Generally, weight loss can be thought of as a balancing act between calories consumed and calories used: If you eat less than you use, you body uses energy reserves from fat cells, and you lose weight.
Most dieting plans are slow and steady—some combination of curtailing calories by a little less every day, like skipping desserts, or exercising a little more while eating the same amount.
Then there are fasting diets, where for one to five days in a row you reduce your caloric intake to about a quarter of what you usually consume, and then eat more than usual on your off days. In small studies (paywalls) that lasted up to three months, both healthy and overweight patients were able to lose more weight adhering to an alternative-day fasting diet compared to those who consistently ate marginally fewer calories.
In the long-run, though, fasting diets may be no better than regular calorie-restricting diets—because they’re harder to follow. A study published May 1 in JAMA Internal Medicine by researchers from the University of Illinois at Chicago, Stanford, and the Nutrition Obesity Research Center in Louisiana followed 100 overweight participants for a year, broken into three groups: one following an alternative-day fasting diet; one following a more typical calorie-restriction diet; and a control group told not to change either eating habits (no group changed their exercise habits). For six months, participants in the two dieting groups tried to lose weight; the next six, they tried to maintain their new weight. By the end of the year, both fasting- and regular-diet participants lost about the same amount of weight.
Although the study’s results don’t make the case against fasting diets in general, researchers did notice some problems with them. First, a lot of people dropped out of the fasting diet group. Of the 100 who were initially recruited, 34 were assigned to fasting diets, and nine quit early—only six dropped out of each other group. And for the fasting dieters who did make it through the full year, the researchers noticed it was a lot harder for them to hit their calorie targets each day; over time, their “fasting” meals looked more like a typical calorie-restricting diet.
The study was divided into phases. For the first six months, participants were supposed to be focused on losing weight. For the fasting group, meals on “fast” days were 25% of recommended caloric intake (based on each person’s weight), and on “feast” days, they were 125% of recommended caloric intake. The regular diet group got meals that were 75% of their recommended caloric intake each day.
Over the following six months, the study participants were told to maintain their weight; the regular dieters switched to a diet that included 100% of recommended caloric intake for their new weight, while the fasting group switched to a 50% calorie diet for “fast” days, and 150% for “feast” days. All participants were enrolled in regular weight-loss counseling to help them stay on track.
For the first three months, researchers provided all the meals for participants to give them a good idea of what they could make on their own. After that, they were supposed to record what they ate themselves. Researchers found that participants on a restrictive diet were able to keep eating just the right amount. Alternative fasters, on the other hand, tended to eat more on their fasting days and less on their feasting days—which appeared to even out to roughly a consistently restrictive diet.
Through the first six months, restrictive dieters lost only a little more weight than fasters, and after six months each group gained a similar amount of weight back. Cumulatively, the groups lost similar amounts of weight—6% of their starting weight for the fasting group and 5.3% for the restrictive group.
To be sure, the fasting diets did work. “This diet just doesn’t happen to do better than a traditional approach, but it still is an effective diet for weight loss,” Krista Varady, a nutritionist from the University of Illinois at Chicago and lead author of the study, told US News. There’s also evidence that intermittent fasting may have other benefits, like lowering diabetes and cancer risk.
But fasting is hard: It takes serious discipline to ignore the gurgling stomach, headaches, and fatigue of which many who try the diet complain. Even if these diets are better in theory, they may not be better in practice because they’re so difficult to do perfectly.
If fasting every other day works for you, great—but no harm if it’s easier to just cut back gradually. The best diet is the one you stick to.