Does eating every other day prevent the metabolic slowing that accompanies weight loss, or does it improve compliance over constant, day-to-day caloric restriction? 

Rather than cutting calories day in and day out, what if you instead ate as much as you wanted every other day or for only a few hours a day? Or, what if you fasted two days a week or five days a month? These are all examples of intermittent fasting regimens, as you can see below and at 0:10 in my video Alternate-Day Intermittent Fasting Put to the Test, and that may even be how we were built. Three meals a day may be a relatively novel behavior for our species. For millennia, “our ancestors could not eat three meals every day. They consumed meals much less frequently, and often consumed one large meal per day or went for several days without food.” 

Intermittent fasting is often presented as a means of stressing your body—in a good way. There is a concept in biology called hormesis, which can be thought of as the “that which doesn’t kill you makes you stronger” principle. Exercise is the classic example: You put stress on your heart and muscles, and as long as there is sufficient recovery time, you are all the healthier for it. Is that the case with intermittent fasting? Mark Twain thought so: “A little starvation can really do more for the average sick man than can the best medicines and the best doctors. I do not mean a restricted diet, I mean total abstention from food for one or two days.” 
But, Twain also said, “Many a small thing has been made large by the right kind of advertising.” Is the craze over intermittent fasting just hype? Many diet fads have their roots “in legitimate science,” but over time, facts can get distorted, benefits exaggerated, and risks downplayed. In other words, “science takes a back seat to marketing.” At the same time, you don’t want to lose out on any potential benefit by dismissing something out of hand based on the absurdist claims of overzealous promoters. You don’t want to throw the baby out with the baby fat. 
Religious fasting is the most studied form of intermittent fasting, specifically Ramadan, a month-long period in which “Muslims abstain from food and drink from sunrise until sunset.” The effects are complicated by a change in sleeping patterns and also thirst. The same dehydration issue arises with Yom Kippur when observant Jews stop eating and drinking for about 25 hours. The most studied form of intermittent fasting that deals only with food restriction is alternate-day fasting, which involves eating every other day, alternating with days consuming little or no calories. 
At rest, we burn about a 50:50 mix of carbohydrates and fat, but we usually run out of glycogen—our carbohydrate stores—within 12 to 36 hours of stopping eating. At that point, our body has to shift to rely more on our fat stores. This metabolic switch may help explain why the greatest rate of breakdown and burning of fat over a three-day fast happens between hours 18 and 24 of the 72 hours. The hope is to reap some of the benefits of taking a break from eating without the risks of prolonged fasting. 
One of the potential benefits of alternate-day fasting over chronic calorie restriction is that you get regular breaks from feeling constant hunger. But might people become so famished on their fasting day that they turn the next into a feasting day? After your fasting day, if you ate more than twice as much as you normally would, that presumably would defeat the whole point of alternate-day fasting. Mice fed every other day don’t lose weight. They just eat roughly twice as much food in one day as non-fasted mice would regularly eat in two days. That is not, however, what happens in people. 
Study participants were randomized to fast for a day and a half—from 8:00 p.m. to 8:00 a.m. the second morning after beginning. Fasting for 36 hours only led to people eating an average of 20 percent more the day after they broke the fast, compared to a control group who didn’t fast at all. That would leave the fasters with a large calorie deficit, equivalent to a daily caloric restriction of nearly a thousand calories a day. This particular study involved lean men and women, but similar results have been found amongst overweight or obese subjects. Researchers typically found only about a 10 to 25 percent compensatory increase in calorie intake over baseline on non-fasting days, and this seems to be the case whether the fasting day was a true zero-calorie fast or a modified fast day of a few hundred calories, which may lead to better compliance. 
Some studies have found that participants appeared to eat no more, or even eat less, on days after a day-long mini-fast. Even within studies, great variability is reported. In a 24-hour fasting study where individuals ate an early dinner and then had a late dinner the next day after skipping breakfast and lunch, the degree of compensation at the second dinner ranged from 7 percent to 110 percent, as you can see in the graph below and at 4:40 in my video. This means that some of the participants got so hungry by the time supper rolled around that they ate more than 24 hours’ worth of calories in a single meal. The researchers suggested that perhaps people first try “test fasts” to see how much their hunger and subsequent intake ramp up before considering an intermittent fasting regimen. Hunger levels can change over time, though, dissipating as your body habituates to the new normal. 

In an eight-week study in which obese subjects were restricted to about 500 calories every other day, after approximately two weeks, they reportedly started feeling very little hunger on their slashed calorie days. This no doubt helped them lose about a dozen pounds on average over the duration of the study, but there was no control group with whom to compare. A similar study that did have a control group found a similar amount of weight loss—about ten pounds—over 12 weeks in a group of “normal weight” individuals, which means overweight on average. For these modified regimens where people are prescribed 500 calories on their “fasting” days, researchers found that, from a weight-loss perspective, it did not appear to matter whether those calories are divided up throughout the day or eaten in a single meal. 
Instead of prescribing a set number of calories on “fasting” days, which many people find difficult to calculate outside of a study setting, a pair of Iranian researchers came upon a brilliant idea of unlimited above-ground vegetables. Starchy root vegetables are relatively calorie-dense compared to other vegetables. Veggies that grow above the ground include stem vegetables (like celery and rhubarb), flowering vegetables (like cauliflower), leafy vegetables (like, well, leafy vegetables), and all of the fruits we tend to think of as vegetables (like tomatoes, peppers, okra, eggplant, string beans, summer squash, and zucchini). So, instead of prescribing a certain number of calories for “fasting” days, researchers had subjects alternate between their regular diet and helping themselves to an all-you-can-eat, above-ground vegetable feast (along with naturally non-caloric beverages, like green tea or black coffee) every other day. After eight weeks, the subjects lost an average of 13 pounds and two inches off their waist, as you can see below and at 6:59 in my video

The same variability discovered for calorie compensation has also been found for weight loss, as seen in the graph below and at 7:10 in my video. In a 12-month trial in which subjects were instructed to eat only one-quarter of their caloric needs every other day, weight changes varied from a loss of about 37 pounds to a gain of about 8 pounds. The biggest factor differentiating the low-weight-loss group from the high-weight-loss group appeared to be not how much they feasted on their regular diet days, but how much they were able to comply with the calorie restriction on their fast days. 

Overall, ten out of ten alternate-day fasting studies showed significant reductions in body fat. Small short-term studies show about a 4 to 8 percent drop in body weight after 3 to 12 weeks. How does that compare with continuous calorie restriction? Researchers compared zero-calorie, alternate-day fasting head-to-head to a daily 400-calorie restriction for eight weeks. Both groups lost the same amount of weight, about 17 pounds, and, in the follow-up check-in six months later after the trial had ended, both groups had maintained a similar degree of weight loss; both were still down about a dozen pounds, as you can see below and at 8:10 in my video

The hope that intermittent fasting would somehow avoid the metabolic adaptations that slow weight loss or improve compliance doesn’t seem to have materialized. The same compensatory reactions in terms of increased appetite and a slower metabolism plague both continuous and intermittent caloric restriction. The longest trial of alternate-day fasting found that “alternate-day fasting may be less sustainable” than more traditional approaches. By the end of the year, the drop-out rate of the alternate-day fasting group was 38 percent, compared to 29 percent in the continuous calorie-restriction group.  

Although alternate-day fasting regimens haven’t been shown to produce superior weight loss to date, for individuals who may prefer this pattern of calorie restriction, are there any downsides? Find out in my video Is Alternate-Day Intermittent Fasting Safe?
I packed a lot into this one. Bottom line: Fasting doesn’t appear to provide an edge over traditional calorie cutting, but if you prefer it, why not give fasting a try? Before you do, first check out Is Alternate-Day Intermittent Fasting Safe?.  

What about total fasting? For that and even more, check out the related videos below.

I have a whole chapter on intermittent fasting in my book How Not to Dietorder now! (All proceeds I receive from my books are donated to charity.)