The practice of fasting dates back to the earliest humans. At a time when food sources were scarce and irregular, eating cycled between periods of hunger and feast. This cycle encouraged—and ultimately, evolved—the body’s ability to survive during periods of food scarcity.

Today, food abundance, not scarcity, is the nutritional problem. We now use fasting selectively as a means to manage body weight, remove harmful toxins in the body, and challenge the body’s stress response pathways.

Fasting works on both sides of the calorie equation. Not eating increases the number of calories burned by boosting the metabolic rate and reduces the number of calories consumed.

When fasting continues for more than 24 hours, the body can start to consume lean muscle mass along with stored fat.

The stress of fasting, without food or intermittently, on the body activates autophagy — the body’s natural cellular regeneration process. Autophagy literally means self-eating.

During autophagy, damaged cells break down; the body recycles their components for energy and as the building blocks of new cells. Autophagy helps the body remove the harmful and toxic compounds released when cells break down, recycles damaged proteins, and increases the production of ketones. It also removes damaged immune system cells and activates stem cells.

True fasting with no food intake is difficult to maintain, however — it causes powerful hunger sensations and isn’t compatible with daily life and work schedules.

A long-term alternative to fasting without eating is intermittent fasting, or eating only within a limited daily timeframe.

The eating window during intermittent fasting typically spans between four and seven hours during the day. It can be reduced or expanded, depending on dietary needs, but the fasting period should be at least 12 hours from the last meal of the evening to the first meal the following day.

During intermittent fasting, calories are not restricted, only the timeframe for consuming them. The diet (preferably healthy with no processed foods or sugar) remains as usual, with no caloric limits.

Even without cutting calories, intermittent fasting can lead to weight loss of 3 to 8 percent of body weight, if it is done consistently over a period of 3 to 24 weeks.

Intermittent fasting with at least 12 hours overnight without eating puts the body into mild ketosis, which can lead to weight loss, greater insulin sensitivity, and improved body composition.