The 60 healthy people in the four-week study were not overweight. Researchers randomly assigned them to either stick to their usual eating habits or switch to alternate-day fasting, with 12 hours of unrestricted food followed by 36 hours of no food.
With alternate-day fasting, people reduced weekly calories by 37% on average and shed an average of 3.5 kilograms (7.7 pounds). That compares with an average calorie reduction of 8.2% and an average weight loss of 0.2 kilograms (0.44 pounds) without this diet.
“We do not recommend this as a general nutrition scheme for everybody, because this is a harsh intervention of which we do not know the long-term effects,” said Frank Madeo, senior author of the study and a researcher at the University of Graz in Austria.
“We feel that it is a good regime for some months for obese people to cut weight,” Madeo said.
To ensure that people assigned to alternate-day fasting didn’t eat on fasting days, researchers asked them to wear continuous glucose monitors. Spikes in blood glucose levels might mean people had a snack. Researchers also asked participants to fill in food diaries documenting their fasting days.
After 4 weeks of alternate-day fasting, people had more lean muscle and less body fat, lower cholesterol levels and improved heart health – all things that can happen with a wide variety of exercise and nutrition programs.
To get a sense of the safety of alternate day fasting, researchers looked at a separate group of 30 people who had been eating this way for at least 6 months, comparing them to healthy people who had not been fasting.
They didn’t find any meaningful negative side effects.
One limitation of the study is that researchers didn’t test the diet in people who needed to lose weight. They also didn’t have any long-term safety data, and many health problems associated with extreme dieting like malnutrition and brittle bones can take much longer than 6 months to develop.
“The ‘starvation mode’ the body goes into during alternate day fasting may have some benefits,” said Susan Roberts, a senior scientist at the USDA Nutrition Center at Tufts University who wasn’t involved in the study.
For example, fasting can improve the body’s ability to use the hormone insulin to transform sugars into energy, a process that can help reduce blood sugar and prevent diabetes, Roberts said by email.
But there isn’t enough safety information about alternate day fasting to recommend it as a regular way of eating to maintain a healthy weight or for weight loss, Roberts said.
“My preferred option to be honest is not to recommend alternate day fasting per se but to use occasional daily fasting as a toolbox option that some people may find helpful,”
Roberts said. “A small percentage of people wanting to lose weight may find it helpful, but we don’t yet know the long-term safety to recommend it with comfort.”