Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

I talked about the benefits of calorie restriction. Well, the greatest caloric restriction is no calories at all. Fasting has been branded the “next big weight loss fad,” but has a long history throughout various spiritual traditions, practiced by Moses, Jesus, Muhammed, and Buddha. In 1732, a noted physician wrote, “He that eats till he is sick must fast till he is well.” Today, about one in seven American adults report taking that advice using some sort of fasting as a means to control body weight.

Case reports of the treatment of obesity through fasting date back more than a century in the medical literature. In 1915, two Harvard docs indelicately described “two extraordinarily fat women,” one of whom was a “veritable pork barrel.” Their success led them to conclude that “moderate periods of starvation constitute a perfectly safe, harmless, and effective method for reducing the weight of those suffering from obesity.”

The longest recorded fast, published in 1973, made it into the Guinness Book of World Records. To reach his ideal body weight, a 27-year-old man fasted for 382 days straight, losing 276 pounds and managed to keep nearly all of it off. He was given vitamin and mineral supplements, so he wouldn’t die, but no calories for more than a year. In their acknowledgements, the researchers thanked him for his “cheerful cooperation and steadfast application to the task of achieving a normal physique.”

A U.S. Air Force study of more than 20 individuals at least 100 pounds overweight, most unable to lose weight on previous diets, were fasted for as long as 84 days. Nine dropped out of the study, but the 16 who remained were unequivocally successful at losing between 40 and 100 pounds. In the first few days, subjects were noted losing as much as four pounds a day. This was mostly water weight as the body starts to adapt, but after a few weeks, they were steadily losing about a pound of mostly straight fat a day. The investigator described their starvation program as a “dramatic and exciting treatment for obesity.”

Of course, the single most successful diet for weight loss—namely no diet at all—is also the single least sustainable. What other diet can cure morbid obesity in a matter of months, but practically be guaranteed to kill you within a year if you stick with it? The reason diets don’t work, almost by definition, is that people go on them, and then they go off of them. Permanent weight loss is only achieved through permanent lifestyle change. So, what’s the point of fasting if you’re just going to go back to your regular diet and gain it all right back?

Fasting proponents cite the psychological benefit of realigning people’s perceptions and motivation. Some individuals have resigned themselves to the belief that weight loss for them is somehow impossible. They may think they’re just “made differently” in some way, and no matter what they do, the pounds don’t come off. But the rapid unequivocal weight loss during fasting demonstrates to them that with a large enough change in eating habits, it’s not just possible, but inevitable. This morale boost may then embolden them to make better food choices once they resume eating.

The break from food may allow some an opportunity to pause and reflect on the role food is playing in their lives—not only the power it has over them, but the power they have over it. In a fasting study entitled “Correction and Control of Intractable Obesity,” a person’s personality was described as changing “from one of desperation, with abandonment of hope, to that of an eager [extrovert] full of plans for a promising future.” She realized that her weight was within her own power to control. They concluded: “This highly intellectual social worker has been returned to a full degree of exceptional usefulness.”

After a fast, newfound commitments to more healthful eating may be facilitated by a reduction in overall appetite reported post-fast, compared to pre-fast, at least temporarily. Even during a fast, hunger may start to dissipate within 36 hours. So, challenging people’s delusions about their exceptionality to the laws of physics with a period of total fasting may seem barbaric, but in reality, this method of weight reduction is remarkably well tolerated by obese patients. That seems to be a recurring theme in these published series of cases. In this influential paper, “Treatment of Obesity by Total Fasting for Up to 249 Days,” the researchers remarked that the most surprising aspect of the study was the ease with which the prolonged fast was tolerated.” All their patients evidently “spontaneously commented on their increased sense of well-being” throughout the process—even, “in some cases frank euphoria.” Though it’s essential that “fasting should only be prescribed under close medical supervision,” they concluded that they were “convinced that it is the treatment of choice, certainly in cases of gross obesity.”

Fasting for a day can make people irritable, moody, and distracted, but a few days in, many report feeling clear, elated, and alert—even euphoric. This may be in part due to the significant rise in endorphins that accompanies fasting. Mood enhancement during fasting is thought to perhaps represent an adaptive survival mechanism to motivate the search for food. This positive outlook towards the future may then facilitate the behavioral change necessary to lock in some of the weight loss benefits.

But is that what happens? Is it actually effective over the long term? And I’m sure you saw titles like this flash by. Is it even safe? We’ll find out, next…

Please consider volunteering to help out on the site.