Is Intermittent Fasting Good or Bad for You?

Karen Ramey-Torres, a retiree from Colorado, was overweight and concerned about her health after setbacks including a small stroke. Then she heard about a type of intermittent fasting called time-restricted eating, where you eat in a shortened time window each day, without necessarily trying to cut calories.

The time limit was more appealing to her than diets requiring constant calorie counting. “It’s just one simple rule to remember,” Ramey-Torres says.

Though it can be simple in practice, the concept of intermittent fasting has led to confusion and exaggerated claims of extended lifespan. Research shows that intermittent fasting may help with weight loss and lower blood sugar, but it depends on the person and how it’s done. 

Experts agree that intermittent fasting can refer to a variety of approaches. They include fasting every other day for 24 hours; modified fasting every other day, where you’re allowed some calories; and time-restricted eating—Ramey-Torres’s strategy—where you don’t eat for at least 14 hours daily, usually from the early evening until early the next morning.

After 18 months of time-restricted eating, Ramey-Torres says her body fat percentage, blood pressure, and energy levels have improved. But what does the science say? Below, researchers weigh in on the benefits and best ways to do it.

Weight loss and blood sugar

Many of us eat almost nonstop in our waking hours, from first-thing granola bars to bedtime refrigerator raids. This behavior would’ve baffled our ancestors. Without refrigeration or artificial light, meals after sundown were rare. The next day, finding breakfast in the wild wasn’t nearly as simple as opening the pantry. Over millennia, the human body probably evolved to benefit from taking breaks from food.

This theory is supported by studies on intermittent fasting. One benefit, confirmed by research, is that people often end up consuming fewer calories just because they have fewer hours to eat.

“A surplus of calories has led to the obesity epidemic,” says Krista Varady, a professor of nutrition at the University of Illinois, Chicago. “Eating less is important.”

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Time-restricted eating may be the most practical form of intermittent fasting. Ramey-Torres eats happily within an eight-hour window. “It’s the most popular type,” compared to less frequent but longer fasts that may involve more hunger, says Courtney Peterson, a researcher of intermittent fasting at the University of Alabama at Birmingham. Some studies find no weight loss with time-restricted eating, but recent research reviews concluded that it helps with shedding pounds. 

Another benefit is lower blood sugar. Last year, Varady published a large study on people with Type 2 diabetes eating from noon to 8 p.m, without being told to cut calories or modify their diet in any other way. Their blood sugar dropped along with their weight. 

These blood sugar benefits were likely driven by weight loss, Varady says. But Peterson thinks people of all body sizes may improve their blood sugar with time-restricted eating, even without losing weight. 

Heart health

Dropping your blood sugar may yield other benefits. If we go long enough without food, the body runs out of glucose to burn for energy and switches to burning fat. The transition releases molecules called ketones that offer several benefits, including fewer harmful waste products and, perhaps, healthier arteries, Peterson has found. People vary in the length of fasting needed to generate ketones.

Some research points to other cardiovascular benefits such as lower cholesterol and blood pressure, as in Ramey-Torres’s case. 

Confusion was created this year by a finding, presented at an American Heart Association conference, linking time-restricted eating to a much higher risk of heart attacks than people who ate throughout the day. The underlying research was unpublished and flawed in several ways, according to a public letter signed by 34 scientists. For example, many of the research participants were sick to begin with, which probably skewed the results. (In an email to TIME, the lead author of the study declined to comment.) 

Lighting the way to healthy fasting 

Time-restricted eating has been researched mostly from about noon to 8 p.m. However, another strategy is early time-restricted eating, from about 8 a.m. to 4 p.m. This schedule could be even more beneficial because it aligns closely to our biological cycles, or circadian rhythms.

Daylight cues the body’s responses to food. Natural light in the morning triggers the brain to adjust hormones in ways that wake up the pancreas. In full swing by 10 a.m., it can respond to food by making lots of insulin, the hormone that controls blood sugar, says Satchin Panda, a biology professor at the Salk Institute and author of The Circadian Code: Lose Weight, Supercharge Your Energy, and Transform Your Health from Morning to Midnight.

The reverse process happens around sunset, as the brain notices the absence of natural light. “In a simplistic way, this helps to put the brain to sleep, and it also puts our pancreas to sleep,” Panda says. But if we eat late at night, the pancreas sleepwalks through its response to food, allowing blood sugar to spike.

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In other words, the body is much better at digesting food in the daytime. If you eat primarily when the sun is bright, “it may have greater health benefits because your body is metabolically primed to take in calories,” says Dr. Victoria Catenacci, an endocrinologist at the University of Colorado Anschutz Medical Campus, who researches intermittent fasting and helps patients with obesity.

More research on early time-restricted eating is needed. Some studies show it’s more beneficial than later, longer eating windows, while others find no difference from later windows. Ramey-Torres eats from 8 a.m. to 4 p.m., which could be ideal. But Panda thinks people benefit as long as they start fasting at least three hours before bedtime. “Nearly all studies reporting benefits independent of weight loss have stopped eating by 6 or 7 p.m.,” Peterson adds.

How to do it 

An outstanding question is how long to fast. Most research that finds benefits involves a 16-hour fasting window, Peterson says. 

14-hour fasts are helpful, Panda says, though the data are more mixed. In a 2024 study, Nisa Maruthur, an associate professor of medicine at Johns Hopkins, found no advantage to 14-hour fasts compared to longer eating windows with the same calorie intake. If 14-hour fasts provide any benefits, it’s probably just because “when people restrict the window, they typically eat less food,” Maruthur says. “The effects are very individualized.”

Fewer studies have examined longer fasts longer than 18 hours. The hunger is a deal-breaker for many. Varady prefers 16-hour fasts. She’s tried longer ones, “but I didn’t get the same benefits for mood or energy levels,” she says.

Aim for consistency with your fasting duration and when you start and stop. “Consistency is important for aligning your circadian rhythm,” Panda says.

Perseverance is also key; hunger may be challenging at first. Panda urged his mother to try early time-restricted eating after she developed prediabetes. After some arguments and resistance, she got used to it while boosting her health and energy levels.

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“Once people adjust, they’re not really hungry at night anymore,” Varady says. This adjustment took Ramey-Torres about two weeks.

Intermittent fasting is not a get-out-of-jail-free card to splurge on goodies during the eating window. To benefit, healthy nutrition must be prioritized. 

This could be especially true when breaking the fast. In a recent study, mice became vulnerable to cancerous mutations during the “refeeding phase” following long fasts. Co-author Omer Yilmaz, a professor of biology and pathologist at MIT and Harvard Medical School, thinks the finding may also apply to humans (though more study is needed). If you regularly break your fast with charred red meat or ultra-processed foods, “you might be at a slightly increased risk,” he says. Instead, start with veggies and slowly increase protein intake as the day goes on.

Another strategy is to drink black coffee or green tea. It lacks the calories to break your fast and may promote “a false sense of fullness” to prolong fasting before breakfast, Varady says. Ramey-Torres doesn’t drink coffee but enjoys warm water during her fasts. Later on, a filling, high-protein dinner may stave off hunger overnight. 

Longevity—maybe

Animal research shows that fasting leads to longer lifespans, but only when cutting 25-30% of calories. The effects of significant calorie-cutting on human aging are inconclusive, and it’s a deal-breaker for most people anyway. Intermittent fasting may help prevent disease, but forms like time-restricted eating probably don’t reduce calories enough to extend lifespan—despite the promises of wellness influencers. 

That said, more research is needed to know how time-restricted eating affects longevity. Yilmaz suspects it triggers autophagy, where the body goes into a restorative mode that repairs cellular damage. The effect would be less dramatic than caloric restriction in animals, but these “smaller benefits could be cumulative” over a human lifetime, Yilmaz says.

Intermittent fasting may promote longevity especially when it’s synced to circadian rhythms. In an influential study, calorically restricted mice, fasting in alignment with their day-night cycles, lived 25% longer than mice eating the same restricted calories, but spread over 24 hours. “Time of eating makes a huge difference in how long lifespan can be extended by caloric restriction,” says Joseph Takahashi, a geneticist at UT Southwestern Medical Center, who co-authored the study.

This much is certain: the benefits of intermittent fasting—including ketones and lower weight and blood glucose—are much greater in mice than humans. Back in 2012, Panda’s team found that mice feeding in an eight-hour window weighed 20% less than mice consuming the same food throughout the day, which inspired research in humans. “Many of the benefits are similar, but you have to divide them by about five,” Panda says. 

Risks

Very long fasts, around 20 hours, may cause unhealthy muscle loss, though research is mixed on this question. “We don’t have clear data on where the cutoff would be,” Peterson says. (Obesity medications, meanwhile, may drive more muscle loss than long bouts of fasting.) Anecdotally, Ramey-Torres says she’s gained muscle because she’s more physically active with energy boosts from her 16-hour fasts.

Another issue with longer daily fasts over 20 hours is that blood glucose could spike during the short eating windows. “You’re eating so much food in such little time, it’s like having Thanksgiving dinner every day,” Peterson says.

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Plus, during lengthy fasts, glucose could drop to harmfully low levels. This issue isn’t observed in studies on shorter fasts of 16 hours. However, if your medications affect blood sugar, consult your doctor before trying any kind of intermittent fasting. 

Pregnant women and new mothers should avoid fasting, Panda says. The risks haven’t been determined in these groups.

Research doesn’t suggest that intermittent fasting leads to eating disorders, but those with histories of these disorders should also avoid it, Varady says. “These diets can exacerbate existing disordered eating behaviors,” which is why people who’ve had eating disorders are typically excluded from research on intermittent fasting, according to a paper co-authored by Varady in 2024.

Fasting and fitting in

If you stop eating around 5 p.m., relatives may tease you a few hours later, as they enjoy dinner while you twiddle your thumbs. In addition to becoming healthier, you may become lonelier. “Time-restricted eating is a difficult paradigm in our current social settings,” Catenacci says.

Some tips for fitting in:

Intermittent fasting isn’t for everyone or every season of life. Some are simply too busy to confine eating to one part of the day. “It’s really about figuring out what you can integrate into your lifestyle, rather than forcing a paradigm that doesn’t work for you,” Catenacci says.