You eat very little one day, sure, but the next day you can eat all the burritos and burgers you desire. Then, not only do you lose weight more quickly, it makes it psychologically easy to hold on in the long run.
This makes intuitive sense. But a new study published today in the journal JAMA Internal Medicine suggests this might not be the case for a specific type of fasting diet called alternate-day fasting. The researchers randomly assigned 100 obese participants to either fast every other day, go on a more traditional diet, or do nothing at all. After a year, the people in the fasting group didn’t lose more weight, or have more health benefits, compared to the normal weight-loss group — and more people dropped out of the fasting group.
That doesn’t mean that alternate-day fasting doesn’t work, says study co-author Krista Varady, a professor of nutrition at the University of Illinois Urbana-Champaign. It just means that it’s not necessarily much more effective than normal dieting. Varady has led the research on alternate-day fasting, and has even written a book promoting it. (This information is included in the paper’s conflict of interest disclosure.) Her previous studies suggested that this regimen helped people lose weight more quickly, was easier to stick to (once you got over the first two weeks, which are challenging), and had good effects for other health indicators like cholesterol. But those studies only followed the participants for two or three months. Today’s study is the first long-term and randomized study we’ve had on alternate-day fasting, following participants for an entire year. “Some people might be disappointed if you’re pro-intermittent fasting, but it’s just exciting to have this type of high-quality data out there,” says Grant Tinsley, a Texas Tech University professor who has studied intermittent fasting and was not involved in the study.
For today’s study, the people in the alternate-day fasting group were told to eat about 25 percent of their normal daily calories on their fast days and 125 percent on their “feast” days. The people in the normal dieting group just cut about 25 percent of their calories every day. Nobody was allowed to join a gym. At the end of the year, both groups had lost about 6 percent of their body weight, and there weren’t a lot of differences when it came to things like cholesterol, insulin resistance, and blood pressure.
When it comes to the lack of other health benefits, Tinsley says this may be because alternate-day fasting is not exactly a true fasting diet. “Individuals usually take their one meal in the middle of the day, so that breaks up the fasting period in half, whereas some other programs have the goal of prolonging that fasting period because of the various metabolic changes,” he says. “In some ways, it’s really just a very low-calorie intake one day, so it’s possible that could play a role.”
It’s important to note that about 30 people dropped out of the study, which always skews the data, especially when there are only 100 people to begin with. Plus, more people dropped out of the alternate-day fasting group, and the people who remained had trouble sticking to instructions. “I was pretty shocked to find out that people had a harder time sticking to alternate-day fasting than they did daily calorie restriction,” says Varady. “They were eating more calories than prescribed on the fast day and also eating less calories than prescribed on the feast days than what we thought, whereas daily calorie restriction was pretty good at sticking to their calorie goals.”
This means that, in the end, the fasting and normal dieting groups ended up eating much more similar amounts of food. It also suggests that sticking to the diet is challenging in the long run. Jason Fung, a doctor who runs an intermittent fasting clinic and was not involved in the study, says that trying to restrict to 500 calories might be harder than not eating at all. “People have to understand that if you start eating, you’re going to want to continue eating,” he says. “It’s like when you have to go to the bathroom — when you start peeing you gotta pee out the whole thing and it takes a lot of willpower to stop.” In his clinic, he frequently counsels people to fast for an entire day or more and just plan their schedule around that, instead of rummaging in the cupboards and trying to control themselves. He also works to create more individualized plans, though he acknowledges that there are high dropout rates there, too.
Fasting research has only picked up in the past couple of decades, so we still don’t know a lot about why some people might find it easier than others. Ultimately, says Fung, the data suggests that both calorie restriction and fasting are viable options for weight loss.
I’ve done alternate-day fasting for years, on and off, along with regular dieting. I always liked the idea of fasting more, especially since I’m not a snacker and have always had erratic eating patterns anyway. I liked not feeling deprived all the time, and knowing I could have a proper meal in 12 hours helped with the fast days. But over time, it became too hard to stick to.
For now, Fung says, fasting’s effectiveness might depend on preference. “I don’t think that there’s one approach that really works for everybody and you really have to find what people like, some people like chocolate and some people like vanilla, you can’t say one is wrong and one’s right,” he adds. “It’s a personal thing, but in a randomized trial like this, you’re taking people and saying ‘you are getting chocolate, do you like it?’ and might be shoehorning them into a dietary method which doesn’t work.”
Fung has a point: it really is incredibly easy to go over the fast day allotment, which for someone of my size is 500 calories — basically two Clif bars. But I also run and just can’t go entire days without eating. The schedule was hard on my social schedule, since people unfortunately don’t invite you out to dinner on neatly alternating days.
Alternate-day fasting isn’t the only type of fasting diet, Tinsley, the Texas Tech professor, points out. There’s the 5:2 diet, where you fast for only two days a week, or various types of intermittent fasting where you only eat within a certain time interval. There’s even a “fasting-mimicking diet” where you fast for five days a month that’s been shown to have good results, though that version of fasting hasn’t been used enough to have a lot to compare it to. Varady adds that today’s results shouldn’t be generalized too broadly. Next, her team is doing research on the effects of alternate-day fasting on diabetics.
In the end, the diet made me too irritable on my fast days and even the promise of a large meal in the future didn’t make me less prone to yelling at everyone. I still go back to ADF every so often, but I’ve become more realistic about what I can handle.