What is intermittent fasting, and how does it work?

Intermittent fasting is an eating pattern that switches between eating and fasting on a regular schedule. Most diets focus on what you eat. Intermittent fasting focuses on when you eat. Johns Hopkins neuroscientist Mark Mattson has studied it for 25 years. He explains that after several hours without food, the body uses up its sugar stores and starts burning fat. He calls this metabolic switching. If you eat three meals and snacks throughout the day, your body keeps running on your last meal and rarely gets to that point.

The two best-known intermittent fasting schedules are the daily approach and 5:2. With the daily approach, you eat within one six- to eight-hour window each day, as in 16:8 fasting: 16 hours of fasting and an eight-hour eating window. With 5:2, you eat normally five days a week and have a single meal of 500–600 calories on the other two. Longer fasts of 24 to 72 hours aren't necessarily better, and they may be dangerous. Expect two to four weeks to get used to a new routine, and some hunger or crankiness along the way. While fasting, stick to water, black coffee and tea. When you eat, a Mediterranean-style diet is a good guide, because filling your eating window with high-calorie food cancels out the benefits. Research so far, some of it in animals, links intermittent fasting to better memory, blood pressure and resting heart rate, weight loss and improvements in type 2 diabetes.

Intermittent fasting isn't for everyone. Avoid it if you're under 18, pregnant or breastfeeding, have type 1 diabetes and take insulin, or have a history of eating disorders. Talk to your doctor before you start, and again if you notice unusual anxiety, headaches or nausea.

General information, not medical advice. Don’t fast without talking to a doctor if you’re pregnant or breastfeeding, under 18, have had an eating disorder, have diabetes, or take insulin or other medicine that lowers blood sugar.