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Rather than cutting calories day in and day out, what if you instead just ate as much as you wanted every other day? Or for only a few hours a day? Or fasted two days a week? Or five days a month? These are all examples of intermittent fasting regimens. That may even be the way we were built. Three meals a day may be a relatively novel behavior for our species. For millennia our ancestors may have only “consumed only one large meal a day or went several days [at a time] 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, where you put stress on your heart and muscles, and as long as there’s sufficient recovery time, you are all the healthier for it. Is that the case with intermittent fasting? Mark Twain thought it was: ‘‘A little starvation can…do more for the average sick man than can the best medicines and the best doctors. [Not just] a restricted diet, [but] 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 devout Muslims abstain from food and drink from sunrise to 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 carbs and fat, but we run usually out of our glycogen—our carbohydrate stores—within 12 and 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 the hours of 18 and 24 of the 72-hour period. 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 becomes so famished on their fasting day that they turn the next into a feasting day, and overeat? If you ate more than twice as much as you normally would, then 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 in one day than non-fasted mice would regularly eat in two. That is not, however, what happens in people.

Randomized to fast from 8 p.m. the night before to 8 a.m. the next day, 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 that didn’t fast at all. That would leave them with a large calorie deficit, equivalent to a daily calorie 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, typically 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 few-hundred calorie so-called “modified fast” day, which may lead to better compliance.

Some studies found study subjects appeared to eat no more or even less on days after a day-long mini-fast. Even within studies, great variability is reported. In a 24-hour fasting study, where folks 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. This means some 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 ramps 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 which to compare. A similar study with a control group found a similar amount of weight loss—about ten pounds—over twelve weeks in a group of normal weight (meaning overweight, on average) individuals. For these modified regimens where people are getting prescribed 500 calories on their quote-unquote “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 instead came upon a brilliant idea of unlimited above-ground vegetables. Starchy root vegetables are relatively calorie-dense, compared to other vegetables. But vegetables that grow above the ground, including stem vegetables like celery and rhubarb, flowering vegetables like cauliflower, leafy vegetables like, well, leafy vegetables, and then all the fruits we tend to think of as vegetables, like tomatoes, peppers, okra, eggplant, string beans, summer squash, and zucchini. So, instead of just prescribing a certain number of calories for their “fasting” days, subjects alternated between their regular diet and helping themselves every other day to an all-you-can-eat above-ground vegetable feast (along with naturally non-caloric beverages, like green tea or black coffee). After eight weeks, subjects lost an average of 13 pounds and two inches off their waist.

The same variability discovered for calorie compensation has also been found for weight loss, though. In a twelve-month trial in which subjects were instructed to eat only one-quarter of their calorie 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 on their fast days with the calorie restriction.

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? Zero-calorie alternate-day fasting was compared 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 ended, both groups had maintained a similar degree of weight loss (still both down about a dozen pounds).

The hope that intermittent fasting would somehow avoid the metabolic adaptations that slow weight loss or improve compliance don’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. And the largest, longest trial of alternate-day fasting found that it may even be less sustainable than more traditional approaches. By the end of a year, the drop-out rate of the alternate-day fasting group was 38 percent, compared to 29 percent in the continuous calorie-restriction group. Though to date, alternate-day fasting regimens haven’t been shown to produce superior weight loss, for the individuals that may prefer this pattern of calorie restriction, are there any downsides? We’ll find out, next.

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