 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's a concept in biology called hormesis, which can be thought of as 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? Or 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 backseat 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 issues arise with yom kapoor 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. In contrast, we burn about a 50-50 mix of carbs and fat, but we usually run out of our glycogen or 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 become so famished on their fasting day that they turn the next into a feasting day and overeat? I view 8 more than twice as much as you'd normally would than 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. That non-fasted mice would regularly eat in two. That is not, however, what happens in people. Randomized a 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% more the day after they broke the fast compared to a control group that didn't fast at all. How 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% 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 appear 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% to 110%. 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 10 pounds over 12 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, a researcher 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 up 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, 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 8 weeks, subjects lost an average of 13 pounds and 2 inches off their waste. The same variability discovered for calorie compensation has also been found for weight loss, though. In a 12-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, 10 out of 10 alternate-day fasting studies showed significant reductions in body fat. Small short-term studies show about a 4% to 8% 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 8 weeks. Both groups lost the same amount of weight, about 17 pounds, and in the follow-up check-in, 6 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 improved 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 calorie restriction, and the largest longest trial of alternate-day fasting found that may even be less sustainable than more traditional approaches. By the end of the year, the dropout rate of the alternate-day fasting group was 38% compared to 29% 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.