9 Types Of Intermittent Fasting + How To Know Which One Is Right For You

Eating nutritious foods is one of the best ways to keep yourself healthy, so it may seem counterintuitive to intentionally refrain from eating—also known as fasting—for a prolonged period of time. But, it turns out, fasting can benefit your health, too.

Intermittent fasting (IF)—an umbrella term that encompasses a number of eating patterns that involve regularly occurring periods of fasting that are longer than a normal overnight fast1—has become a popular way to lose weight and improve metabolic health.

There are a number of fasting programs to choose from, which can make it difficult to know which one is right for you. In this article, we break down different types of IF and chat with experts about the benefits and possible risks of IF to help you decide whether IF could benefit your health.

There are a few different types of IF programs1 to choose from, but they all have one thing in common: abstaining from calories over a set period of time. There are some programs that involve eating a small number of calories rather than not eating at all, but those are considered "modified2" fasts and differ from traditional types of IF.

Here's a little more about the types of IF and modified fasts that are currently most popular, including pros and cons, and who they'd benefit most:

Time-restricted feeding (TRF) is one of the major forms of IF. It involves eating as much or as often as you like within a defined 1period of time, usually between three and 12 hours per day. This results in a fasting window of 12 to 21 hours per day.

It's associated with a number of health benefits, including promoting weight loss6 and improving blood sugar levels7.

If sustained weight loss is your goal, you'll want to stick with shorter and more manageable fasting windows, says Grant Tinsley, Ph.D., a professor at Texas Tech University and intermittent fasting researcher.

"I believe the best fasting method for weight loss is whichever one someone can stick to indefinitely," Tinsley tells mindbodygreen. "Since sustainable weight loss and long-term weight loss maintenance should be the goal of those seeking to lose weight, a conservative but feasible program is preferred over an extreme, short-term program. While more extreme fasting programs can lead to additional weight loss in the short term, they don't typically help in establishing long-term habits in the same way a more balanced, conservative program can." 

Tinsley recommends time-restricted eating for IF newbies. "TRF seems to be the easiest to follow because it allows for a consistent daily schedule and often involves relatively short fasting periods," Tinsley says.

However, it's inappropriate for some people, including children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight.

Circadian fasting is a way to ensure your time-restricted feeding lines up with your body's internal clock. It involves timing your meals with the rise and fall of the sun and typically eating larger meals earlier in the day and fasting during the evening when your digestion may not be as strong. See here for a guide to circadian fasting, complete with a sample eating schedule.

One randomized controlled trial11 found that when participants with obesity went on a calorie-restricted diet, those who took in more calories at breakfast and lunch and fewer at dinner lost more weight and experienced higher reductions in blood pressure than those who ate more calories later in the day. Fasting in the hours leading up to bed can also help improve your sleep.

Circadian fasting is another eating strategy that is suitable for IF beginners. Those who are prone to unhealthy late-night snacking might find it especially beneficial. It's inappropriate for children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight.

This is one of the most popular types of TRF. The 16:8 method involves fasting for 16 hours and consuming all of your calories within an eight-hour window each day. This window can be earlier in the day or later in the day, depending on your schedule. See here for a guide to 16:8 fasting, complete with a sample meal plan. Those who are looking for a slightly more restrictive plan can also look into 18:6 fasting.

This type of fasting is associated with all the benefits of TRF, and research finds that eating within an eight-hour window might lower blood pressure12 too.

This method fits in well with most people's schedules. However, fasting beginners might want to start with a longer eating window (say, 12 hours) and gradually work their way down to eight hours.

It's inappropriate for some people, like children, pregnant and breastfeeding women, those taking certain medications, and people who are underweight.

Another major type of IF, alternate-day fasting (ADF) involves eating food every other day. A person will follow a normal day of eating one day and then abstain from eating any calories the following day.

When followed for eight to 12 weeks16, ADF has been shown to decrease LDL cholesterol by as much as 25% and triglycerides by as much as 30%.

Since ADF is a more restrictive type of fasting, you should consult with your doctor before giving it a try. It's inappropriate for some people, like children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders19, and people who are underweight.

Modified ADF is similar to ADF but allows you to eat a very small amount of calories—typically under 500—during your fasting days. This can make it easier for a person to stick to ADF long term.

It's been shown to be effective in promoting weight loss18 and reducing certain inflammatory markers, like high-sensitivity C-reactive protein (hs-CRP). 

While this type of fasting is slightly more approachable than strict ADF, you'll still want to talk to your doctor before trying it. It's inappropriate for some people, like children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight.

A 12-hour fast is a type of TRF in which you consume all of your calories within a 12-hour window and then fast for 12 hours. This is the least restrictive type of TRF because it allows for a fairly normal eating pattern. Still, it may help you reduce your overall calorie intake, which can help you lose weight and improve your metabolic health. 

This 12-hour fast is a safe way to dip your toes into fasting if you're a beginner. You'll want to stick with a circadian-friendly eating pattern (where you restrict calories in the evenings close to bedtime) for the best results. It's generally safe for most people since it mimics our natural eating patterns.

The 5:2 diet is considered a type of periodic fasting (PF). PF refers to methods where fasting doesn't occur every day or every other day. The 5:2 diet involves eating normally five days per week and then following a modified fast for two consecutive or nonconsecutive days. Learn more about the 5:2 diet here.

During the fasting days, energy intake is typically kept to around 600 calories. Studies show that the 5:2 diet may be helpful for weight loss21 and improving certain health conditions, like nonalcoholic fatty liver disease22 (NAFLD). Learn more about how to do it here.

Since 5:2 fasting is a more restrictive type of fasting, you should consult with your doctor before giving it a try. It's inappropriate for some people, like children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight.

"Dirty fasting" is a term that's used to describe eating patterns that allow for some calorie intake during a fasting window. There's no set definition of dirty fasting, but people using this method usually take in around 100 calories during their fast.

Some people using dirty fasting may put some cream in their coffee or sip on some soup during their fast, for example.

Because modified fasts that allow for some calorie intake during fasting windows seem to offer some health benefits—like weight loss—it's likely that dirty fasting does, too—as long as you don't go overboard. Learn more about dirty fasting here.

Those who have trouble sticking with strict fasting plans may benefit from the less rigid approach of dirty fasting. Dirty fasting is inappropriate for children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders, and people who are underweight.

A 24-hour fast is a type of PF where you consume zero calories for 24 hours. Some people fast for 24 hours a few times a month, while others use this method once or twice per week. This method could help you lose weight23 and improve other aspects of health, but it can be difficult to stick to. Learn more about 24-hour fasting here.

Since 24-hour fasting is a more restrictive type of fasting, you should consult with your doctor before giving it a try. It's inappropriate for some people, like children, pregnant and breastfeeding women, those taking certain medications, people with eating disorders19, and people who are underweight. For those with Type 1 diabetes, this type of fasting may throw off blood sugar levels.

Many women are eager to jump on the fasting bandwagon because they've heard that it can help them lose weight, improve fertility, and more. But is fasting safe and effective for women? We asked Sara Gottfried, M.D., a board-certified physician who specializes in women's health and integrative medicine for the details. 

"While fasting is safe for women who are not pregnant or breastfeeding, I usually advise most of my female patients to ease into it," she says. "This is because fasting can trigger a rise25 in the level of the stress hormone cortisol, which can then impede weight loss attempts." 

Studies show that different fasting methods influence cortisol in different ways, so if you’re concerned about your fertility or fasting’s effect on cortisol, it’s best to speak with a healthcare provider before trying out an IF regimen.

She also warns that fasting methods that involve more extreme calorie restriction can impact fertility. "A state of severely reduced calorie intake is a signal to the ancient wisdom of the body's endocrine system that the body is facing a period of famine, which can impact the menstrual cycle and hence, fertility," says Gottfried. 

Gottfried recommends that instead of jumping into more extreme fasting methods, like ADF, women—particularly women over the age of 50—should start with a gentler approach. In her book, Women, Food, and Hormones, Gottfried advises readers to slowly introduce time-restricted eating with an overnight fast of 12 to 14 hours on nonconsecutive days—creating an eating window of 12 to 10 hours—and to build from there. 

Gottfried adds that IF can be particularly effective for women with certain medical conditions, like PCOS.

A 2021 study26 published in the Journal of Translational Medicine found that following a 16:8 fasting regimen for six weeks led to significant improvements in body weight, menstrual cycle irregularity, insulin resistance, and testosterone levels in women with PCOS. 

Research shows that IF can benefit brain health in a number of ways, as long as it's done safely. 

However, these benefits have mostly been seen in people with cognitive impairment or medical conditions like Alzheimer's disease, epilepsy, and multiple sclerosis (MS).

A 2021 review27 found no significant short-term benefits of IF on brain function in healthy people but did find evidence that IF could benefit people with epilepsy, Alzheimer's disease, and multiple sclerosis by improving symptoms and slowing disease progression.  

Some studies show that IF can be a helpful tool for improving brain function in older people with cognitive impairment. A 2020 study28 found that older adults with mild cognitive impairment who regularly practiced IF had better cognitive function and higher scores on brain function tests at 36 months of follow-up. 

Fasting may help reduce neuroinflammation29, improve insulin resistance, and prevent the formation of amyloid plaques27 in the brain, all of which could improve brain health and help protect against cognitive decline. 

IF has also been shown to increase levels of brain-derived neurotrophic factor (BDNF)30, a molecule that's involved in learning, memory, nerve function, nerve cell formation, and the regulation of brain inflammation. 

According to Jason Fung, M.D., a nephrologist and intermittent fasting expert, fasting may be beneficial for concentration and memory. "There is limited research on fasting and brain health, although theoretically, it may be beneficial," he tells mindbodygreen. "Insulin resistance, obesity, and metabolic syndrome are all associated with cognitive decline...and fasting reduces those conditions. In the short term, fasting may improve concentration, memory, and other cognitive functions because of the increase in sympathetic tone31."

Intermittent fasting can be an effective way to improve a number of health parameters, from body weight to blood lipid levels. If you're new to IF, consider starting with a gentler method—like an overnight fast of 12 to 14 hours on nonconsecutive days—before moving on to longer fasts.

If you have questions about IF, consider speaking with a trusted health care provider. They can help you decide if IF is right for you and can help you choose a safe IF program. If you're new to fasting, these tips can make it a bit easier.