You’ve likely heard personal trainers or nutritionists say it’s important to eat breakfast right when you wake up to jump-start your metabolism and begin the day with your engine revving. But a new diet trend called time-restricted eating, which is basically a form of fasting in which you refrain from eating for several hours of the day, flips that thinking on its head: On this plan, you could be awake for several hours before you eat anything.
It might sound counter to conventional dieting advice (not to mention difficult to stick to), but research has found it can be an effective way to stay slim—even if you don’t restrict what you’re eating. Read on for more about the science behind this craze and whether it might be a good meal strategy for you.
To follow a time-restricted diet (which is also known as intermittent fasting), you can continue eating the same foods you normally do, you just only eat them during a certain number of hours of the day. You start by restricting all eating to an 11- or 12-hour period—so, if you have breakfast at 8 a.m., you’ll have to set forks down by 7 or 8 p.m. You’re allowed to drink water, coffee, or tea (with no milk or sugar) during the fasting periods between bedtime and in the a.m., but you have to steer clear of ingesting anything else.
If you want to give it a shot, it’s a good idea to start slowly, advises Jaclyn Jacobsen, a nutritionist with the Vitamin Shoppe. Begin by fasting for about 12 hours of the day (including the time you sleep). Once you get used to it and it feels fairly easy, try dialing it back even further, allowing yourself to eat only 10 or maybe even eight hours of the day. “Theoretically, the longer you fast, the more fat you are burning for energy in the absence of glucose,” Jacobsen says.
A few animal-based studies suggest that time restriction might be all that’s necessary to help you slim down. Recent research published in the journal Cell Metabolism, for instance, found that this rule can help you maintain a healthy weight—even if you cheat on the weekends. And the study also found that it didn’t matter what you eat: If you eat it during a small window of the day, you’ll still be slimmer than if you eat it at all hours.
In addition to helping you maintain a healthy weight, restrictive eating might also help you have less inflammation and lower your risk of diabetes, heart disease, and other chronic illnesses. Longer fasting periods and less eating in the evening may lower inflammation and risk of breast cancer as well, according to a study published in PLoS One.
It makes your body burn fat more efficiently, too, says Jacobsen: “When glucose is absent, the body will break down fat stores into free fatty acids for energy. This use of free fatty acids enhance fat burning as you are using stored fat for fuel.” And though it may seem contradictory, time-restriction can actually make you less hungry—by normalizing your body’s level of ghrelin (the "hunger hormone"). “Fasting helps the regulation of this hormone so we feel hungry only when we actually need to eat,” Jacobsen says.
And this is where the trainer's advice to "jump-start" your metabolism by eating breakfast takes a beating: “Much research has shown that your metabolic rate actually increases when fasting, so you burn calories more efficiently when you do start eating,” Jacobsen says. Your metabolic function won’t decrease significantly, she says, unless you go at least three full days without eating.
Cutting back on the hours you’re allowed to eat might not come naturally. Some people will adapt easily and get used to eating during a small window of time after just a few days, according to Jacobsen, while others will take up to a month to fully adjust. And you might never feel fully acclimated.
Another big drawback: It might not get you the slim-down results you want. “Some individuals, especially women, are very sensitive to any type of hormonal changes in the body, so fasting may offer no additional benefit when compared to eating a whole-foods diet,” she says.
You’ll likely see better weight-loss results and feel better if you make healthier food choices in general, Jacobson says; whether you’re doing time-restricted eating or not, she recommends a diet consisting of about 80 percent whole foods (like vegetables, whole grains, and lean protein) and letting yourself indulge in treats 20 percent of the time.
So intermittent fasting might be worth a shot, but keep in mind that skipping midnight snacks or early-morning coffee with cream might not necessarily be ideal for you. “There is no ‘one size fits all’ when it comes to this concept or any other health-related concept,” Jacobsen says. “Nutrition is so individual, so what works for one group of people, may not work for others. It’s all trial and error.”