Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It’s currently very popular in the health and fitness community.

IF doesn’t specify which foods you should eat but rather when you should eat them. This means it’s not really a diet — it’s more accurately described as an eating pattern.

Common IF methods involve fasting for 16 hours and having an 8-hour eating window. Some people do 24-hour fasts twice per week.

Fasting has been a practice throughout human evolution. As a result, humans evolved to be able to function without food for extended periods of time.

People also commonly fast for religious or spiritual reasons, including in Islam, Christianity, Judaism, and Buddhism.

You can practice IF in several different ways, though they all involve splitting the day or week into eating and fasting periods.

During the fasting periods, you won’t eat anything. You can usually drink calorie-free beverages like water, herbal tea, and black coffee.

These are the most popular methods:

• The 16/8 method: Also called the Leangains protocol, this involves an 8-hour eating period and a 16-hour fasting period. Some people do this by skipping breakfast, but you can also do the reverse and skip dinner or eat early.
• Eat-stop-eat: This involves fasting for 24 hours once or twice per week. Because this is a slightly more advanced version of fasting, you should only try this if you’re pretty comfortable with the practice.
• The 5:2 diet: With this method, you consume only 500–600 calories on two nonconsecutive days of the week but eat as usual the other 5 days.

For many people, reducing their eating period also reduces the number of calories they eat on a daily basis. Over time, this can contribute to weight loss.

When you fast, several things happen in your body on the cellular and molecular level.

First, human growth hormone (HGH) levels go up and insulin levels go down. Your body’s cells also change the expression of genes and initiate important cellular repair processes.

Here are some changes that occur in your body when you fast:

• HGH: Your levels of HGH increaseTrusted Source. This can help with both fat loss and muscle growth.
• Insulin: When you fast, insulin sensitivity improves and levels of insulin decreaseTrusted Source. Lower insulin levels make stored body fat more accessible.
• Cellular repair: When fasted, your cells initiateTrusted Source cellular repair processes. This includes autophagy, which means cells digest and remove old and dysfunctional proteins that build up inside them.
• Gene expression: Fasting can causeTrusted Source changes in the function of genes related to longevity and disease prevention.