This form of intermittent fasting involves consuming significantly fewer calories than usual every other day. People may do this to achieve or maintain a healthful weight or boost other aspects of health.

Alternate day fasting (ADF) may be suitable for some people, but people who are pregnant or breastfeeding or have any ongoing health issues should consult a doctor before trying it.

This article describes how ADF works, specific foods and drinks involved, and whether it is safe and effective. We also look into who may benefit from this type of fasting and who should avoid it.

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ADF is a form of intermittent fasting. It involves having fasting days and “feasting” days. On fasting days, a person significantly reduces their caloric intake, and on feasting days they exceed the recommended daily intake of calories.

A 2017 study reports that a person trying ADF consumes 25% of their required daily calories on fasting days and 125% on feasting days. If a person’s recommended daily intake is 2,000 calories, this would mean consuming 500 calories on fasting days and 2,500 calories on feasting days.

ADF may be an attractive option for people who want to try intermittent fasting without having daily caloric reductions or timed fasting.

According to a 2017 review, ADF appears to be safe for healthy individuals, regardless of whether they have a healthy weight, overweight, or obesity.

It is not clear whether ADF is safe for people with ongoing health conditions. Anyone with a health issue who wants to try ADF should talk to their doctor first.

ADF basically involves switching between fasting and feasting days.

Fasting days

On these days, a person consumes about 25% of their regular caloric intake. They can also have as many noncaloric drinks as they like, including water, teas, and herbal infusions.

Some ADF plans advise people to have only liquids in the morning and get most of their calories from a single small meal that may be lunch or dinner.

Other plans recommend eating a few very small, low-calorie meals throughout the day.

According to a 2014 study, both of these ADF plans yield similar weight loss results and heart health benefits among people with obesity.

Ultimately, a person should choose the plan that is most feasible for them.

Feasting days

On feasting days, a person should consume about 125% of their daily recommended calorie intake. Generally, a person can choose the diet and pattern of eating that works best for them.

The excess calories partially make up for the lack on fasting days. But overall, a person’s caloric intake is reduced.

Having these feasting days may make it easier to achieve a caloric deficit if weight loss is a goal.

Reducing the overall intake of calories is one of the first things that doctors recommend to people looking to lose weight. Reducing the intake encourages the body to use up its energy reserves in the form of stored fats.

A person trying ADF typically eats about 25% of their regular calories on fasting days and 125% on feasting days.

As the 2018 review cited above notes, people with larger bodies typically burn more calories. As a person loses weight on the ADF diet, they go on to burn fewer calories. This is a natural adaptation, not a sign of starvation.

One analysis found that people with obesity lost about 6.8% of their body weight after 6 months of following an ADF plan.

The 2017 review notes, however, that intermittent fasting may have similar weight loss results as continuous caloric restriction. ADF did not produce more significant results.

Any type of caloric restriction can cause hunger while the body is learning to adapt to the reduced intake.

ADF can cause intense hunger on fasting days, and some people may find it more difficult to maintain than a steady reduced-calorie diet.

ADF may produce similar weight loss results as other forms of caloric restriction, such as a low-calorie diet.

The 2017 reviewobserves that ADF does not lead to more significant weight loss, compared with these diets.

Below are some potential health benefits of ADF.

Diabetes control

The review from 2017 analyzed several studies in which ADF was associated with reductions in both blood glucose and insulin concentrations.

However, determining whether intermittent fasting leads to clinically significant improvements in diabetes and other metabolic diseases requires more research.

Anyone with diabetes who is thinking of changing their diet should seek advice from their doctor or nutritionist. Those who opt for intermittent fasting should take extra care to monitor their blood glucose levels. This reduces the risk of having dangerously low levels on fasting days.

Heart health

As the Centers for Disease Control and Prevention (CDC) observe, having a healthy weight is an important step toward preventing heart disease. This is because carrying extra weight puts excess stress on the heart and blood vessels.

For some people, maintaining a healthy weight involves losing weight. This involves reducing the number of calories consumed, but the way a person does this may not so important — intermittent fasting may be equally as effective as having a steady calorie-restricted diet.

Autophagy

Autophagy is a cellular process that involves cleaning up or recycling damaged cell components.

A 2018 review notes that autophagy protects healthy cells from damage while eliminating potentially harmful cells that may cause disease. The researchers also found that caloric restriction and fasting are the most effective ways to induce autophagy.

ADF may work well for people who are considering a calorie-restricted diet but do not want to reduce their calorie intake every day. Having feast days, which involve eating extra calories, may be easier or more satisfying than following a steady low-calorie diet.

However, some people may want to avoid the intense hunger that can result from fasting. A 2017 study comparing ADF with daily caloric restriction found that people trying the ADF diet had a higher dropout rate.

Overall, there are many ways to consume fewer calories. If this is a goal, a person should try different approaches and go with what works best for them.

ADF may not be suitable for:

  • People who find following a diet challenging due to increased hunger: There are less drastic approaches to caloric restriction than ADF. A person may prefer a steady diet that reduces the intake of calories gradually.
  • People who are insulin dependant: Anyone with diabetes should talk to a doctor before starting any calorie-restrictive diet, including ADF.
  • People who require more calories to meet their daily needs: Some examples include:
    • athletes
    • people who are pregnant or nursing
    • people with certain underlying health conditions, such as cystic fibrosis

Intermittent fasting may instead involve:

  • fasting on specific days of the week
  • fasting for specific hours of the day
  • avoiding eating after a certain time each day
  • skipping certain meals on specific days

A person can have as many noncaloric drinks as they like, and drinking plenty of fluids may help stave off hunger pangs or help keep the mind off eating.

Examples of beverages with very few or no calories include:

Also, low-calorie protein or fiber drinks may help increase feelings of fullness on fasting days. A 2019 study found that drinking whey protein isolate reduced appetite and improved feelings of fullness in young adult female participants with obesity.

On fasting days, some people opt to eat low-calorie foods regularly throughout the day. Such foods generally include fibrous, water-rich fruits and vegetables. A person might try:

ADF is a form of intermittent fasting. It involves alternating between fasting days and feasting days. Research suggests that this encourages weight loss about as well as having a steady low-calorie diet.

Some people may find ADF suitable because they prefer the variety of feasting and fasting. Others may want to avoid the hunger that results from fasting. To increase feelings of fullness during fasting, a person might try low-calorie protein or fiber drinks and fibrous, water-rich fruits and vegetables.

ADF is not suitable for everyone. Anyone who is pregnant or nursing or who has an underlying health condition should talk to a doctor before trying ADF or any other type of intermittent fasting.