Intermittent Fasting

Medically Reviewed by Kathleen M. Zelman, RD, LD, MPH on September 18, 2024
Written by Laurel Leicht, Timothy Gower
10 min read

Intermittent fasting is an eating plan in which you alternate between periods when you eat normally and extended breaks when you eat little or no food. Some people try intermittent fasting to lose weight, while others believe it will improve their health.

There are different versions of intermittent fasting. They include alternate day fasting, 5:2 intermittent fasting (fasting 2 days each week), and daily time-restricted feeding (in which you eat only during specified hours of the day). 

Some research shows that intermittent fasting works, at least in the short term. In some studies, people who followed this diet lost weight and had improvements in common health measures, such as cholesterol levels and markers of inflammation.

Studies of intermittent fasting in people with chronic disease have resulted in better control of their conditions. However, most of these studies have been relatively brief, lasting only a few months. It’s unclear if the benefits would last years if the diet is continued throughout life.

While intermittent fasting is becoming more popular, always talk to your doctor before you start a new diet.

Your body’s main source of energy is a form of sugar called glucose, which circulates in the blood. When you eat a typical diet, your body has a steady supply of sugar, primarily from carbohydrates. But when you go hours without eating, you burn up all of your stored sugar. When that happens, your body burns stored fat as fuel, which is called metabolic switching. That’s why intermittent fasting can help you lose weight and body fat.

Researchers are still studying intermittent fasting to understand the possible secrets behind its overall health-boosting benefits. One theory suggests that fasting puts your cells under mild stress, prompting them to adapt. Scientists think that this process strengthens your cells’ ability to deal with stress and potentially fight off some diseases.

Keep in mind that no one is 100% certain how intermittent fasting works. Furthermore, no two people respond the same to any given diet, so there’s no guarantee that any claims about the benefits of intermittent fasting will prove true for you. That’s another good reason to check in with your doctor before starting any new diet. 

WebMD does not back claims made about intermittent fasting or any other eating plan.

Studies in humans have shown that intermittent fasting appears to have some important health benefits, including:

  • Weight loss
  • Less insulin resistance, a condition that causes and worsens type 2 diabetes
  • Reduced levels of unhealthy blood fats, such as cholesterol and triglycerides, which increase the risk for cardiovascular disease
  • Lower blood pressure
  • Better control of inflammation, which is linked to many diseases, from Alzheimer’s disease to asthma

You may also find that intermittent fasting has some additional upsides, including:

Simplicity. Intermittent fasting doesn't require you to count calories or carbs. You simply don’t eat (or eat very little) during certain hours or on certain days.

Adaptability. With intermittent fasting, you decide what to eat. However, it’s preferable to select healthy foods for your non-fasting periods, as you may undermine the benefits of the diet if you choose highly processed foods. You can make intermittent fasting work with food restrictions, such as if you’re vegetarian or vegan. And you can eat the type of diet you like, whether it’s high- or low-carb, or low in fat.

Savings. You’re reducing the amount of food you eat each week, so your grocery costs should go down.

Studies on intermittent fasting have found that this approach to eating may carry some risks along with its potential benefits. They can range from simply making you feel lousy while you’re fasting to more serious threats to your health.

Side effects of intermittent fasting

Going without food for long periods can have some downsides. Some side effects that have been linked to intermittent fasting in studies include:

  • Dizziness
  • Nausea
  • Insomnia
  • Headache
  • Weakness
  • Mood swings
  • Needing to pee frequently

Some studies have also found that participants on intermittent fasting regimens lost muscle. Doctors also caution that some people who try intermittent fasting overeat on non-fasting days or hours. If you’re older, losing too much weight could affect your bone health, immune system, and your energy level.

Intermittent fasting and heart disease

Intermittent fasting appears to improve cholesterol levels and other factors that increase the risk for heart disease. But there are some clues that some forms of this diet may actually be bad for your ticker. One form of intermittent fasting called time-restricted eating limits the period you can eat to an 8-hour window each day. A large study reported at an American Heart Association conference in 2024 found that people who followed a time-restricted eating plan were 91% more likely to die of cardiovascular disease than others who ate a typical diet. More research on the link between intermittent fasting and heart health is necessary, but this study raises important questions.

You can eat mostly what you want on days when you don’t fast. But to lose weight and get the nutrients you need, you should stick to healthy foods and limit treats such as desserts, alcohol, and processed foods.

On fasting days, you’ll eat very little food or none at all.

For example, the Every Other Day Diet says to eat no more than 500 calories during each fast day. Nuts are often eaten on fasting days for protein and satiety.

Another program called the 5:2 Fast Diet involves eating 5 days a week and fasting for the other 2 days. For this plan, women get no more than 500 calories and men no more than 600 calories per day. That’s a quarter of the amount you likely eat on the days when you don’t fast. Whether you eat those calories in one sitting or spread them across micro-meals throughout the day is up to you.

If you decide to try intermittent fasting, choose your plan carefully. Most intermittent fasting diets recommend cutting back to 500-600 calories on fasting days. In general, for many people, this would be medically safer and easier than not eating at all on those days.

It’s not easy to skip most of your calories a few days a week and rely mostly on water, coffee, and tea to keep you feeling full. You’ll need a balanced meal plan to eat in moderation even on your so-called “feast” days, despite their name. You can indulge in a treat occasionally, but that's about it if you want to see results.

Remember to drink enough water and other fluids on fasting days to prevent dehydration. And you’ll need to eat a healthy diet on days that you don’t fast.

How much you exercise is up to you. But obviously, you’re not going to have a lot of energy for that on your fasting days, so consider lighter forms of exercise, such as walking. The creators of the Every Other Day diet studied people who did cardiovascular exercise (such as biking) while on the alternate-day fasting plan. The study found that they were able to maintain muscle mass while fasting.

Does intermittent fasting work?

Most studies have found that intermittent fasting can help you lose weight. However, there is also scientific evidence that taking scheduled breaks from eating causes changes in the body that go beyond the benefits of weight loss and could potentially improve your health. For example, lab studies show that periods of fasting induce stress on human cells. When you try intermittent fasting, your cells adapt to the stress of a food-free environment by producing more protective antioxidants, turning down inflammation, and undergoing other changes that fight disease.

Is it good for certain conditions?

Researchers have studied whether intermittent fasting benefits certain medical conditions, with mixed results. 

Obesity. A review of 40 studies found that intermittent fasting can help people lose weight. The review showed that the typical person who tries intermittent fasting loses about 7-11 pounds over 10 weeks. However, there are a few important things to keep in mind. That same study found that people who went on conventional low-calorie diets instead of fasting lost about the same amount of weight and that lots of people quit both types of diets because they found them too hard to stick with. Some nutritionists say that intermittent fasting may shed pounds quickly, but it may not be a sustainable approach to weight loss over the long term for some people.

Heart disease. Studies show that intermittent fasting improves some key factors that increase the risk for heart disease, including artery-clogging cholesterol and triglycerides. (However, one study found that LDL, or “bad,” cholesterol went up in people on alternate-day fasting diets for a year.) Intermittent fasting has also been shown to improve blood pressure, another risk factor for heart problems. However, conventional low-calorie diets have a similar benefit. Also, a recent study raised concerns that intermittent fasting may actually increase the risk for deadly heart attacks and strokes.

Type 2 diabetes. Some studies, but not all, show that intermittent fasting improves how the body uses the hormone insulin, which could help prevent or improve control of type 2 diabetes by lowering blood sugar levels. One 6-month study found that the form of intermittent fasting called time-restricted eating reduced blood sugar levels in people with type 2 diabetes. However, a conventional reduced-calorie diet had the same effect, and some experts think intermittent fasting isn't safe for people with diabetes.

Asthma. Intermittent fasting has been shown to reduce inflammation, which triggers this condition. One initial study found that people with asthma who fasted for 24 hours reduced their levels of immune system proteins that caused inflammation. But other research failed to show that fasting improves the ability to breathe in people with asthma.

Alzheimer’s disease. In lab studies, animals that are deprived of food for certain periods are less likely to develop brain changes that cause Alzheimer’s disease than animals fed regularly. However, it’s not clear that humans who follow an intermittent fasting plan are protected against this disease or any other form of dementia.

If you have any medical conditions, talk with your doctor before you try intermittent fasting.

Who shouldn’t try intermittent fasting?

Intermittent fasting is off-limits for you if:

  • You have an eating disorder
  • You use medications that must be taken with food
  • You’re pregnant or breastfeeding
  • You’re still growing

If you have diabetes, ask your doctor whether intermittent fasting is safe for you. Many experts don't recommend fasting for anyone with diabetes.

There are many different recommended schedules for intermittent fasting, including:

16/8 fasting. Eat normally for 8 hours, such as from 10 a.m. to 6 p.m., then fast for 16 hours. This approach is often called time-restricted eating.

5:2 fasting. Eat normally for 5 days each week and fast for 2 days, eating no more than 500-600 calories. Some people break that down to a 200-calorie meal and a 300-calorie meal. You can pick whatever 2 days you like.

Alternate-day fasting. Every other day, limit yourself to 500 calories. Some people opt to go entirely food-free on fasting days.

When you’re fasting, make the 500-600 calories you’re allowed during that time count by choosing high-fiber and high-protein foods, which will fill you up. Apples and almonds are a few good high-fiber choices, while plain Greek yogurt and nuts are top sources of protein.

During the days or blocks of hours when you’re eating as usual, stick to a healthy diet. Nutrition experts often recommend the Mediterranean diet, which is rich in lean protein such as fish and poultry, olive oil, nuts, beans, leafy greens and other vegetables, fruit, and whole grains.

Intermittent fasting is rapidly growing in popularity, but it may not be the right choice for you if you’re trying to lose weight or improve your health. Studies show that it can help you shed some pounds, but intermittent fasting doesn’t seem to be any more effective than conventional low-calorie diets. There may be some potential risks involved in this eating approach, so check with your doctor before giving it a try.

What are the different methods of intermittent fasting?

There are many different methods of intermittent fasting. For example, in the method known as 16/8, you eat a normal healthy diet for 8 hours a day, then fast for 16 hours. In 5:2 fasting, you eat your usual diet 5 days a week but fast for 2 days. Alternate-day fasting is just what it sounds like — you fast every other day.

How does intermittent fasting benefit your health?

Research shows that intermittent fasting can help you lose weight. Most, though not all, studies show that this eating approach also lowers blood pressure, blood sugar, and blood fats such as LDL (“bad”) cholesterol. Research also indicates that intermittent fasting reduces inflammation, which is linked to many diseases.

Is intermittent fasting safe for everyone?

No, intermittent fasting is unsafe if you’re pregnant or breastfeeding, have an eating disorder such as anorexia nervosa or bulimia, have diabetes, or are still growing. Also, if you use medication that must be taken with food, intermittent fasting is not an option.

What should you eat during intermittent fasting?

During the hours or days when you are eating, choose a diet full of healthy foods. After all, filling up on high-calorie, low-nutrition foods, such as fast food, soda pop, and desserts, could hurt your efforts to lose weight and get healthier. Some experts recommend the Mediterranean diet, which includes plenty of healthy foods such as fish, fruit and vegetables, olive oil, whole grains, and nuts.