The science behind intermittent fasting — and how you can make it work for you

This post is part of TED’s “How to Be a Better Human” series, each of which contains a piece of helpful advice from people in the TED community; browse through all the posts here.

You’ve probably heard the hype: Intermittent fasting has been hailed as the secret to weight loss.

But what’s the reality?

While there is credible scientific evidence for intermittent fasting’s benefits, it’s neither a quick nor a guaranteed fix, according to leading researcher Satchin Panda. Panda, professor of circadian biology at the Salk Institute for Biological Studies in La Jolla, California, has spent his career studying the complex biochemical processes of the human body. His research — in mice and people — appears to suggest that intermittent fasting could benefit human health in a variety of different ways, including losing weight.

Before we dive into the science, let’s put one thing up front: There’s no one way to do intermittent fasting. If you google it, you’ll find a menu of options, each with their own proponents. There’s the 5: 2 diet, which involves eating very few calories (roughly 500-600) for two days of the week, followed by five days of normal eating. Or, there’s alternate-day fasting, which means eating normally one day and then eating either nothing or just 500 calories the next.

All intermittent fasting methods are essentially based on the same idea: When you reduce your caloric intake, your body will use its stored fat for energy. But what makes intermittent fasting different from simply cutting calories is the possibility that it’s easier for people to restrict calories for limited stretches of time rather than for the days, weeks and months demanded by conventional diets. Plus, the specific type of intermittent fasting that Panda has studied may have additional positive effects.

Panda has focused on an intermittent fasting method known as time-restricted eating. In this format, a person consumes all of their calories for the day within an 8-to-12-hour window. Let’s say you usually start your day with a first cup of coffee at 7AM and eventually wind down with popcorn and a drink around 11PM. With time-restricted eating, you might switch to eating breakfast at 8AM, including coffee, and finishing your dinner by 6PM. That way, you’re eating all your meals within a 10-hour window — and you’re most likely forgoing calories from desserts, evening snacks and alcohol. But that’s not the whole story.

Time-restricted eating seems to be doing more for the body than simply reducing calorie intake. This was first suggested by a 2012 study that Panda and colleagues did with mice. They took two genetically identical sets of mice and fed them the same diet — a lab-mice version of the standard American diet that’s high in fat and simple sugar and low in protein.

While both groups were given the exact same amount of food, one group had access to the food for 24 hours and the other group had access to it for only 8 hours. Mice are nocturnal, typically sleeping during the day and eating at night. But when one group was given round-the-clock access to food, those mice began eating some of it during the day as well, when they’d normally be sleeping.

After 18 weeks, the mice who could eat at all hours showed signs of insulin resistance and also had liver damage. But the mice who ate in an 8-hour window did not have these conditions. They also weighed 28 percent less than the mice with 24-hour access to food — even though both groups of mice ate the same number of calories a day. “It was kind of earth-shattering,” Panda recalls. Until then, he says that he and other researchers had thought the total number of calories, rather than when they were eaten, were what determined weight gain.

His team repeated the experiment with three additional sets of mice and got the same results. The outcomes also held steady for different types of food and for eating windows of up to 15 hours — although, interestingly, the shorter the window, the less weight the mice gained. When the time-restricted mice were switched over to unrestricted eating for two days a week, or what Panda calls “having the weekend off”, they still gained less weight than the mice allowed to eat 24 hours a day.

Then, Panda’s team also tried it another way: They took mice that had gained weight because of unrestricted feeding and switched them to time-restricted eating. Despite eating the same amount of calories, these mice lost weight and maintained it for 12 weeks until the end of the study. They also reduced their insulin resistance, which is thought to be linked to obesity, although scientists still don’t understand the association. Of course, the human body is more complex than that of a mouse, Panda says, but these experiments were the first indication of how important timing could be when it comes to how our bodies use food.

In recent years, scientists have been discovering that so many of the human body’s processes are tied to our circadian rhythms. For example, most of us know that getting sunlight early in the morning is beneficial to our mood and sleep and that being exposed to light at 9PM via our cell phones or laptops can disrupt our night’s sleep. “Similarly, food at the right time can nurture us, and healthy food at the wrong time can be junk food,” Panda says. Instead of being used as fuel, it gets stored as fat, which makes sense once you examine the basics of how human metabolism works.

Time-restricted eating gives our body more time to use up fat. When we eat, our body uses carbohydrates for energy, and if we don’t need them right away, they get stored in the liver as glycogen or converted into fat. After we’ve finished eating for the day, our body continues to run on glucose from the carbohydrates that we’ve just eaten for a few hours before tapping into stored carbohydrates, or glycogen, in the liver. That glycogen lasts for several hours before running out roughly eight hours after we’ve stopped eating, which is when our body begins to tap into its stored fat.

When we shorten our eating window and extend our fasting window, we spend longer in this fat-burning mode of our metabolism. But the moment we ingest food again — even if it’s just coffee with a bit of sugar and milk — we switch back into the other mode and start burning carbohydrates and storing glycogen and fat. So if you finish eating at 10PM with your evening snack, your body will run out of glycogen and start burning fat at around 6AM. If you usually eat breakfast at 6AM but you change that to 9AM, you’ve given your body three extra hours to use fat as fuel.

Panda followed up his time-restricted eating experiments in humans — and found it showed promise there, too. In 2015, he and his colleagues tried putting small group of people on a time-restricted eating plan for 16 weeks. Intriguingly, the researchers gave these people no diet instructions or advice at all. Instead, the subjects were told to choose a 10-to-12 hour window in which to do all their eating. When they ate, they took pictures of their food and texted it to the researchers. After 16 weeks, the subjects showed a small amount of weight loss — an average of just over 8 pounds each. But they reported experiencing better sleep, more energy in the mornings and less hunger at bedtime, suggesting time-restricted eating “actually has a systemic impact all over the body,” according to Panda. While it was much too small a group of people to be able to draw definitive conclusions, the researchers found it encouraging that this simple intervention seemed easy for subjects to implement and sustain.

Time-restricted eating has shown some potential to prevent diabetes. In a study of 15 men at risk for type-2 diabetes that was run by Panda, he and his team found that after one week of limiting them to eating within a nine-hour window, the men showed a lower spike in blood glucose after a test meal, a sign of improved insulin sensitivity. It might also help lower cholesterol. In another experiment, Panda and colleagues had 19 people — most of whom were on medication to lower cholesterol or blood pressure or treat diabetes — time-restrict their eating. After 12 weeks of eating within a 10-hour window, they lowered their total cholesterol by about 11 percent on average. What’s more, Panda checked in one year later and found that roughly ¾ of the subjects were still voluntarily eating in an 8-11 hour window. “It was gratifying that they could self-sustain this for a period of time,” Panda says. This is good news given that by some estimates, ⅓ to ½ of dieters eventually regain more weight than they lose.

Here’s how you can practice time-restricted eating, according to Panda. While some intermittent fasting plans allow people to have unlimited quantities of coffee and tea during the day, he says you should consume only water during your fasting window. This means no coffee, tea or herbal tea, which can all change blood chemistry and which is why they’re not allowed during fasts for medical blood tests.

Panda recommends that you drink plain hot water after you wake up; it can give you some of the same soothing feeling as coffee. Of course, if it’s important for you to be alert in the morning, he says it’s OK to have some black coffee — but stay away from any adding creamer, sugar, honey or other sweeteners. “Just one teaspoon of sugar is enough to double our blood sugar,” he says, and switches your body out of fat-burning mode and back into carb-burning mode.

As to when to have your meals, Panda recommends that you wait to eat breakfast until you’ve been awake for a couple of hours. About 45 minutes after you wake up, the hormone cortisol spikes and high cortisol levels can impede your glucose regulation. Plus, the hormone melatonin, which prepares our body for sleep, only wears off about two hours after waking. This means that, for those first two hours, your pancreas, which produces the insulin needed to use carbohydrates in food, is also just waking up. Then you should try to finish your last meal about two to three hours before your bedtime since that’s when the melatonin begins to prepare the body, including your pancreas, for sleep.

While intermittent fasting, and time-restricted eating in particular, holds tantalizing promise, it’s still early days. Since Panda began his research, other research groups have backed up some of his results. For example, a study published in July in Cell Metabolism found that people on a time-restricted eating program reduced their calorie intake, even though they weren’t asked to, and lost a modest amount of weight.

There’s a need for more research about time-restricted eating. So far, there haven’t been any studies with human subjects that lasted longer than a few months. Researchers also need to understand the ways in which fasting affects the human body. For example, the gut microbiome has been shown to actually change in mice that restrict their eating to an eight-nine hour window so that they digest nutrients differently, absorbing less sugar and fat. Is this possible in humans? That remains to be seen. Panda is not the only one investigating the effects of time-restricted eating that go beyond weight loss; other researchers are also beginning to explore whether intermittent fasting might also protect the brain from neurodegenerative diseases.

Intermittent fasting is not a silver bullet for weight loss. Some research even suggests that people practicing the 5:2 diet or alternate-day fasting might instinctively eat more before and after their fasting days or reduce their activity on fasting days, negating the calorie-reducing benefits. In his studies of time-restricted eating, Panda says he’s seen some participants gain weight after they’ve taken the idea of eating whatever they wanted within a window to the extreme, bingeing on the foods they usually abstained from. Also, unlike mice, the human body may have ways of slowing down metabolism so that as you consume fewer calories, you also burn fewer. Finally, it’s unclear whether intermittent fasting is beneficial for people who aren’t trying to lose weight. In fact, there’s a potential danger for people who struggle with binge-eating disorder or anorexia; it’s not hard to see how attempting intermittent fasting could encourage these harmful behaviors.

Time-restricted eating has practical advantages over other dieting options: It’s easy and accessible. Many people don’t have the time or resources to count calories — planning their meals, buying certain foods, tracking their calories — so that diets are often the privilege of people who can afford them. Time-restricted eating can be done by anyone who can count time and limit eating and drinking to specific periods.

Panda and his colleagues are now conducting a study of time-restricted eating to 120 participants. They’re also investigating whether firefighters might improve their health by eating in a 10-hour window. Firefighters and other shift workers are more prone to disease due to the constant disruption to their circadian rhythms. (Editor’s note: If you’d like to participate in Panda’s research, download a free app that will ask you to log your sleep, exercise, medications and everything that you eat and drink. Seven groups of scientists around the world are also currently doing studies using the app’s platform.)

For a long time, people who want to lose weight have had to focus on changing the foods on their daily menus. Time-restricted eating has the potential of expanding the factors that we could control. “When it comes to health, we have a menu” of options, says Panda, who adheres to a 10-hour window of eating. “Now we can add the timing of food to the menu.”

Watch Satchin Panda’s TEDxBeaconStreet talk here: