For some people, fasting is a way to cleanse the body of toxins. For others, it's a religious or spiritual practice.
While some health experts have questioned its safety — particularly when used in the extreme — fasting proponents have praised its value. Fasting has been linked in several studies to weight loss, immune system benefits and brain function.
Recently, researchers developed a five-day monthly diet that they called the "Fasting Mimicking Diet." In the study, published in the journal Cell Metabolism, scientists said the 19 participants who followed the fasting regimen for three months had reduced risk factors for aging, cancer, diabetes and cardiovascular disease.
“Strict fasting is hard for people to stick to, and it can also be dangerous,’’ study co-author Valter Longo, professor of gerontology and biological science at the University of Southern California, told The Telegraph. ''So we developed a complex diet that triggers the same effects in the body.’’
On Day One of the pseudo-fast, dieters eat 1,090 calories made up of 10 percent protein, 56 percent fat and 34 percent carbohydrates. Days Two through Five each have 725 calories with 9 percent protein, 44 percent fat and 47 percent carbohydrates. The rest of the month, dieters ate whatever they want.
Is fasting normal?
Our "normal" meal pattern of three daily meals (plus snacks) is highly abnormal from the perspective of human evolution, wrote an international group of researchers in a paper published in the journal Proceedings of the National Academy of Sciences. Early hunter-gatherers often ate intermittently, depending on how often they were able to capture prey.
"The ability to function at a high level, both physically and mentally, during extended periods without food may have been of fundamental importance in our evolutionary history," the researchers wrote. They believe the human body may have adapted to perform at its peak with occasional fasting.
"Intermittent fasting helps the body to rejuvenate and repair, thereby promoting overall health," article co-author Satchidananda Panda, associate professor of regulatory biology at the Salk Institute for Biological Studies in San Diego, told LiveScience.
"Fasting alone is more powerful in preventing and reversing some diseases than drugs," he said.
Panda and his colleagues performed a time-restricted feeding study with mice several years ago where they only fed one group during an eight-hour period each day. They gave those mice lots of fat to eat while a second control group was allowed to eat whatever whenever they wanted.
After 100 days, the control group developed high cholesterol and blood glucose, gained weight, had liver damage and had issues with motor control. The fasting mice weighed 28 percent less and performed better on exercise tests.
The researchers concluded that restricted fasting can have an impact (at least on mice) on the prevention of metabolic diseases.
What happens to your body when you fast?
Technically, most of us fast every night when we sleep. Fasting is defined as going without food for eight or more hours — so when you wake up in the morning, your body is in a fasting state. That's when your body has stopped absorbing nutrients from the last meal you've eaten.
Next, your body uses stored-up blood sugars (glucose) for energy. You may start to feel tired and rundown when your body depletes these stores for fuel.
After an extended time without food — typically 48 hours or more — your body enters a state called ketosis, where it starts breaking down stored fat for energy.
Here's a more in-depth look at the whole process:
Some people go on special fast diets — drinking just juice or eating just cabbage soup, for example — hoping to rid the body of chemicals and pollutants accumulated from food, the environment and everyday life.
"There is no scientific evidence it will detox the body. The issue of fasting to cleanse the body has no biological basis because the body is real good at that by itself," Madelyn Fernstrom, Ph.D., CNS, founder and director of the University of Pittsburgh Medical Center's Weight Loss Management Center, told WebMD. "The liver is a natural detox center; the lungs, the colon, the kidneys, [the lymph glands] and the skin get rid of toxins."
Fasting typically doesn't mean having to abstain from all food and drink. Whether for religious, spiritual or health reasons, people typically fast by limiting their food to certain hours or types or by cutting back dramatically on their overall food intake.
There are several types of "intermittent fasting" diets that have been popular recently.
5:2 diet — People on this fasting diet fast for two non-consecutive days and then eat whatever they want for the remaining five days a week.
Alternate day fasting — This is an every-other-day diet where you eat what you want one day, then fast (about 500-600 calories) the next day.
8-hour diet — Here, you eat in any 8-hour window during the day and then fast for the remaining 16 hours. It's sometimes referred to as the 16:8 diet.
Should fasting be used for weight loss?
When fasting can be dangerous is when it is used as a weight loss tool, says Melinda Johnson, MS, RDN, clinical assistant professor at Arizona State University and president of the Arizona Academy of Nutrition and Dietetics.
"Many people who practice fasting are doing it to lose weight, and this type of weight loss behavior is linked to problems, such as a lower body image, lower self esteem, and a poor relationship with food," says Johnson.
"I particularly worry when fasting is done by someone who is already chronically dieting or highly focused on their appearance, because research shows that it can be further damaging to the person's well-being. However, when people fast for other reasons — such as for a religious holiday — there does not seem to be that psychological damage, and it is not unsafe."
In a small 2018 study, however, researchers put 23 obese volunteers on a 16:8 fasting diet where they ate between 10 a.m. and 6 p.m. and drank only water the rest of the time. After 12 weeks, the fasting volunteers lost about 3 percent body weight and their blood pressure dropped.
"The take-home message from this study is that there are options for weight loss that do not include calorie counting or eliminating certain foods," said study co-author Krista Varady, associate professor of kinesiology and nutrition in the University of Illinois at Chicago College of Applied Health Sciences, in a statement.
"The 16:8 diet is another tool for weight loss that we now have preliminary scientific evidence to support. When it comes to weight loss, people need to find what works for them because even small amounts of success can lead to improvements in metabolic health."
Editor's note: This story was written in June 2015 and has been updated with new information.