science of us

The Ultimate Guide to Intermittent Fasting

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Intermittent fasting is having a moment. The first thing you should know is that it’s an umbrella term for a variety of eating patterns that cycle between eating and … not. Silicon Valley is interested, and new companies shilling fast-aiding supplements, like HVMN (pronounced “human,” of course) have popped up as a result. A forum called WeFast, organized by HVMN, boasts thousands of intermittently fasting members, who meet in Facebook and Slack chats to share tips and, presumably, to complain about being hungry.

There are a lot of ways to do intermittent fasting, and a lot of self-proclaimed experts attempting to brand their specific formulations: there’s the 18/6 model (18 hours of fasting to a six-hour window in which you can eat normally), and the 16/8; there’s the 5/2 model espoused by Jimmy Kimmel, in which fasters eat normally for five days of the week and eat only 500 to 600 calories a day on the other two; and there’s alternate-day fasting, which is mostly what it sounds like: one day, you eat normally, and the next, you eat very little, or nothing. As to why anyone would do this, motives vary, from weight loss to better cognitive function to enhanced creativity — makeup mogul Bobbi Brown says it helps her digestion and gives her more energy.

As is the case with most trendy diets, there’s a lot of information online about intermittent fasting, and it’s not all good. Here you can find a solid, science-backed beginner’s guide to the practice, a few different ways to do it, and the health benefits supported by evidence thus far.

So what is intermittent fasting?

According to Dorothy Sears, the associate director of the Center for Circadian Biology at UCSD, intermittent fasting can mean a lot of things. The one thing all forms of intermittent fasting have in common is the cycling pattern between eating, and not eating. But, says Sears, even what is meant by “not eating” can vary — for some people it’s very literal, as in water only, and for some people it might mean only a certain, low number of calories. There are a variety of approaches to intermittent fasting, each with their own supporters and detractors.

Sears suggests a practical model called time-restricted eating. While some studies have shown promising results in participants who eat only during short time frames, like six hours a day, Sears fears such a small window wouldn’t be sustainable over the long term, and might make fasters more miserable than it’s worth. If, for instance, you experience work stress during a fasting period, you’re likely to be less able to handle it than you would be if you’d eaten recently.

“We have these fight-or-flight hormones, like epinephrine and norepinephrine, that get secreted when you have a stressful experience, and then you can get a very quick drop in the blood glucose, where you feel a little shaky. Some people call it hanger,” says Sears. “I think that happening a lot over a long period of time is probably not good for us.”

Instead, Sears suggests a 14/10 fasting-to-eating ratio — meaning you’d eat over a ten-hour period (say, between the hours of 8 a.m. and 6 p.m.), and fast the rest of the time, for example. That way you can still have a social life, and still function at work, while getting all the benefits time-restricted eating provides.

Does intermittent fasting work?

In short: all signs point to yes.

Dr. Mark Mattson, a neuroscientist at the National Institute on Aging, has been studying intermittent fasting for decades, predominantly in mice. In the late ’80s, says Mattson, there were a couple of studies that found that alternate-day fasting in mice extended their life spans by 30 percent. That finding made researchers wonder if there might be a connection between alternate-day fasting and brain function. So in studies published in the late ’90s and early aughts, Mattson tested this hypothesis, and found strong support for it. “Over two decades, essentially, we found that the intermittent fasting enhances the ability of nerve cells to cope with and resist stress, the kinds of stress that we think are leading to degeneration of nerve cells,” he says. The way this seems to work, says Mattson, is that intermittent fasting aids in the production of a protein called brain-derived neurotrophic factor, or BDNF, which is known to be critical for learning and memory.

Now, an important caveat: Most, though not all, of the conclusive research done on intermittent fasting thus far has been on mice. Very few such studies exist for humans, and those that do often rely on very small sample groups. According to Sears, at the time of the review she published last year, there were only 17 studies done on intermittent fasting in humans, and most were very small. “Humans are just so different from one another that you really need to do studies on men and women, and you need to do people of different ages, and people of different disease states,” says Sears. “To do really rigorous testing of health benefits in populations you need to do large studies of lots of kinds of people.”

Part of the problem so far, Sears says, is that the NIH hasn’t funded many of the larger studies on the subject. But she hopes that will change. “When they are funded, and the studies are done, then I feel very strongly that we will start to see strong support of the health benefits of intermittent fasting.”

Mattson agrees that more support can only be forthcoming. He, along with Dr. Michelle Harvie, published a study that found that overweight women assigned to the 5/2 fasting diet lost more belly fat and had greater improvements in glucose regulation than their counterparts, who ate meals as usual but reduced their calorie intake by 20 percent. (Both groups lost equivalent amounts of weight.)

Will intermittent fasting help me lose weight?


As with any other form of calorie restriction, evidence suggests that intermittent fasting will lead to weight loss — but again, there are some important caveats: most of the research on humans was done on those who were severely overweight or obese, and most measured weight loss over a period of a few months or less. To date, there aren’t any longitudinal studies on humans who fast intermittently, and we know that most people who lose weight eventually gain it back. This, suggests Sears, is all the more reason to pick a fasting model that you’re able to maintain.

Should I skip breakfast as part of my fast?

The verdict is still out.

While Mattson is in favor of the skipped breakfast (he claims he hasn’t eaten one in 35 years), there’s some reason to believe it really is the most important meal of the day, says Sears. “When you eat a meal, you have what’s called a thermogenic response, where your body produces a little bit of heat. You do that more readily in the morning than you do at night, so when you eat at night, you’re not liberating some of the fuels or energy that you’re eating as heat. You’re keeping all that energy in and storing it as fat,” she explains.

“We have a special hormone system in our bodies that helps us clear the sugar out of the blood, and that hormone is called insulin,” Sears says. “Insulin works really really well in the morning, and then it starts to not work so well in the afternoon, and it doesn’t work so well at night.” In other words, our bodies seem set up to process food more favorably in the morning than they are at night. “When we eat a large meal at breakfast, our bodies can handle it really well. So when it comes to intermittent-fasting regimens, I think the ones that are going to show the most promise moving forward are the ones where the food consumption is in the daytime.”

Is it okay to exercise while intermittently fasting?

Yes — in fact, it’s even better for you.

Most people work out when they can, so it might depend on where you can fit things in, but the ideal fasting and exercise setup might be to fast overnight — i.e., from 8 p.m. to 8 a.m. for a 12-hour fast, or 8 p.m. to 10 a.m. if you’re going for a 14-hour fast — and exercise first thing in the morning, before you eat, says Mattson. “When you’re sleeping you’re not using as much energy, so when you’re sleeping, the rate of depletion of your energy stores is slower than if you’re active,” he says. “So if you exercise in the morning, then you’re definitely going to start burning fat during the exercise. This is one thing we’ve been studying in animals, and there’s some indirect evidence in humans, that exercising while in the fasted state may amplify or enhance the beneficial effects of exercise on health.”

What am I allowed to eat or drink while I’m fasting?

Again, that depends on your fasting model.

Importantly, both Mattson and Sears say that their research suggests that animals and humans benefit from intermittent fasting even when it’s only done most of the time — whether that means doing the 5/2 model, or only doing time restricted eating during the week. “We think you don’t have to have a perfect adherence to the regimen, you just need to practice it for the most part. Five out of seven days is probably enough to see a long-term benefit,” says Sears.

So, while you’re fasting, you can have water, but you can also chew gum, or have coffee, if you want — maybe even a Diet Coke, says Sears (though she wouldn’t encourage it, either). In a study she’s currently piloting, Sears says she told subjects, “When you’re waiting for your fast to end, and you have one more hour to go, and you want your coffee, have it, but just don’t put anything in it that has calories. So if a woman wanted to have a black coffee with an artificial sweetener, she could do that.” You don’t have to be a true ascetic to make intermittent fasting work for you.

The Ultimate Guide to Intermittent Fasting