Fasting is a longstanding part of many religious traditions, including the Jewish and Muslim observances of Yom Kippur and Ramadan. A form of fasting known as intermittent fasting has also gained popularity as a weight-loss tool.
Many studies have examined the benefits and risks of giving up food for a day, including how it affects weight loss.
In this article, we look at what happens to the body during fasting, as well as what a person can do to make fasting safer.
What happens during fasting?
Whether a person is fasting or not, the body still needs energy. Its primary energy source is a sugar called glucose, which usually comes from carbohydrates, including grains, dairy products, fruits, certain vegetables, beans, and even sweets.
The liver and muscles store the glucose and release it into the bloodstream whenever the body needs it.
However, during fasting, this process changes. After about 8 hours of fasting, the liver will use the last of its glucose reserves. At this point, the body enters into a state called gluconeogenesis, marking the body’s transition into fasting mode.
Studies have shown that gluconeogenesis increases the number of calories the body burns. With no carbohydrates coming in, the body creates its own glucose using mainly fat.
Eventually, the body runs out of these energy sources as well. Fasting mode then becomes the more serious starvation mode.
At this point, a person’s metabolism slows down, and their body begins burning muscle tissue for energy.
Although it is a well-known term in dieting culture, true starvation mode only occurs after several consecutive days or even weeks without food.
So, for those breaking their fast after 24 hours, it is generally safe to go without eating for a day unless other health conditions are present.
Can fasting promote weight loss?
It does appear that fasting can help with weight loss. However, studies make it clear that this is not the case for everyone.
Popular diet plans include 12-hour or 16-hour fasting periods, as well as the 24-hour fast. Some diets require people to drink only water during the fast, while others allowed any zero-calorie beverage.
Fasting is not necessarily better than any other weight-loss method, including reducing daily calorie intake by a small amount.
In a recent study, people with obesity who fasted intermittently for 12 months lost slightly more weight than those who dieted in a more traditional way, but the results were not statistically significant.
The limits of fasting appear to have less to do with its physical effects than how it fits into a given lifestyle.
For example, the same study found that people who fasted were more likely to give up on weight-loss efforts than those who dieted in a more traditional way, such as counting calories. The researchers concluded that fasting might be harder to maintain over time.
Another possible concern is post-fast binging. Some fasting experts agree that it is easy to derail weight-loss successes by overeating after the fasting period.
Fasting days can also offer a false sense of security, leading people to disregard positive eating habits on non-fasting days.