Alternate-day fasting is one way to do intermittent fasting.
On this diet, you fast every other day, but eat whatever you want on the non-fasting days.
The most common version of this diet actually involves "modified" fasting where you can eat 500 calories on fasting day.
Alternate-day fasting is a very powerful weight loss tool, and can help lower your risk of heart disease and type 2 diabetes.
Here is a detailed beginner's guide to alternate-day fasting.
How to Do Alternate-Day Fasting
Alternate-day fasting (ADF) is an intermittent fasting approach.
The basic idea is that you fast on one day, and then eat what you want the next day.
This way you only need to restrict what you eat half of the time.
On fasting days, you're allowed to drink as many calorie-free beverages as you like. Examples include water, unsweetened coffee and tea.
If you're following a modified ADF approach, you're also allowed to eat about 500 calories on fasting days, or 20–25% of your energy requirements (, , ).
The most popular version of this diet is called "" by Dr. Krista Varady. She also did most of the studies on ADF.
The health and weight loss benefits seem to be the same regardless of whether the fasting-day calories are consumed at lunch, dinner or as small meals throughout the day ().
Studies show that many people find alternate-day fasting much easier to stick to than traditional, everyday calorie restriction (, , ).
Most of the studies on alternate-day fasting used the modified version, with 500 calories on fasting days. This is considered much more sustainable than doing full fasts on fasting days, but it's just as effective.
In this article, the terms "alternate-day fasting" or "ADF" generally apply to the modified approach with about 500 calories on fasting days.
Bottom Line: Alternate-day fasting cycles between days of fasting and normal eating. The most popular version allows for about 500 calories on fasting days.
Alternate-Day Fasting and Weight Loss
ADF is very effective for weight loss.
Studies among overweight and obese adults show it may make you lose 3-8% of your body weight in 2–12 weeks (, , ).
Interestingly, ADF seems to be particularly effective for weight loss among middle-aged people ().
Studies have shown that ADF and daily calorie restriction are equally effective at reducing harmful belly fat and inflammatory markers in obese individuals ().
However, a 2016 review study concluded that ADF may be superior to daily calorie restriction diets, given that it's easier to stick to, produces greater fat loss and preserve more muscle mass ().
Furthermore, combining ADF with endurance exercise may cause twice as much weight loss than ADF alone and six times as much weight loss as endurance exercise alone ().
In regards to diet composition, ADF seems to be equally effective whether it's done with a high-fat or low-fat diet ().
Bottom Line: Alternate-day fasting is very effective for weight loss and may be easier to stick to than traditional calorie restriction.
Alternate-Day Fasting and Hunger
The effects of ADF on hunger are rather inconsistent.
Some studies show that hunger ultimately goes down on fasting days, while others state that hunger remains unchanged (, , ).
However, research agrees that modified ADF with 500 calories on fasting days is much more tolerable than full fasts on fasting days ().
Similarly, animal studies have shown that modified ADF resulted in decreased amounts of hunger hormones and increased amounts of satiety hormones than other diets (, , ).
Another factor to consider is compensatory hunger, which is a frequent downside of traditional, daily calorie restriction (, , ).
Compensatory hunger refers to increased levels of hunger in response to calorie restriction, which cause people to eat more than they need to when they finally "allow" themselves to eat.
Studies have shown that ADF does not increase compensatory hunger as much as continuous calorie restriction (, , ).
In fact, many people who try modified ADF claim that their hunger diminishes after the first two weeks or so. After a while, some find that the fasting days are nearly effortless ().
However, the effects of ADF on hunger most likely vary by individual.
Bottom Line: The effects of alternate-day fasting on hunger are inconsistent. Studies on modified alternate-day fasting show that hunger decreases as you adapt to the diet.
Alternate-Day Fasting and Body Composition
ADF has been shown to have unique effects on body composition, both while you are dieting and during your weight-maintenance period.
Studies comparing traditional calorie-restricted diets and ADF show that they are equally as effective at decreasing weight and fat mass.
However, ADF seems to be more effective at preserving muscle mass (, , ).
This is really important, as losing muscle mass along with fat decreases the amount of calories your body burns on a daily basis.
One randomized controlled study compared ADF to a traditional, calorie-restricted diet with a 400-calorie deficit ().
Both after an eight-week study and 24 unsupervised weeks, no difference was observed in weight regain between the groups.
However, after the 24 unsupervised weeks, the ADF group had preserved more muscle mass and lost more fat mass than the calorie-restricted group ().
Bottom Line: Alternate-day fasting is more effective at preserving muscle mass during weight loss than other weight loss methods.
Health Benefits of Alternate-Day Fasting
ADF has several health benefits aside from weight loss.
Type 2 Diabetes
Type 2 diabetes accounts for 90–95% of diabetes cases in the US (). What's more, more than a third of Americans have pre-diabetes, a condition in which blood sugar levels are higher than normal but not high enough to be considered diabetes ().
Losing weight and restricting calories is usually an effective way to improve or reverse many symptoms of type 2 diabetes ().
Similarly to continuous calorie restriction, ADF seems to cause mild reductions in risk factors for type 2 diabetes among overweight and obese individuals (, , ).
However, ADF seems most effective at lowering insulin levels and reducing insulin resistance, while only having a minor effect on blood sugar control (, , ).
Having high insulin levels (hyperinsulinemia) has been linked to obesity and chronic diseases, such as heart disease and cancer (, ).
Among pre-diabetic individuals, 8–12 weeks of ADF have been shown to decrease fasting insulin by about 20–31% (, , ).
A reduction in insulin levels and insulin resistance should lead to a significantly reduced risk of type 2 diabetes, especially when combined with weight loss.
Bottom Line: Alternate-day fasting may reduce risk factors for type 2 diabetes. It can reduce fasting insulin levels by 20-31% in pre-diabetic individuals.
Heart disease is the leading cause of death in the world, responsible for about 1 in 4 deaths (, ).
Many studies have shown that ADF is a good option for overweight and obese individuals to lose weight and reduce heart disease risk factors (, , , ).
Most studies on the subject range from 8–12 weeks and are done in overweight and obese individuals.
The most common health benefits include (, , , , , ):
- Reduced waist circumference (2–2.8 in or 5–7 cm).
- Decreased blood pressure.
- Lower LDL cholesterol concentration (20–25%).
- Increased number of large LDL-particles and reduction in the dangerous small, dense LDL particles.
- Decreased blood triglycerides (up to 30%).
Bottom Line: Alternate-day fasting may reduce waist circumference, decrease blood pressure, lower LDL cholesterol and decrease blood triglycerides.
Alternate-Day Fasting and Autophagy
One of the most common effects of fasting on the body is the stimulation of autophagy.
Autophagy is a process in which old parts of cells are degraded and recycled. It plays a key role in preventing diseases such as cancer, neurodegeneration, heart disease and infections (, ).
Animal studies have consistently shown that both long-term and short-term fasting increase autophagy and are linked to a delay in aging and reduced risk of tumors (, , , ).
Furthermore, fasting has been shown to increase lifespan in rodents, flies, yeasts and worms ().
Moreover, cell studies have shown that fasting stimulates autophagy, resulting in effects that may help keep your body healthy and help you live longer (, , ).
This has been confirmed by human studies showing that ADF diets reduce oxidative damage and promote changes that may be linked to longevity (, , , ).
The findings look very promising, but the effects of ADF on autophagy and longevity need to be studied more extensively.
Bottom Line: Alternate-day fasting stimulates autophagy in animal and cell studies. This process may slow down aging and help prevent diseases such as cancer and heart disease.
Does Alternate-Day Fasting Induce Starvation Mode?
Nearly all weight loss methods cause a slight drop in resting metabolic rate (, ).
This effect is often called "starvation mode," but the technical term is adaptive thermogenesis.
When you severely restrict your calories, your body starts conserving energy by reducing the amount of calories it burns. It can make you stop losing weight and feel miserable ().
However, ADF does not seem to cause this drop in metabolic rate.
One study compared the effects of standard calorie restriction and ADF for eight weeks.
The results showed that continuous calorie restriction significantly decreased resting metabolic rate by 6%, while ADF only caused an insignificant 1% reduction ().
What's more, after 24 unsupervised weeks, the calorie restriction group still had a 4.5% lower resting metabolic rate than at the beginning of the study. Meanwhile, the ADF participants maintained their original metabolic rate.
Several effects of ADF may be responsible for counteracting this drop in metabolic rate, including the preservation of muscle mass.
Bottom Line: Alternate-day fasting does not seem to decrease metabolic rate in the same way as continuous calorie restriction. This may be because ADF helps to preserve muscle mass.
Is It Also Good for Normal-Weight People?
ADF is good for more than just weight loss — it can also have health benefits for normal-weight people.
A 3-week study analyzed normal-weight individuals following a strict ADF diet, with zero calories on fasting days.
The researchers found that it resulted in increased fat burning, decreased fasting insulin and a 4% decrease in fat mass ().
However, hunger levels remained quite high throughout the study, and they speculated whether a modified ADF diet with one small meal on fasting days might be more tolerable for normal-weight people.
Another controlled study in both normal-weight and overweight individuals showed that following an ADF diet for 12 weeks reduced fat mass and produced favorable changes in risk factors for heart disease ().
That being said, ADF generally provides you with much fewer calories than you need to maintain weight, which is the reason why people ultimately lose weight.
If you're not looking to lose weight or fat mass or are underweight to begin with, other dietary methods will probably suit you better.
Bottom Line: Alternate-day fasting increases fat burning and reduces risk factors for heart disease in normal-weight people.
What to Eat and Drink on Fasting Days
There is no general rule on what you should eat or drink on fasting days, except that your total calorie intake should not exceed 500 calories.
It's best to drink low-calorie or calorie-free drinks on fasting days, such as water, coffee and tea.
Most people find it best to eat one "big" meal late in the day, while others prefer to eat early or split the amount between 2–3 meals.
Soups may also be a good option on fasting days, as they tend to make you feel fuller than if you ate the ingredients on their own (, ).
Here are a few examples of meals that are suitable for fasting days:
- Eggs and vegetables.
- Yogurt with berries.
- Grilled fish or lean meat with vegetables.
- Soup and a piece of fruit.
- A generous salad with lean meat.
You can find numerous recipes for quick 500-calorie meals and healthy low-calorie snacks on Google and Pinterest.
Bottom Line: There are no strict guidelines regarding what to eat and drink on fasting days. It's best to stick to high-protein foods and vegetables, as well as low-calorie or calorie-free beverages.
Is Alternate-Day Fasting Safe?
Studies have shown that alternate-day fasting is safe for most people.
It does not result in a greater risk of weight regain than traditional, calorie-restricted diets. On the contrary, it may even be better for long-term weight loss than continuous calorie restriction ().
Some think that ADF increases your risk of binge eating, but studies found that it actually decreased depression and binge eating. It also improved restrictive eating and body image perception among people with obesity ().
That being said, there are always groups of people who should not adhere to any weight loss diet.
These include people with eating disorders, pregnant and lactating mothers, children and those who are underweight or have certain medical conditions.
You should check with a doctor before trying this if you have a medical condition or are currently taking any medications.
Bottom Line: Alternate-day fasting has an outstanding safety profile. It does not increase binge eating or raise your risk of weight regain after you stop the diet.
Take Home Message
Alternate-day fasting is a very effective way to lose weight.
It has several benefits over traditional calorie-restricted diets, and is linked to major improvements in many health markers.
The best part of all is that it is surprisingly easy to stick to, because you only need to "diet" every other day.