Alternate-day fasting: how to do it, benefits, effects

Fasting diets have become popular in the U.S. and elsewhere. Several of these diets—including alternate-day fasting—have some solid research backing their benefits. 

Alternate-day fasting is one of the most heavily scrutinized of the various fasting diets. In a nutshell, it involves fasting one day and eating as usual the next. Rinse and repeat. Studies have found that alternate-day fasting can promote lower body weight, improved body composition, and other health benefits (Trepanowski, 2017).

Let’s take a closer look at this form of fasting. 

Researchers define “fasting” in a variety of ways. But generally speaking, to fast means that you’re voluntarily choosing to restrict your food or caloric intake for a longer-than-usual period of time (Sen, 2018). 

Alternate-day fasting—often abbreviated as ADF—involves reducing or eliminating your calorie intake every other day, with days of normal eating sandwiched in-between (Trepanowski, 2017). 

There are a lot of different ways to practice alternate-day fasting. Some permit only low-calorie liquids (water, black coffee, plain tea, etc.) on fasting days, while others allow you to consume 25% or more of a typical day’s calories. Most allow you to eat as you usually would on non-fasting days (Trepanowski, 2017; Stekovic, 2019). 

Before starting, it’s a good idea to speak with a medical professional—such as a nutritionist or physician—to make sure you’re not at risk for any adverse events or side effects (Stekovic, 2019). While research suggests that ADF is safe, some studies looking into ADF have excluded people with Type 2 diabetes, cardiovascular disease, or other chronic medical conditions (Hoddy, 2015).  

In most of the published research to date, including a 2019 study in the journal Cell Metabolism, the practice of alternate-day fasting has been simple and straightforward (Patterson, 2017; Stekovic, 2019):

• On fasting days: You consume essentially zero calories. You’re allowed to drink water, black coffee, plain tea, and non-caloric liquids. Foods are off-limits. 

• On non-fasting days: You can eat what you want when you want.

Part of the appeal of ADF is its simplicity. While you’re permitted to eat whatever you want on non-fasting days (a.k.a., feast days or feeding days), experts recommend that you try to eat a wholesome and balanced diet (Stekovic, 2019).

While not all the research is consistent, most of the trials on ADF suggest that it can promote weight loss, weight maintenance, and reduced waist circumference. There’s also evidence that it can improve scores on tests of metabolic and cardiovascular health, including those related to blood pressure, blood cholesterol (lipids and triglycerides), and blood glucose (Patterson, 2017; Stekovic, 2019). 

Some animal research suggests that ADF may also improve liver health and may reduce your risk for heart disease and some cancers. But this hasn’t been shown in human studies (Patterson, 2017). 

Finally, some work has found that ADF may counteract cell-related processes that are linked with aging. In other words, it may help you live longer. But there’s not yet evidence from clinical trials that ADF will improve longevity (Heilbronn, 2019). 

Most studies that have looked at alternate-day fasting for weight loss have found that it works. One review of the research on ADF found that, depending on the study, people lost on average between six and 12 pounds. Most of these studies were looking at ADF over a period of 8–12 weeks. 

That review also found that the weight-loss benefits of ADF were roughly the same as those associated with intense forms of daily dieting. (Specifically, it compared ADF plans to diets that required people to eat no more than 800 calories per day, every day, for several weeks or more.) 

"The weight loss isn't because there's something magical about fasting", says Krista A. Varady, Ph.D., a Professor of Nutrition at the University of Illinois Chicago and co-author of The Every Other Day Diet: The Diet That Lets You Eat All You Want (Half the Time) and Keep All the Weight Off. "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."

Meanwhile, ADF seemed to promote more fat loss while helping people better hold on to fat-free mass compared to those on very low-cal daily diets (Alhamdan, 2015).

Recently, researchers examined both the short-term (eight weeks) and long-term (six months) effects of alternate-day fasting plans among adults without obesity and underlying conditions. It found that, in both cases, ADF was not associated with any significant side-effects or adverse effects—apart from hunger (Stekovic, 2019).

On the other hand, some review studies have turned up some side effects. These include (Hoddy, 2015):

• Constipation

• Bad breath

• Problems sleeping through the night

• Dizziness

• Weakness

Most of these side effects were minor. However, the existing research hasn’t taken a hard look at cognitive effects—such as problems thinking or concentrating. Finally, and most importantly, there is very little work on the long-term effects of ADF, especially among people with obesity or other health conditions (Hoddy, 2015).

"I would say fasting likely isn't safe for pregnant women, children, and seniors, who tend to suffer from muscle wasting and 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", explains Dr. Varady.

It’s tough to go an entire day without eating. Some studies have found that, even after several weeks of ADF, people don’t feel any less hungry on their fasting days. In other words, they don’t get used to the diet (Heilbronn, 2005). 

In order to make things somewhat easier, researchers have examined the effects of so-called “modified” ADF plans. These permit some eating on fasting days, as opposed to the traditional zero-calorie approach.

Some of the modified-ADF research has found that it beats out old-school dieting (trying to restrict calorie intake daily) when it comes to weight loss, waist circumference, and blood pressure. Compared to traditional dieting, modified-ADF plans tend to involve more severe caloric restriction on fast days and more-relaxed eating on non-fast days (Parvaresh, 2019). 

On the other hand, some studies have found that, while they can help people lose weight, modified ADF plans are no more effective than old-school calorie restriction diets among healthy adults with obesity (Trepanowski, 2017).

While the research on them is mixed, here’s what one research-validated modified ADF protocol looks like (Parvaresh, 2019): 

• On fast days: You’re permitted to eat 25% of a typical day’s calories. You must eat at midday—between the hours of noon and 2 pm. Your fasting days are Saturday, Monday, and Tuesday. 

• On non-fast days: On Sunday, Tuesday, and Thursday, you’re permitted to eat 100% of a typical day’s calories. On Friday, you can eat as much as you want.

If you’re going with this “modified” approach, you may be wondering how to figure out what 25% and 100% of a day’s calories are. Researchers use sophisticated measurements to work this out, but you can come up with a ballpark figure by using this free calculator from the U.S. Department of Agriculture (USDA). 

Again, it’s not clear which of these—traditional ADF or modified ADF—is best. There’s not much research comparing the different approaches. 

The bottom line is that alternate-day fasting appears to promote some weight-loss benefits, as well as other health improvements—at least in the short term. But it’s not easy, and its long-term effects aren’t well studied, so it’s good to reach out to a healthcare provider or registered dietician before starting a new fasting diet, especially if you have any underlying health conditions.

Dr. Varady says that 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."

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Current version

November 22, 2021

Written by

Health Guide Team

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.