Lately, people love to brag about how long they can go without eating. Jack Dorsey is as famous for skipping food all weekend as he is for being the CEO of Twitter. Brooklyn Nine-Nine star Terry Crews says intermittent fasting, or forgoing meals for lengths at a time, helps him maintain that enviable six pack.
Proponents believe the practice improves heart health and may even lower cancer risk, but many use intermittent fasting as a weight-loss tool.
Are any of these claims legit, and more importantly, should you try it?
How long should you fast?
The concept calls for going extended periods of time without eating, and people generally follow three common fasting schedules:
- Alternate-day fasting entails switching from days without food restrictions and days that include one meal that provides about 25 percent of your daily calorie needs, according to Harvard Health Publications. For example, you might eat one 700-calorie meal on Monday and resume your normal diet on Tuesday.
- Whole-day fasting is also known as the 5:2 fasting schedule, which means you eat roughly 500-calories two days a week. The other five days have no food restrictions.
- Time-restricted fasting is typically following in a 16:8 format, meaning you only eat for 8 hours throughout the day and fast for 16. For example, noon to 8 p.m. is a good guide as many people only eat lunch and dinner.
Everyone’s preferred fasting schedule will vary, but New York-based Alexandra Sowa, M.D., says it’s best to start slow with just a 12-hour fast. If that’s not unbearable, move onto a 16-hour fast several times a week.
“If you like it then see how you do extending it to 24-hour fast,” she tells Men’s Health.
What happens to your body when you fast?
There’s not a lot of evidence to show exactly what happens when we fast, says Nathalie Sessions, R.D., at Houston Methodist.
“There’s only been a few viable scientific studies that have been done with humans on intermittent fasting,” she tells Men’s Health.
Most research has been conducted in animals, and human bodies don’t function in the same way.
Many theorize that giving your body a break from eating helps your body repair damaged cells, a process known as autophagy, says Sowa. However, most of this research has been conducted in mice, so the science isn’t 100 percent clear.
However, there is evidence that fasting lowers insulin levels, according to Sowa. Healthy adults experienced a decline in insulin levels after fasting, according to a 2005 study published in The American Journal of Clinical Nutrition. Insulin, a hormone made by the pancreas, regulates blood sugar and other hormones. Too much insulin has been linked to obesity and health conditions like heart disease.
How much weight can you lose while fasting?
Let’s be clear about one thing: intermittent fasting doesn’t guarantee weight loss. In theory, you’ll consume less calories because you’re eating fewer meals, but it’s entirely possible to overeat while fasting.
You don’t want to gorge on a burger, fries, and shake right after fasting, says Sowa.
“In order to continue the benefits of the fast you want to choose foods that will not spike your insulin,” she says.
She recommends eating foods that are high in fiber and protein, like leafy green vegetables, eggs, and chicken.
Research about intermittent fasting for weight loss is mixed.
“Some studies have demonstrated that it can result in as much as a 7 percent weight loss in body weight,” says Sessions.
However, a randomized clinical trial of obese adults showed that intermittent fasting was no better than standard calorie restriction, according to a 2017 paper published in JAMA Internal Medicine.
What can you eat or drink while fasting?
Very little, according to Sowa. Generally, you’ll want to stick to water or beverages that have virtually zero calories, like black coffee.
You may have seen people adding butter or MCT oil to coffee, and these are actually okay in small quantities because they don’t have protein or carbs, says Sowa.
If you just can’t stomach black coffee, she recommends adding a tablespoon of full-fat whipping cream to your coffee.
“That will allow most people to stay in a technically fasted state,” she says. Just don’t go overboard.
“If you’re drinking a ton of whole fat whipping cream or eating a lot of butter you’re not going to lose weight,” she says.
Aim for no more than two tablespoons per day.
Should I try intermittent fasting?
Sessions says this plan probably works best for people who are regimented and says yo-yo dieters will likely have a hard time fasting. The best way to lose weight is by finding something you can stick to.
“The long-term key to weight loss is to establish a lifestyle that you can sustain,” says Sessions.
If you think this will work for you, it’s best to work with a doctor or dietitian who can ensure you’re getting enough nutrients.