Is Fasting Good for You?

By Temma Ehrenfeld @temmaehrenfeld
April 27, 2016

Most people who try a regular routine find they become less hungry on their fast days. But is fasting good for you?

Americans aren’t used to the idea of fasting. But there’s quite a bit of evidence that it can be good for your health. It’s one way to lose weight and also regulate blood sugar levels and possibly fight a variety of illnesses. 


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The argument goes like this: back in our hunter-gatherer days, human beings frequently went hungry. Our ancestors didn’t snack all day, that’s for sure. So we evolved to store sustenance through our muscles and fat tissues and can stay alive for weeks without food. Some people have likened the benefits of fasting to those of exercise: short-term wear and tear on the body makes it healthier. 

Fasting is required for specific periods in Judaism, Catholicism, and Islam, and many people have experienced it as rough. Fasting for a religious holiday, however, isn’t the same as keeping to a regular fasting regime over weeks or months. The body gets used to going without food for a spell when it’s a more ordinary occurrence. 

People who do well on low-carb, high-fat diets may be good candidates, since fasting has a similar effect, forcing the body to burn fat for fuel.  

The fasting weight loss regimes with known results are strict. One approach, called the 5:2 diet, is to eat whatever you want for five days and only 500 calories — about one light meal — on two days a week. Some people eat 500 calories every other day and normally on the other days. Some fast most of every day, eating within only a window of hours. None of these regimes would be advisable without first talking to your doctor, especially for pregnant women or people taking medication for blood pressure or diabetes problems. 


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When obese people have been studied on alternate-day, intermittent fasting weight loss regimes, they lost weight even if they ate a high-fat diet on their eating days and up to 500 calories on their fasting day, according to some research. In these studies, 10 percent to 20 percent of people usually drop out but after several weeks, the others adjust. According to nutritionist Krista Varady, who has worked on a series of trials, most people gradually become less hungry on their fast days and don’t gorge on their eating days, either. They’ll eat about 10 percent more than what would be recommended, she found.  

In a study of 107 overweight and obese women ages 30 to 45, half were put on the 5:2 program and half on a low-calorie diet with no timing requirements. After six months, the women on the 5:2 program had lost more weight and belly fat, retained more muscle, and saw more improvements in their blood sugar regulation.

People on fasting weight loss routines have seen some relief from symptoms of asthma and arthritis, possibly because breaks from peak consumption reduce inflammation and give the body a chance to rejuvenate and repair. Fasting lowers insulin and another hormone called insulin-like growth factor, or IGF-1, making the body stop pushing for new cell growth, which could in effect slow aging. 

The results from experiments with animals are more dramatic still. An early mouse study established that two to five days of fasting each month reduced biomarkers for cancer and heart disease. Other data from animal studies suggests that intermittent eating could counteract multiple sclerosis, lupus erythematosus, and type I diabetes. 

Mark Mattson, a neuroscientist at the National Institute on Aging in Maryland, believes that fasting may slow cognitive decline. He himself has been eating meals only later in the day for years. 

If you decide to keep your eating within a time window, 12 hours is a less extreme limit that has been backed up by studies with mice. A 7:30 a.m. breakfast would keep you away from food all evening after an early dinner — which might serve you well, since people who eat at night tend to eat more. 

Blogger James Clear reports that he eats one meal at 1 p.m. and another at 8 p.m. each day, and doesn’t eat again for the next 16 hours. After his first year on this regime, he decreased his body fat even though he cut his training time at the gym from 7.5 hours to 2.5. The internet abounds in similar anecdotal reports; you might browse this active Reddit board, this site by a personal trainer, and this one from a diet coach, and see if you’re inspired to try. Be prepared to be uncomfortable for at least two weeks, but the results may motivate you to stick with it.   


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April 09, 2020

Reviewed By:  

Christopher Nystuen, MD, MBA