What Is Intermittent Fasting?

By 2030, experts estimate that 3 out of 4 Americans will be overweight or obese. Obesity is linked to many health risks, including type 2 diabetes and heart disease. But intermittent fasting could be a powerful tool for weight loss and better health outcomes. Here, we look at the science behind intermittent fasting.^1,2

What is intermittent fasting?

Fasting means not eating or limiting the amount you eat for a certain period of time. Intermittent fasting is a way of eating that involves fasting on and off. Intermittent fasting is based on research that shows your health is impacted by not just what you eat but also when you eat. Intermittent fasting has shown promise for managing obesity and other health problems.^1,3

How does intermittent fasting work?

Fasting causes the body to change how it uses energy. This results in decreased blood sugar and improved insulin sensitivity. Fasting also helps the body use fat stores as an energy source.³

There are the 3 main approaches to intermittent fasting:^1,4

• Alternate-day fasting – This involves not eating (or eating around 500 calories) every other day. On the other days, you eat a regular healthy diet.
• 5:2 fasting – The 5:2 approach involves eating normally 5 days per week. On the other 2 days, you limit your eating to a single 500- to 700-calorie meal. Fasting days are spaced out during the week. For example, you might fast on Monday and Thursday and eat normally the rest of the week.
• Daily fasting / time-restricted feeding – Here, eating is limited to a single 6 to 12-hour period each day. For the rest of the day, you do not eat. For example, you might eat a regular healthy diet from 10 AM to 6 PM and fast the rest of the day.

More research is needed to determine the best approach to fasting. Fasting for more than 24 hours is not better than shorter fasts. In fact, it could be dangerous. To avoid dehydration during fasting periods, drink plenty of calorie-free fluids like water or unsweetened tea.^1,3,5

What are the benefits of intermittent fasting?

There may be several health benefits of intermittent fasting. There is strong evidence that intermittent fasting could be a helpful treatment for the following health problems:^3,6-10

• Obesity
• Inflammation
• Heart disease
• High blood pressure
• High cholesterol
• Insulin resistance and type 2 diabetes

Although more research is needed, studies also suggest that intermittent fasting may help prevent brain cancer and improve brain function. Experts believe that intermittent fasting may even help you live longer.^3,11

What are the risks of intermittent fasting?

Intermittent fasting can be hard to maintain in the long term. Also, some people report unpleasant side effects, including:⁵

• Hunger
• Fatigue
• Trouble sleeping
• Irritability
• Trouble concentrating
• Nausea
• Constipation
• Headaches

Experts warn that frequent or long periods of fasting can increase the risk of gallstones. And some research suggests that skipping breakfast could increase the risk of heart attack and death. However, fasting for about 12 hours seems to have benefits without increasing risks.³

Fasting is not recommended for children, people with a history of eating disorders, or people with type 1 diabetes. Talk with your doctor before trying intermittent fasting if you have other health problems or are taking prescription drugs.⁵

Intermittent fasting and weight loss

Numerous studies have shown that intermittent fasting may have many health benefits, including weight loss. Studies of alternate-day fasting and 5:2 fasting have shown weight loss ranging from 1 to 13 percent of body weight after several weeks. The weight loss caused by intermittent fasting is similar to that caused by cutting calories. But fasting may be more flexible and so easier to maintain.^3,7

A 2018 study looked at people who fasted for 16 hours per day. Study participants were allowed to eat between 10 AM and 6 PM and fasted for the rest of the day. After 12 weeks, the fasting group lost nearly 3 percent of their body weight. Their blood pressure also improved.⁸

One of the keys to getting benefits from intermittent fasting is eating a healthy, well-balanced diet. It should include plenty of vegetables, fruits, whole grains, and high-quality protein. Be sure to avoid overeating on your non-fasting days.⁵

Intermittent fasting may not be right for everyone. Talk with your doctor if you want to try intermittent fasting for weight loss to make sure it is safe for you.