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Could fasting every other day help you lose more weight?

As much as you might hate the daily restrictions of a conventional diet, a new study shows that fasting one day and eating what you want the next may not be a better way to lose weight.

After one year, researchers found that weight loss on either type of diet was about the same — about 6 percent of body weight on an alternate-day fasting diet and 5.3 percent for those on the daily restricted-calorie diet.

"We thought the alternate-day fasting group would do better. It allows people to have a break from dieting every other day, so we thought their adherence would be better," explained lead author Krista Varady. "But it turns out people in both diets lost the same amount of weight." 

Diet trends: What to try, what to skip
Diet trends: What to try, what to skip

Varady, who is an associate professor of kinesiology and nutrition at the University of Illinois at Chicago, said people seemed to have trouble sticking to the fasting requirements. Dieters were supposed to consume only 500 calories on fast days. 

"People in the alternate-day fasting group were eating more than the 500 calories prescribed on the fast day, but a lot less than the calories prescribed on the feast day. That's why they lost the same amount of weight," she explained.

However, "people who stuck to the [alternate-day fasting] diet lost 20 to 50 pounds in a year," Varady added. "It does work for some people."

In the study, Varady's team randomly assigned 100 obese people to either an alternate-day fasting diet, a conventional diet involving a 25 percent reduction in calories every day, or no diet at all (the "control" group).

People on the conventional diet were able to stick to their goals better than the alternate-day fasting group, the researchers found.

The dropout rates proved the point: 38 percent of the alternate-day fasting group quit, while only 29 percent of the conventional diet group and 26 percent of the control group gave up.

It's hard for people to stick to only 500 calories in one day, Varady explained. "Certain people are suited to this type of diet. If someone were to pick this diet for themselves, they probably would do better," she said.

The fasting diet appeared to be safe, she added. People on the plan were encouraged to eat a lot of protein on their 500-calorie days, because protein makes you feel full, she said.

In terms of weight loss, all calories are the same, but not all calories are healthy ones, Varady said. On days when people could eat what they chose, some ate bags of chips and still lost weight, she noted.

"If you are reducing your food intake, you are going to lose weight, but in terms of health benefits, people should try to eat less processed foods and more fruits and vegetables," Varady said.

Why "ultra-processed" foods pose health hazards 03:03

Some people really love this lifestyle and have been on the fasting diet for years, but it's not for everyone, Varady said. "People should find what works for them," she added.

The report was published online May 1 in the journal JAMA Internal Medicine.

One specialist not involved in the study, however, isn't convinced that a long-term fasting diet is healthy.

"There are some experts who suggest that intermittent fasting may help with weight control, but for markers for heart disease and diabetes management, the jury is still out — especially on how healthy and sustainable this approach is," said Samantha Heller, a senior clinical nutritionist at New York University Medical Center.

Fasting on alternate days feels punitive to many, and may exacerbate an already difficult and complex relationship someone has with food, Heller explained.

In addition, the body does not know that restricting food is a choice and views severe calorie restriction as a crisis, Heller said.

"Intermittent fasting does not teach strategies for making healthy choices and managing life's ups and downs," she added.

"A lifestyle overhaul — one that a person can maintain for long term that provides a healthy, balanced diet as well as pleasurable foods — is what I would like to see people embrace," Heller said. "These kinds of changes take time, motivation and ongoing support."

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