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Q. What is alternate day fasting?
A. Although there are three main ways to fast, intermittent fasting ultimately comes down to limiting your eating for a certain amount of time. For some forms of fasting, that means shrinking the amount of time you’re allowed to eat during a day down to a small window. The popular pattern right now is to fast for 16 hours and eat all of your meals during the remaining 8 hours. The Warrior diet is another form of this with a smaller eating window.
For alternate day fasting, that means one fasting day during which you cut calories very low, followed by a “feasting” day or non-fasting day during which you eat whatever you want. Then you just repeat. The 5:2 method and eat-stop-eat are variations on alternating your days. In 5:2, you have calorie restriction to very low levels two days per week and eat normally for the remaining five days. Eat-stop-eat requires a full 24-hour fast once or twice a week and, again, regular eating on the remaining days.
Fasting historically comes from religion. Think of the Ramadan fast, during which believers don’t eat or drink from dawn to sundown for a month every year. Multiple religions have periods of fasting. But despite the long history, fasting didn’t become popular until about two years ago. We’ve only been researching the potential health benefits of fasting for a short time. I’ve been researching fasting for 15 years, though it’s been easier to conduct more research in the last two years. Fasting may potentially help with weight loss, lowering insulin levels, and preventing heart disease, though there are some benefits people talk about that simply haven’t been proven yet.
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Generally, in alternate-day fasting, people eat 500 calories on a fast day, followed by a day of eating normally. You obviously need to watch your calories on the fasting days to ensure you’re not going over, but the next day is untracked. During studies, we allow fasting group participants black coffee, black tea, water, and up to two diet sodas a day to help with the hunger. We limit sodas because artificial sweeteners can make cravings for sweets worse, and the first ten days tend to be a challenge for many people. But if it makes sticking to the plan easier, we’ll allow people to have a tablespoon of cream in their coffee, for example. Chewing gum is also a good trick for getting through hunger while your body gets used to the new style of eating.
Styles of alternate-day fasting differ. Some people do a complete fast for 24-hours with only calorie-free drinks. Other people eat very low calories, around 500 for the entire day. But in both styles, you’re allowed to return to how you normally eat the following day. It’s a clear departure from other types of dieting that generally rely on daily calorie restriction.
We are seeing that alternate-day fasting may be a good tool for some people to help them lose weight. But I tell people to try it if they can typically go for long periods of time without eating or if their jobs make it easy to do that. If you’re someone who likes to eat every two or three hours, it’s probably not a plan that will work for you. People who try it for a limited amount of time and then return to how they were eating before generally gain the weight back. Ideally, you fast one day, feast one day, and repeat forever.
But the weight loss isn’t because there’s something magical about fasting. It simply works because, in general, you’re creating a large enough calorie deficit on the fasting days that you can’t make it up on the feasting days. Most people end up reducing their calorie intake by 10%. So they’re eating fewer calories without tracking their feasting days. Some people report an increased feeling of fullness and less hunger, but that doesn’t happen for everyone.
In terms of your metabolism, it will lower—as it does on every diet—as you lose weight. Fasting isn’t a magic bullet for a fast metabolism. There is some indication that fasting may preserve more muscle when losing weight than other diet plans, though. When you lose weight, it’s typical for 75% of that weight to be body fat and 25% of it muscle. In some research, the weight loss in people doing alternate-day fasting is only 10% muscle as opposed to this typical 25%.
There are some preliminary clinical trials that suggest it may help lower insulin resistance and so help people with type 2 diabetes. It may also lower your risk of heart disease, but again, these are preliminary findings. Most of the health benefits of fasting that people talk about a lot, like increased longevity, are based on studies done in animals. We don’t know at this point that those benefits will hold true in humans.
Another common claim about fasting is that it promotes autophagy. People simply don’t know that the one study we have on this is done in worms. We don’t have a way to measure autophagy in other animals or humans, so it’s impossible for us to say that this happens or to what degree when people fast. There are researchers working on ways to measure this, and we may be able to in the near future. It just isn’t possible yet.
I would say fasting likely isn’t safe for pregnant women, children, and seniors, who tend to suffer from muscle wasting and so don’t need to lose any more muscle mass through weight loss. People with a history of eating disorders, especially binge eating disorder, shouldn’t try this type of eating as the restriction or larger meals may be triggering. Past research on people with a history of bingeing shows they also don’t lose weight through alternate-day fasting. These individuals tend to eat more on the “feast” days than other people, eliminating the calorie deficit that leads to weight loss.
While you can try fasting if you’re on medication, you need the approval of your doctor and need to work with them closely throughout the process. Losing weight can change how much medication you need, so any weight loss effect needs to be monitored by a medical professional.
But I’d also say that people who don’t eat similarly to this now shouldn’t try it. The most effective weight loss plan is something similar to your current lifestyle since it’s easier to maintain. Ultimately, people who lose weight through fasting don’t lose more weight than those who slim down on other plans. Again, fasting isn’t magic. It comes down to what you can keep doing.