Does Alternate-Day Fasting Work? It Depends on the Person


Does a feast-and-fast cycle of dieting work? Chris Ryan/Getty Images\”Does a feast-and-fast cycle of dieting work? Chris Ryan/Getty Images\”Does a feast-and-fast cycle of dieting work? Chris Ryan/Getty Images

Alternate-day fasting is a hot dieting trend. But does it work? That\’s what researchers at the University of Illinois at Chicago wanted to find out.

"I\’ve been studying this diet for about 10 years now," says Dr. Krista Varady, one of the researchers. She notes that they had already run six to eight short-term clinical trials, which yielded positive weight loss results for alternate-day fasting. "The main point of this study was to see if people can stick to this diet long-term — to compare it to a traditional weight loss approach like daily calorie restriction."

Alternate-day fasting (ADF) involves restricting your calorie intake to 25 percent of the recommended amount, which works out to around 500 calories, every other day. On your "feast" days, you eat up to 125 percent of your recommended calorie intake. The idea behind it is that fasting activates a gene called SIRT1 that aids in revving up your weight loss.

Little research had been done, though, to show whether this diet actually works over time. For this project, the researchers assigned 100 obese adults to one of three groups: the ADF group, the daily calorie restriction group (75 percent of calorie needs every day) and the control group, which had no particular diet. The groups were followed for one year.

The scientists had thought, based on their previous trials, that the ADF diet would result in greater weight loss than standard day-in-day-out calorie restriction diets. But the length of study revealed the true facts. The ADF diet performed about evenly with traditional daily calorie restrictions over the one-year period (six months of active weight loss and six months of maintenance).

The ADF group lost about 6 percent of their body weight over the study period while the daily calorie restriction group lost 5.3 percent. There were no major differences in their blood pressure, fasting glucose, fasting insulin and triglycerides, all measures of cardiovascular disease.

 "We found that the alternate-fasting group had a hard time sticking to the fasting calories. The other group stuck to calorie goals better day in and day out," says Varady, a nutrition professor at University of Illinois, Chicago. People in the ADF group tended to eat more than prescribed on fast days and less than prescribed on feast days. 

However, she points out that the members of the ADF group who really committed to the diet experienced greater results. "Fifteen people out of the alternate fasting group stuck to it really well, and they lost a lot more weight," she says, noting that their weight loss ranged between 20 and 50 pounds. "It is effective; it\’s just a question of adherence. People need to find something they can incorporate into their lifestyle."

Indeed, alternate-day plans tend to be much simpler and less restrictive than some other diet plans which eliminate whole food groups. The study participants could have any kind of background diet, Varady says. "Of course, we recommended people eat healthier, more fruits and veggies, less processed foods." The diet also seemed to work better for people who could wait longer between meals. "People who can go for five to six hours without eating do well with alternate-day fasting, whereas people who eat every two to three hours tend to quit the diet," she explains.

What Some Experts Think of ADF

But the experts are divided on whether alternate-day fasting is a good idea. "Skipping meals, restricting calories and not giving your body the right kind of nourishment will end up damaging the body," says Mumbai-based nutritionist Tehzeeb Lalani in an email. "A few downsides include, but are not limited to, a metabolism which slows down and leads to weight gain eventually, sugar cravings, nutrient deficiencies, headaches, dizziness, fatigue, lethargy [and] food fears."

Lalani, who did her training at NYU, notes that all her clients who used this diet said that while they did lose weight, they put it back on, plus some extra pounds. "Their metabolism has slowed down and in our counseling sessions, they have a hard time eating any food without feeling guilty." 

Other experts expressed concern that ADF could trigger eating disorders — people struggling with bulimia may binge on feast days and those struggling with anorexia may continue restricting full-time. "Even those without a history of an eating disorder may binge on non-fasting days due to an increase in hunger, which could lead to weight gain as well as blood sugar control problems," emails therapist Kimberly Hershenson, who treats people with eating disorders in New York City. "[ADF] also may lower your metabolism because your body goes into starvation mode." 

However, Caroline Apovian, M.D., director of the Nutrition and Weight Management Center at the Boston Medical Center, says she highly recommends a form of ADF called intermittent fasting. (In her version, the person fasts one day a week.) "In my practice, I have found that while intermittent fasting is incredibly effective, it is also difficult to sustain for many who have a significant amount of weight to lose," she emails. "As a result, I combine the principle of intermittent fasting with high protein smoothies."

Now That\’s Interesting

There are several versions of intermittent fasting. One involves eating all your meals within an eight- or 10-hour window. Atlanta-based Gin Stephens used this approach to lose more than 80 pounds. "An eating-window approach is much easier for me [than ADF], because I get to eat whatever I love every day of my life," she says in an email. Stephens wrote a book about intermittent fasting in 2016.


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