Fasting and weight loss

Fasting can certainly trigger rapid and significant weight loss, but how well does it hold up over the long term? Some research groups have reported surprisingly discouraging results. In one study, participants started at an average weight of about 270 pounds and, as is common with obesity, continued gaining weight in the six months leading up to the fast. After 24 days of what researchers called “inpatient starvation,” they lost a dramatic 27 pounds. But what happened afterward? They regained all the weight—and then some. One could argue, though, that without the fast they might have ended up even heavier.

Other research teams found more promising outcomes. In a group of about 100 individuals, roughly 60 percent either maintained some of the weight loss or continued losing at follow-up. However, follow-up periods ranged widely from 1 to 32 months, and the data didn’t break down who fasted for how long, making interpretation difficult. In another study, one year after fasting, 62 patients who had lost 16 pounds in 10 days retained some of that loss, and 40 percent kept off at least seven pounds.

Researchers also compared the long-term sustainability of weight loss achieved through different rates of dieting: six days of fasting, three weeks of a 600-calorie-per-day very-low-calorie diet, or six weeks of a 1,200-calorie-per-day low-calorie diet. One year later, only the fasting group maintained a significant weight loss. But what about much further out—say, nine years?

The largest and longest follow-up study available shows that while most participants maintained at least some of their fasting-related weight loss after one year—about 90 percent were still lighter than when they started—this success faded over time. After two, three, four, seven, and eventually nine years, the number who maintained the loss dwindled to fewer than one in ten. Nearly everyone regained the weight they had initially lost. For the vast majority, fasting did not lead to lasting changes in eating behavior.

Those few who did maintain long-term weight loss all reported making major, permanent changes to their eating habits. In other words, fasting only works as a long-term strategy if it serves as a catalyst for adopting a healthier diet.

Although fasting for a week or two can produce more weight loss than simple calorie restriction, it may paradoxically result in less actual fat loss. During fasting, the body begins breaking down its own tissues for fuel, burning proportionally more protein. Even a small amount of carbohydrate—such as a bit of honey added to water—can cut this protein loss by up to 50 percent.

What about exercising during a fast to preserve lean mass? Surprisingly, it may actually worsen muscle loss. While at rest the heart and muscles can rely heavily on fat for energy, exercising increases their demand for blood sugar, which is otherwise reserved for the brain. To compensate, the body may have to break down even more protein.