Is Fasting Beneficial for Weight Loss?

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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.

In another study with follow-ups ranging up to 50 months, only 4 out of 25 so-called “superobese” patients achieved even partial sustained success. Based on these kinds of data, some investigators concluded that “complete starvation is of no value in the long-term treatment of obese patients.”

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.

Put six such studies together, and hundreds of obese subjects fasted for an average of 44 days, lost an average of 52 pounds, and around one or two years later, 40 percent retained at least some of the weight loss. So, most gained all their weight back, but 40 percent is extraordinary for a weight-loss study. Following 100 obese individuals getting treated at a weight loss clinic with a standard low-calorie diet, researchers found only 1 out of 100 lost more than 40 pounds, and only about 1 in 10 even lost 20 pounds, with the overall successful weight maintenance at only two patients over two years. That’s why having a control group is so important. What may look like a general failure may actually be a relative success, compared to more traditional weight loss techniques.

Researchers new to the field may find it clearly disappointing that a year later, two-thirds were “failures,” with more than a third regaining all the weight they had initially lost. But 12 percent were labeled successes, maintaining 59 pounds of weight loss two years later. They lost massive amounts of excess weight and kept it off. In a direct comparison of different weight loss approaches at the same clinic, five years after initiating a conventional low-calorie approach, only about one in five was down 20 pounds, compared to nearly half in the group who instead had undergone a few weeks of fasting years previously. By year seven, most of those instructed on daily caloric restriction were back to, or had exceeded, their original weight, but that was only true for about one in ten of the fasted group. In an influential paper in the New England Journal of Medicine on seven myths about obesity, fallacy #3 was that “Large, rapid weight loss is associated with poorer long-term weight-loss outcomes, as compared with slow, gradual weight loss.” In reality, the opposite is true. The hare may end up skinnier than the turtle.

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. Many patients reported they thought the temporary loss was worth it, though. As a group, they lost an average of about 60 pounds. They described improved health and quality of life, claiming re-employment was facilitated, and earnings increased during that period of time. But 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. In a retrospective long-term comparison of weight reduction after an inpatient stay at a naturopathic center, those who were fasted lost more weight at the time, but at around seven years were back to the same weight. No surprise, since most reported returning to the same diet they were on before. Those who were instead placed on a healthier, more whole food plant-based diet were more likely to make persistent changes in their diet, and seven years later were lighter than when they started. Why can’t you have it both ways, though? You could use fasting to kickstart a big drop, and then start a healthier diet. The problem is that big drop is largely illusory.

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. Wait, how can eating fewer calories lead to less fat loss? Because during fasting, your body starts cannibalizing itself and burning more of your protein for fuel. Emperor penguins, elephant seals, and hibernating bears can survive just burning fat without dipping into their muscles, but our voracious big brains appear to need at least a trickle of blood sugar, and if we’re not eating any carbohydrates, our body is forced to start turning our protein into sugar to burn. 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.

Less than half of the weight loss during the first few weeks of fasting ends up coming from your fat stores. So, even if you double your daily weight loss on a fast, you may be actually losing less body fat. An NIH-funded study placed obese individuals on an 800- calorie-a-day diet for two weeks, and they steadily lost about a pound of body fat a day. Then they switched them to about two weeks of zero calories, and they started losing more protein and water, but, on average, only lost a few ounces of fat a day. When they were subsequently switched back to the initial 800-calories-a-day for a week, they rapidly replaced the protein and water. And so, the scale registered their weight going up, but their body fat loss accelerated back to the approximate pound a day. The scale made it look as though they were doing better when they were completely fasting, but the reality is they were doing worse. So, during the five-week experiment, they would have lost even more body fat sticking to their calorie-restricted diet than completely stopping eating in the middle. They would have lost more body fat, eating more calories. Fasting for a week or two can interfere with the loss of body fat, rather than accelerate it.

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• body fat
• caloric restriction
• calories
• fasting
• obesity
• weight gain
• weight loss