Intermittent Fasting

Last Updated July 2023 | This article was created by editorial staff and reviewed by Kyle Bradford Jones, MD, FAAFP

You may have heard of the term intermittent fasting. It’s a popular topic nowadays. Some people talk about it when looking for a way to lose weight. Others talk about it as a way to improve their overall health. But what is intermittent fasting? Is it good for your body? What are the benefits? Are there side effects? Is it right for everyone?

Path to improved health

Intermittent fasting is not a diet plan. While it may have some of the same health benefits as a diet, it’s really an eating pattern. It means you fast (don’t eat) during a certain period of time each day (usually an extended period of time). You then eat during another period of time each day (usually a smaller period of time). While you’re fasting, you can drink beverages that don’t contain calories, including water, black coffee, and unsweetened tea.

How fasting and eating is divided each day is called an eating schedule. One of the most common, easy-to-follow schedules is 16:8. This means you fast for a 16-hour period of time and eat your daily meals during an 8-hour period of time. For example, you may want to fast from 7 p.m. until 11 a.m. the next day. You would then eat a healthy lunch and dinner between 11 a.m. and 7 p.m. You wouldn’t eat anything after 7 p.m. until 11 a.m. the next day. This is just an example of times. You can select any 16-hour and 8-hour block of time that works best for your schedule. But it’s important to keep your eating window at the same every day.

Other intermittent fasting schedules include 18:6 (when you fast for 18 hours and eat for 6 hours) or alternating days. With alternating days, you fast for 24 hours, then eat a healthy diet for the next 24 hours, then fast again for the following 24 hours. This schedule continues using the every-other-day format. Another schedule option is 5:2. This is when you fast for two days a week, and eat a normal, healthy diet the other five days. This is a little different, though, as this schedule allows you eat one small meal of 500 to 600 calories on your fasting days.

The time you’re allowed to eat is called your eating window. During your eating window, focus on eating a healthy diet and maintaining portion control. Don’t eat too many calories and avoid junk food and fast food. While you don’t need to eat anything in particular, you do need to make sure you’re getting the nutrition you need. Some people choose to use the Mediterranean diet as a guideline for what to eat. This plan focuses on fruits, vegetables, beans, whole grains, and fish. You can also choose lean proteins and healthy fats.

How intermittent fasting works

Fasting for at least 12 hours changes how your metabolic system works. Your metabolic system is how your body changes the foods and beverages you consume into energy. Most of the time, your body gets its energy from a sugar called glucose. Glucose is found in the foods you eat and beverages you drink. When you eat three meals during a day, your body maintains a steady glucose level because you’re eating and drinking frequently.

However, when you fast for more than 12 hours, your body’s glucose levels start to dip because you’re not eating as frequently. When your body doesn’t have the glucose it needs for energy, it taps into your body’s fat for energy. When this happens, the fatty acids in your body are absorbed into your blood. They produce a chemical called ketones. Your body then uses the ketones as its energy source. This is called a metabolic switch. Your body is switching from glucose to ketones.

When your body uses ketones instead of fat, you may lose weight. But, behind the scenes, the ketones also may be having a positive effect on your body’s organs and cells.

To get the benefits of intermittent fasting, you need to fast for at least 12 hours. That’s how long it takes your body to switch from using glucose for energy to using fat for energy. Additionally, it will take your body a while to get used to this new eating schedule. So don’t expect results right away. You may need to wait between 2 and 4 weeks to see or feel any results.

Health benefits of intermittent fasting

While researchers are still studying intermittent fasting, some research has shown it offers some health benefits. For starters, it’s common to lose weight when following intermittent fasting. That’s because your body is using fat—not glucose—as its energy source. Additionally, if you make wise food choices when you do eat, you’re likely eating fewer calories than before you started intermittent fasting. If you add exercise to the mix, it’s a great combination for not only weight loss, but also improved health benefits. Intermittent fasting may help people who have cardiovascular disease, neurological disorders, and some cancers. Intermittent fasting may also help lower your bad cholesterol and improve symptoms of arthritis.

Be sure to talk with your doctor before you begin intermittent fasting. They will help you decide if it’s a good fit for you. They will consider your current health, medicines, and health history when making their recommendation. If you have certain chronic health problems such as diabetes or heart disease, you may need to adjust or monitor your eating patterns.

Things to consider

It may take 2 to 4 weeks for your body to get used to eating on an intermittent fasting schedule. During those first few weeks, you may have headaches and feel hungry, grouchy, or tired. Know you may feel this way before you start and make a plan to push through these feelings. After a few weeks, your body will get used to this eating pattern and those symptoms should go away. In the end, many people say that feel better following an intermittent fasting lifestyle.

When determining an eating schedule to follow, don’t fast for many days in a row. That can be dangerous. It can trigger your body into thinking it’s starving. That can send it into starvation mode, meaning your body will store fat—not use it—for use later.

Intermittent fasting isn’t for everyone. It’s not recommended for people under 18 years of age, women who are pregnant or breastfeeding, or some people with diabetes, kidney disease, or heart issues. Talk to your doctor to see if it’s safe for you.

There is still much for scientists and doctors to learn about intermittent fasting. They’re researching how it affects the body and its systems. They’re also looking to see if there are long-term benefits to following this eating schedule for many years.

Questions to ask your doctor

  • Is intermittent fasting a good choice for me?
  • How do I get past being hungry during the fasting time?
  • What beverages can I drink during the fasting time?
  • What beverages do I need to avoid during fasting time?


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