Fasting diets are trendy these days, but they may be no better for weight loss than traditional diets, according to a new study.
Researchers looked at a weight-loss method called "alternate-day fasting," in which people drastically reduce their calorie intake every other day, but eat more than usual on nonfasting days.
The researchers randomly assigned 100 obese adults to one of three groups: an alternate-day fasting group, a traditional diet group and a group that did not diet at all. Participants in the alternate-day fasting group consumed just 25 percent of their typical calorie intake — about 500 calories — on fasting days, and 125 percent of their typical intake on nonfasting days. In contrast, those in the traditional diet group consumed 75 percent of their typical calorie intake every day.
After six months, the people in both the fasting group and the traditional diet group had lost about 7 percent more of their body weight, on average, compared with the group that did not diet. And after a year, the participants in both diet groups had maintained a weight loss of 5 to 6 percent of their original body weight. There was no significant difference between the group that did the alternate-day fasting and the group that followed the traditional weight-loss method, the researchers said.
What's more, 38 percent of the participants in the fasting group dropped out of the study before the one-year mark, in most cases because they were dissatisfied with the diet, compared with 29 percent who dropped out in the traditional diet group. Participants in the fasting group also tended to "cheat" on fasting days by eating more than their diet allowed, and they consumed slightly less than they were allowed on nonfasting days, the researchers noted.
"Alternate-day fasting has been promoted as a potentially superior alternative to daily calorie restriction under the assumption that it is easier to restrict calories every other day," the researchers, from the University of Illinois at Chicago. But the new findings show that this is not the case. "These findings suggest that alternate-day fasting may be less sustainable in the long term, compared with daily calorie restriction, for most obese individuals," the researchers said.
The study also found no difference in blood pressure, heart rate, triglyceride levels, blood sugar levels or insulin levels, between the two diet groups.
Fasting diets such as the "5:2 diet," which involves fasting just two days a week and eating normally on the other five, have risen in popularity in recent years. Some previous research suggested that fasting diets lead to just as much weight loss, and are easier to stick with, than traditional diets. But these studies have tended to be small and short term. The new study is one of the largest and longest-running trials to look at the effects of alternate-day fasting, the researchers said.
Still, some obese people may prefer this type of fasting diet over a traditional diet that restricts calories every day, the researchers said. Future studies could examine traits that make alternate-day fasting more tolerable for some people than others — for instance, it may be that some people find it easier than others do to go for long periods without eating, the researchers said.
It's also important to note that the study involved obese people who were "metabolically healthy," meaning they did not have any of the typical risk factors for heart disease or diabetes. It's not clear if the findings would be the same in other groups of people, the researchers said.