Basically, intermittent fasting calls for restricting eating to certain time periods within a given day or week. Some plans call for certain fasting windows (time periods where a person can and cannot eat), while others have people eating every day but some days have a lower caloric intake than others.
The concept of skipping meals or limiting how much you eat at given times sounds…well, dicey. But proponents of the eating plan posit that by putting the body into a fasting state for short periods of time, people can potentially boost their metabolism, kickstart healthy weight loss, and see other intermittent fasting benefits like enhanced cognition and improved energy and mood.
However, exactly how the diet works is a hot debate among researchers, says James Mitchell, Ph.D., associate professor at Harvard T.H. Chan School of Public Health, whose research focuses on dietary restriction. Many people believe that both the potential metabolic benefits and weight loss are just a result of calorie restriction (eating less overall), while again, others believe that going set periods of time without eating plays a role. And while people who practice IF swear by its benefits, what they don’t talk about is how that anecdotal evidence isn’t totally backed up by research…yet.
So yeah, there’s a lot to unpack with the buzzy eating plan. Here’s what you should know about intermittent fasting before considering it for yourself:
The three most popular types of intermittent fasting are 16:8, 5:2, and alternate-day fasting.
In 16:8 fasting (also known as Leangains), you restrict eating to a specific eight-hour window every day, so that you’re essentially fasting for 16 hours every day. You choose what your hours are—if you like to eat dinner late, for example, you could choose to have your first meal of the day at 1 p.m., and not eat any more food for the day after 9 p.m. No foods are off-limits, but the idea is that you shouldn’t eat bigger meals than you normally would during the eight hours.
“I think this time-restricted feeding paradigm is the one with highest compliance,” says Dr. Mitchell. “It’s compatible with a busy lifestyle—you get up, skip breakfast, have a late lunch, and eat dinner with everybody. That’s doable most days.”
On the 5:2 diet, you eat normally five days per week, and reduce calorie intake to 500-600 calories for two non-consecutive days per week of your choosing. The diet authors emphasize that during the five “normal” eating days you should eat as you would if you weren’t fasting part of the time, and there are no rules on what you can and can’t eat. The diet can be tough to stick to—500 calories doesn’t go far in a day, especially if you’re active or busy.
Strictest of all is alternate-day fasting, or ADF. It’s just as it sounds: You fast every other day, continuously. Some people do full-on water fasts, while others choose to eat around 500 calories on fasting days. Because this version of IF is so restrictive, it’s not recommended for most people unless they’re under the advisement of a doctor and a dietitian. Plus, studies have shown that in alternate-day fasting, people don’t typically adapt to be less hungry during the fasting periods—making it very difficult to stick with.
Watch the video below to see a registered dietitian weigh in on intermittent fasting:
Intermittent fasting may help with weight management, but isn’t a guarantee.
Intermittent fasting will likely lead to some weight loss in the short-term because people generally eat fewer calories on this plan, whether or not they are following a form of IF that specifically calls for limited-calorie days. “With time-restricted feeding, the thinking is that you can eat as much of whatever you want, as long as it’s during a narrower window,” Dr. Mitchell says. “Of course, what happens in reality is that people generally don’t eat as much—it feels good to think that you can, but you really can’t, consistently, if you’re eating during a shorter window.”
However, current research doesn’t support any long-term weight management potential with intermittent fasting. Although a 2017 review found that a majority of studies reviewed (11 out of 17) showed statistically significant weight loss, none were long-term or large-scale, meaning their results aren’t entirely conclusive. (The longest of these 11 lasted 20 weeks and included just 54 subjects; the shortest lasted one day (one day!) and included 30 people, which, kay). The longest and largest study examined in the review lasted for six months and included 107 young, overweight females…but no significant weight-loss results were reported.
So in short: “IF may help reduce weight, but it works because in the end, it’s a low-calorie diet,” says Abby Langer, R.D.
Intermittent fasting benefits for hormones and metabolism are promising, but inconclusive.
Although anecdotal evidence—what you’ve heard from friends and wellness influencers—make IF seem like a magic bullet for improved health, the actual research is still in early stages. Since there isn’t just one definition for IF (see above with the different types of plans), it’s up to individual research teams to define its parameters for different studies. “Researchers really haven’t compared the different types of intermittent fasting,” says Dr. Mitchell. It’s difficult to find funding for such granular, descriptive studies, he says.
One agreed-upon benefit, says Dr. Mitchell, is that occasional fasting can improve insulin sensitivity—which is key to metabolic health, diabetes prevention, and weight management.
Many other studies looking at IF’s impact on hormones have been done on animals, or on very small (generally less than ten) groups of healthy people—making the results not super conclusive. Likewise, a 2015 review of the literature found that, while IF definitely has potential and warrants further study, there’s little published data that effectively links this eating style to better health outcomes in terms of diabetes, heart health, cancer, or other chronic diseases. But there is promise: A 2017 study of 100 people found that those who did a fasting-style diet for five days in a row per month lost weight, lowered their blood pressure, and saw other improvement in markers for age-related diseases.
As for the purported brain-boosting benefits of intermittent fasting? Take those with a grain of salt; published studies have been only done on animals. But there is some evidence that switching back and forth from a fasted state could boost brain function and help it fight off disease.
Although IF might be OK for some people, it’s definitely not for everybody.
For anyone who has a history of eating disorders or disordered eating: “Stay far away [from intermittent fasting],” says Langer. The National Eating Disorder Association (NEDA) lists both history of dieting and a negative energy balance (burning more calories than you’re taking in) as biological risk factors for an eating disorder. “Many people report that their disorder began with deliberate efforts to diet or restrict the amount and/or type of food they were eating in the form of dieting,” the group says. Although IF doesn’t restrict types of food, it’s definitely a form of food restriction.
As with any diet, timing matters—if someone is sick or recovering from an injury, IF should be put to the side. “If you’re trying to heal a wound, it might work against you,” Dr. Mitchell says. Adequate nutrition, especially protein, is important for healing, so whether you’re healing from surgery or just a scrape, the best thing to do is eat without restriction until you’re back to normal. Same goes for those who have a preexisting health condition like a thyroid disorder—going without nutrients for longer periods of time might be riskier for them.
Also keep in mind that social isolation might be an issue. “Some days you’ll finish eating before typical dinner reservations! The timing can get tricky,” says Langer.
Bottom line: While we’ve still got plenty to learn about IF, the potential weight management and health benefits might be worth it for some people. But given its restrictive nature, it’s definitely not an eating plan for everyone.
Another popular eating plan this time of year: Whole30. Check out the video below to see what a registered dietitian thinks of it:
Originally published March 24, 2019. Updated January 2, 2020.
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