Intermittent fasting is a pattern of eating based on time limits. For a set time of hours or days, you eat a typical diet. At the end of the set time, you switch to very few or no calories, called fasting.
When practiced for health benefits, intermittent fasting can be done in different ways. Fasts can last hours or a full day.
Full-day fasting
Alternate-day fasting means eating a typical diet one day and no meals the next day. One example is 5:2 fasting, also called twice-a-week fasting. This means you eat very little or nothing two days a week. The other five days you eat a typical amount of calories.
Alternate-day modified fasting is eating a typical diet one day and only about 25% of your typical calories the next day.
Time-restricted eating
Limiting what you eat to a small part of each day is called time-restricted eating. This can mean fasting for up to 16 hours a day. But fasts of 6 to 8 hours also are common.
Some people eat earlier in the day and fast in the afternoon and evening. Others fast in the morning, eat around midafternoon then fast again in the evening.
Health effects of intermittent fasting
The idea is that intermittent fasting causes the body's cells to change how they work. Timed eating may push cells to focus on repair, energy use and balancing body-wide functions.
Research shows that intermittent fasting may improve some signs of health in the short term. These include:
- Blood sugar.
- Weight.
- Blood cholesterol.
- Blood pressure.
- Chronic inflammation.
But the long-term health effects of intermittent fasting aren't clear. And some studies find just limiting calories in general may have the same benefit as intermittent fasting.
Also, researchers don't know if different fasting cycles have the same benefits. Some evidence suggests that a 16-hour fasting, 8-hour eating cycle may raise heart disease risk compared with other time-restricted eating cycles.
Intermittent fasting also may cause people to feel very tired and dizzy. It can affect diabetes management and cause headaches or mood swings. Intermittent fasting also may cause constipation and affect the menstrual cycle.
Intermittent fasting is not for everyone
Only eating during a short window often means skipping breakfast or dinner. If those meals are important parts of your social life, then intermittent fasting may not work for you.
While intermittent fasting is safe for many people, it may not be a healthy pattern for people who:
- Have an eating disorder.
- Are pregnant or breastfeeding.
- Are at high risk of bone loss and falls.
If you're thinking about intermittent fasting, talk to a healthcare professional about the pluses and minuses. Other eating patterns might work as well or better depending on your health goals.