Fasting and weight loss

Fasting obviously causes consistent, dramatic weight loss, but how do fasted individuals do long-term? Some research groups reported extremely disappointing results. Here’s what they saw. Average subject started out at about 270 pounds and, in the six months before the fast, continued to gain weight as obese persons tend to do. Then, after 24 days of what they called “inpatient starvation,” a dramatic 27-pound weight loss. Then what do you think happened? They gained it all back and more, though one could argue if they had not fasted, they might have been up around here at that point.

Other research teams reported better outcomes. One series of about 100 individuals found that 60 percent either retained at least some weight loss at follow-up or even continued losing. The follow-up periods varied from 1 to 32 months, with no breakdown as to who fasted how long, though—making the data hard to interpret. One year after fasting, 62 patients down 16 pounds in 10 days. In another study, 40 percent retained at least 7 pounds of that weight loss.

Researchers set up a study comparing the sustainability of weight loss at three different speeds: six days of fasting versus three weeks of a very-low-calorie diet, 600 calories a day, or six weeks of a low-calorie diet, 1,200 calories a day. The question is, what happened a year later? A year later, the fasting group was the only one that sustained a significant loss of weight. That was just one year, though; how about nine years later?

This is the largest, longest follow-up study I could find. At least some of the fast-induced weight losses were maintained a year later by the great majority. A year later, 90 percent remained lighter than they started out, but then two years later, three years, four years, seven years, and by nine years later, that number dropped to fewer than 1 in 10. By then, almost everyone had regained the weight they had initially fasted away. The fasting didn’t appear to result in any permanent change in eating habits for the vast majority.

The small minority for which fasting led to sustainable weight loss “all admit to a radical change in previous eating habits.” Fasting only works long-term if it can act as a jumpstart to a healthier diet.

Fasting for a week or two can cause more weight loss than calorie restriction, but paradoxically, it may actually lead to less loss of body fat. Because during fasting, your body starts cannibalizing itself and burning more of your protein for fuel. Even just a few grams of carbs, like people who add honey to their water when they fast, can cut protein loss up to 50 percent.

What about adding exercise to prevent loss of lean tissues during a fast? It may make it worse! At rest, most of your heart and muscle energy needs can be met with fat, but if you start exercising, they start grabbing some of the blood sugar meant for your brain, and your body may have to break down even more protein.