Intermittent Fasting
Key Points
Intermittent fasting focuses on when you eat instead of how much or what you eat.
Research shows that intermittent fasting is a way to manage your weight and prevent — or even reverse — some forms of disease.
There are several fasting schedules available with intermittent fasting.
Intermittent fasting isn’t for everyone, so talk to your doctor before you consider starting.
Intermittent fasting is an eating plan that switches between fasting and eating on a regular schedule. Johns Hopkins neuroscientist Mark Mattson,who has studied intermittent fasting for 25 years, explains this dietary approach.
What is intermittent fasting?
Many diets focus on what to eat, but intermittent fasting is all about when you eat. It limits eating to specific time windows to support metabolic changes like burning fat after sugar stores in the body are depleted. With intermittent fasting, you only eat during a specific time of the day or on certain days of the week. Research shows fasting for a certain number of hours each day or eating just one meal a couple days a week may have health benefits.
How does intermittent fasting work?
Intermittent fasting works by prolonging the period when your body has burned through the calories consumed during your last meal and begins burning fat.
“Intermittent fasting contrasts with the normal eating pattern for most Americans, who eat throughout their waking hours,” Mattson says. “If someone is eating three meals a day, plus snacks, and they’re not exercising, then every time they eat, they’re running on those calories and not burning their fat stores.”
In prehistoric times, before humans learned to farm, they were hunters and gatherers who evolved to survive — and thrive — for long periods without eating. Our bodies still operate in this same mode. Mattson says that after hours without food, the body exhausts its sugar stores and starts burning fat. He refers to this as metabolic switching.
What are the different intermittent fasting plans?
Daily Approach
The daily approach restricts daily eating to one six- to eight-hour period each day. For instance, you may choose to try 16/8 fasting: eating during an eight-hour window and fasting for 16 hours.
5:2 Approach
The 5:2 approach involves eating regularly five days a week. For the other two days, you limit yourself to one 500–600 calorie meal. An example would be if you chose to eat normally on every day of the week except Mondays and Thursdays, which would be your one-meal days.
Longer periods without food, such as 24-, 36-, 48- and 72-hour fasting periods, are not necessarily better for you, and may be dangerous. Going too long without eating might actually encourage your body to start storing more fat in response to starvation.
How long does it take for intermittent fasting to work?
Mattson’s research shows that it can take two to four weeks before the body becomes accustomed to intermittent fasting. You might feel hungry or cranky while you’re getting used to the new routine. But, he observes, research subjects who make it through the adjustment period tend to stick with the plan because they notice they feel better.
What can I eat while intermittent fasting?
While fasting:
Water
Zero-calorie beverages such as black coffee and tea
When eating normally:
Mediterranean diet: Most nutrition experts regard the Mediterranean diet as a good blueprint of what to eat, whether you’re trying intermittent fasting or not. You can hardly go wrong when you pick leafy greens, healthy fats, lean protein and complex, unrefined carbohydrates such as whole grains.
Wide range of foods: What some experts like about intermittent fasting is that it allows for a range of different foods to be eaten — and enjoyed. Sharing good, nutritious food with others and savoring the mealtime experience adds satisfaction and supports good health.
Avoid high-calorie foods: Research shows that you’re not likely to lose weight or get healthier if you pack your feeding times with high-calorie food or a much larger volume than you typically eat for a meal.
What are the benefits of intermittent fasting?
Here are some intermittent fasting benefits research has revealed so far:
Thinking and memory. Studies discovered that intermittent fasting boosts working memory in animals and verbal memory in adult humans.
Heart health. Intermittent fasting improved blood pressure and resting heart rates as well as other heart-related measurements.
Physical performance. Young men who fasted for 16 hours showed fat loss while maintaining muscle mass. Mice who were fed on alternate days showed better endurance in running.
Type 2 diabetes and obesity. In animal studies, intermittent fasting prevented obesity. And, in six brief studies, obese adult humans lost weight through intermittent fasting. People with type 2 diabetes may benefit: Most of the available research shows that intermittent fasting can help people lose body weight and lower their levels of fasting glucose, fasting insulin and leptin while reducing insulin resistance, decreasing levels of leptin and increasing levels of adiponectin. Certain studies found that some patients practicing intermittent fasting with supervision by their doctors were able to reverse their need for insulin therapy.
Tissue health. In animals, intermittent fasting reduced tissue damage during surgery and improved results.
“Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers,” Matton adds.
Is intermittent fasting safe?
Some people try intermitting fasting for weight management, and others use the method to address chronic conditions such as irritable bowel syndrome, high cholesterol or arthritis. Overall, research continues to indicate that it is a safe way to reduce weight. However, it’s important to check with your doctor before trying intermittent fasting.
Who shouldn’t try intermittent fasting?
Children and teens under age 18.
Women who are pregnant or breastfeeding.
People with type 1 diabetes who take insulin. While an increasing number of clinical trials have shown that intermittent fasting is safe in people with type 2 diabetes, there have been no studies among people with type 1 diabetes. Mattson explains, “Because those with type 1 diabetes take insulin, there is a concern that an intermittent fasting eating pattern may result in unsafe levels of hypoglycemia during the fasting period.”
Those with a history of eating disorders. People with a history of eating disorders are advised to avoid intermittent fasting because the strict fasting windows can trigger or worsen restrictive or obsessive food behaviors.
Are there side effects to intermittent fasting?
Keep in mind that intermittent fasting may have different effects on different people. Talk to your doctor if you start experiencing unusual anxiety, headaches, nausea or other symptoms after you start intermittent fasting.
How long can I practice intermittent fasting?
People not in the categories listed above, and who have discussed the diet with their doctor, can practice intermittent fasting safely on an ongoing basis. It can be a lifestyle change ― and one with benefits.
Medically reviewed by Mark Mattson, Ph.D.