Intermittent fasting is a centuries-old practice that exploded in popularity following the release of The Obesity Code in 2012. Celebrities from Jennifer Aniston to Jimmy Kimmel endorsed it as a better way to manage weight, reduce inflammation, and increase lifespan. Since then, proponents have flooded social media with anecdotal evidence for the 16/8 method versus the 5:2 method, and everything in between.
But are the benefits of intermittent fasting confirmed by science?
Research on intermittent fasting is only mildly helpful, as most studies are small and short-term. Some research has shown that it can lead to weight loss and may lower certain health risks. In the long term, however, intermittent fasting doesn’t appear significantly more effective than other thoughtful approaches to eating.
“Intermittent fasting has the same issue as any other diet: sustainability is usually not achievable and, therefore, weight lost is regained,” says Caroline Apovian, MD, co-director of the Center for Weight Management and Wellness at Brigham and Women’s Hospital and professor at Harvard Medical School.
Here’s what science tells us about the pros and cons of intermittent fasting, for discussion with your health care provider or dietitian.
1 of 4
What Is Intermittent Fasting?
Intermittent fasting is an approach to eating that alternates between periods of eating no (or little) food, followed by periods of eating normally. During the eating periods, there are typically no restrictions on what or how much to eat, though priority is given to nutrient-rich food. There are three main types of intermittent fasting:
- Time-restricted eating. The most popular type of intermittent fasting, this involves fasting for part of the day (typically 16 hours) and eating during the rest of the day (eight hours).
- 5:2 fasting: You fast for two full days and eat for five days. The fasting days are not consecutive. Variations allow for eating a small amount, about 500 to 600 calories, on fasting days.
- Alternate-day fasting: You fast (or consume 500 to 600 calories) every other day and eat on non-fasting days. Studies show that this is the most effective type for weight loss and improving health risks but also the most difficult to sustain over the long term.
2 of 4
What Are the Benefits of Intermittent Fasting?
Research shows that the benefits of intermittent fasting can include:
Short-term weight loss. Intermittent fasting can help you lose weight in the short term, in part because shortening the window of time in which you eat leads to lower calorie consumption. One six-month study of people with diabetes found that those in the fasting group ate about 100 fewer calories a day than those in a calorie-restriction group, and they lost twice as much weight. When you fast, your body depletes fat stores for energy, which is similar to what happens in the popular keto diet. But there’s limited research on whether people keep the weight off over the long term.
Reduced sense of deprivation. During the windows that you can eat, there are few restrictions on what you can eat. That means there’s no calorie counting and no agonizing over what you are (and aren’t) allowed to eat. Some people say that intermittent fasting reduces the sense of deprivation and ensuing cravings that are common with restrictive dieting.
Cardiovascular and metabolic benefits. There’s a deep body of research in animals and humans showing that fasting can extend life. Fasting for a day or longer was a natural part of existence in hunter-gatherer days, helping the body protect itself. Intermittent fasting can help regulate blood sugar and insulin levels, release antioxidants to protect against oxidative stress, reduce inflammation, and improve DNA repair.
Intermittent fasting has been shown to reduce risk factors for cardiovascular disease, like blood pressure, cholesterol, and insulin resistance. These same risk factors tend to improve with any healthy eating plan, and no large randomized, controlled studies have proven that intermittent fasting is superior at boosting health.
3 of 4
Who Should Avoid Intermittent Fasting?
Some people may not be able to fast because of medical or other reasons. Talk to your health care provider before trying intermittent fasting if you:
- Are pregnant or nursing
- Have diabetes, kidney disease, or heart disease, or are immunocompromised
- Are on medications that need to be taken with food during fasting windows
- Are at risk of eating disorders. Some, but not all, studies show that fasting increases the likelihood of developing an eating disorder, such as anorexia, and binge eating. The reason: It trains you to disregard hunger cues, which can lead to disordered eating.
4 of 4
How to Practice Intermittent Fasting
Choose a type of fasting: Remember that no eating plan works if you can’t stick with it, so the best approach is one that you can sustain. Apovian recommends time-restricted eating because it’s easier to fast for part of a day than it is to not eat for 24 hours.
Pick a fasting window: For time-restricted eating, you’ll eat for eight to 10 hours and fast for the rest of the 24-hour day. You may get the most bang for your buck by eating during daylight hours. “That is the time when metabolic genes are programmed to metabolize calories to benefit the body,” says Apovian. Since we tend to eat more carbohydrates in the evening, fasting at night can head off some of your unhealthier eating habits, too.
She recommends eating from 8 a.m. to 5 p.m. and fasting until the next morning. But what works best is what you can do. If another time slot works better for you, then choose that.
Eat nutrient-packed foods in the eating window. Intermittent fasting’s short eating window carries the risk of undernourishment, so it’s important to eat nutrient-rich food. Aim for protein, vegetables and fruits, and high-fiber foods. While there are no specific restrictions, some experts recommend limiting high-carb, high-sugar, and ultra-processed foods, which tend to be low in nutrients.
