What Is Intermittent Fasting?

Intermittent fasting is a term used to describe eating patterns that involve alternating periods of fasting—abstinence from eating—and normal calorie intake, on a recurring basis. Intermittent fasting is a popular way of eating that can be used as a tool to promote health no matter what diet you follow.   

It’s been linked to a number of health benefits, from boosting weight loss to promoting heart health, which is why it’s often recommended by health experts like doctors and registered dietitians. 

However, there’s several types of intermittent fasting methods—each with its own set of rules—which can be confusing to those interested in using intermittent fasting to support their health.

Here’s what you need to know about intermittent fasting, including different types, benefits, and potential risks.

Types of Intermittent Fasting

There are different types of intermittent fasting, some of which are more restrictive than others. Each type has its own set of guidelines.

Here’s some more information about a few of the most common types of intermittent fasting:

• Time-Restricted Feeding (TRF): TRF involves eating within a specific period of time—usually within a four to twelve-hour window—and then fasting for the rest of the day. The 16/8 method is one of the most popular types of TRF. 
• Alternate Day Fasting (ADF): This is another major form of intermittent fasting that involves fasting one day and then eating regularly the following day. Traditional ADF involves 100% calorie restriction on fasting days while Modified Alternate Day Fasting involves eating a small amount—usually around 500 calories—on fasting days. 
• Periodic Fasting:  Periodic fasting involves extended fasting periods that occur periodically, such as twice per week or once a month. The 5:2 method is a type of periodic fasting that involves eating normally five days out of the week and then fasting for two.

While all intermittent fasting regimens have different rules, they all have some things in common. Traditional intermittent fasting methods require completely abstaining from food intake for a set period of time, while modified fasting methods—like modified ADF—allow for a small number of calories on fasting days.

No matter the type of fasting program you choose, there’s no dietary restriction, meaning there are no special rules regarding what you can and can’t eat. Some people combine intermittent fasting with popular diets like low-carb diets or paleo diets, but intermittent fasting can be used by anyone, no matter the dietary pattern you choose to follow.

If you’re following a traditional fasting method, you’ll completely abstain from calorie intake during the fasting window, but you can drink non-caloric beverages like black coffee, tea, and water. 

When the fasting period is over, you can resume your normal diet during the eating window.

If you’re following a modified fast, like modified alternate-day fasting, you can consume a small number of calories—usually around 500—during the fasting window.

Benefits of Intermittent Fasting

Intermittent fasting is a popular weight loss method, but it’s also associated with other health benefits, many of which have nothing to do with fat loss. 

Helps With Weight Loss

Intermittent fasting has been shown to be an effective way to lose weight. When you shorten your daily eating window, such as when following a 16/8 regimen, you usually end up eating fewer calories and fewer meals. Methods like ADF result in large calorie deficits because you’re only taking in calories every other day.

A 2020 review that included 27 intermittent fasting studies found that fasting regimens lasting for 2 to 26 weeks resulted in a weight loss ranging from 0.8% to 13.0% of baseline weight. 

However, it’s unclear if intermittent fasting is more effective than continuous energy restriction, which most weight loss diets use.

Some studies show that intermittent fasting may result in greater weight loss compared to continuous dieting, while others have found no difference between the two.

Benefits Metabolic Health

Following an intermittent fasting regimen could help you improve your blood lipid levels and blood sugar, which can improve your metabolic health and help lower your disease risk. 

Metabolic syndrome is a cluster of health conditions including elevated blood sugar and blood lipid levels. 

A 2022 review of 10 studies in people with metabolic syndrome found that intermittent fasting protocols led to reductions in fasting blood sugar, hemoglobin A1C (HbA1c) (a long-term marker of blood sugar control), total cholesterol, LDL cholesterol, and triglyceride levels and significantly improved insulin resistance. 

Insulin resistance is when the body stops responding to the hormone insulin, which shuttles blood sugar into cells. Insulin resistance can increase your risk of diabetes and heart disease.

Improves Circadian Rhythms

The Circadian rhythm is an internal clock that regulates your sleep-wake cycle by responding to light changes. This clock resets every day based on the sun’s cycle. It’s important for overall health, including the function of organ systems, like your digestive system.

Some researchers suggest that, ideally, food intake should be kept between 8 a.m .and 6 p.m., and that eating outside of this window can disrupt your circadian rhythm and interfere with proper digestion, blood sugar regulation, and more.  

Intermittent fasting regimens like TRF can help align food consumption with the body’s circadian rhythm, which may improve health. 

However, it’s unclear if eating within this ideal window is more beneficial to health compared to eating within a later window. One study did show that, when compared to a TRF program that used an eating window later in the day—12 p.m. to 9 p.m.—men who followed an earlier TRF program—8 a.m. to 5 p.m.—had lower average fasting blood sugar levels. 

Boosts Digestive Health

Some research suggests that intermittent fasting could help improve gut health by increasing levels of beneficial bacteria in the digestive tract.

Intermittent fasting has been shown to increase the growth of bacteria that help reduce gut inflammation, like Lactobacillus and Bifidobacterium, while decreasing levels of disease-causing bacteria.

Even though it’s likely that intermittent fasting has positive effects on gut health, human research is limited and more well-designed studies are needed to understand the full effects of fasting on the digestive system.

Risks of Intermittent Fasting

As long as it’s done safely, intermittent fasting is considered relatively safe and isn’t associated with dangerous side effects.

Depending on the length of the fast, some people can experience side effects like headaches, extreme hunger, lightheadedness, irritability, fatigue, and other mild symptoms.

Longer fasts are more likely to cause more significant side effects like dizziness and extreme hunger.

Even though intermittent fasting isn’t associated with dangerous side effects, it’s not appropriate or safe for some people. Pregnant and breastfeeding women, children and teens, those taking certain medications, people who are underweight, and people with eating disorders should generally avoid fasting. 

Sometimes, physicians recommend fasting for medical purposes to people with conditions like cancer, but you shouldn't try intermittent fasting if you have a medical condition without clearing it with your healthcare provider first. 

A Quick Review

Intermittent fasting is a way of eating that involves cycling periods of fasting with regular eating. Most intermittent fasting methods involve completely abstaining from calorie intake during the fasting window, while others allow for a small number of calories. 

Intermittent fasting has been linked to some impressive health benefits, including boosting fat loss, improving blood sugar and blood lipid levels, and positively influencing gut health. 

If you’re interested in trying out intermittent fasting, you may want to start with a shorter fast before moving on to a longer fasting period to see how your body reacts. If you have a medical condition like diabetes, it’s important to clear any dietary changes, including intermittent fasting, with your doctor first to make sure it’s a safe choice for you.