The study, published in JAMA Internal Medicine, followed people through six months of trying to lose weight and six months of a maintenance diet. During the first six months, one third of the 100 subjects could eat whatever they wanted; one third had three meals a day provided, making up 75 percent of their calorie needs (so, 1500 calories a day if they would normally eat 2000); and the fasting group alternated between a 25 percent (500 calories) day and a 125 percent (2500 calories) day.
By the end, both groups kept off the same amount of weight (just 5-6 percent, which is 10-12 pounds for a 200-pound person) and had similar numbers for blood pressure, heart rate, cholesterol, insulin resistance, fasting glucose, and more.
The biggest difference between groups? The dropout rate. The fasting group lost 13 out of 34 people (38 percent), with five of those saying they were quitting because they hated the diet. The group on the steady diet only saw 29 percent of their members leave, and none of those cited the diet as the reason. The control group lost 26 percent of people. Remember, these folks all had to keep in touch with the researchers for a year, and the dropout numbers include people who just plain flaked out. The averages above, like the 10-12 pounds lost, include the people who dropped out. So that means weight loss may have worked a little better for the intermittent fasters who stuck with it.
The diets ended up being more similar than intended. People ended up eating more than just the provided food, and they ate too much on fast days and too little on feast days. That’s another way in which this diet was hard for people to stick to.
So does this study prove intermittent fasting is nothing special? Yes and no. The subjects were “metabolically healthy” obese women, while proponents of IF often say its value is in fixing a broken metabolism. And the food they ate was pretty standard, carb-heavy fare: 55 percent carbs, 30 percent fat, 15 percent protein. Many intermittent fasters combine the regimen with lower carb food, relying on protein, fat, and fiber to provide most of their calories.
Finally, it’s just one type of fasting. The 5:2 diet gives you a little more time between fasts. Another common way to fast is to go 18 hours of each day without eating: basically, skip breakfast and eat nothing between yesterday’s dinner and today’s lunch. We don’t know if these other formats would be better or worse than alternate-day fasting, but you can always try them and see.
Bottom line, intermittent fasting isn’t good enough to blow traditional dieting out of the water. But it’s worth a try, if you think it might work for you.