens pose special challenges to people with families, physically demanding jobs, or work-related meals (sales, business meetings, etc.).

There are some people who should never fast

Debra Safer, a professor of psychiatry and behavioral sciences at Stanford, also warned 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, old 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 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 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."