Proponents say intermittent fasting is a safe and effective way to lose weight and improve your overall health in a number of ways. Supporters also claim it’s easier to adhere to than other diets and offers more flexibility than traditional calorie-restricted diets.
Intermittent fasting is defined as restricting one’s food intake, and therefore also one’s calorie intake, for a period of time, and then eating regularly for the rest of the time, “rather than focusing on permanent caloric restriction,” says Lisa Jones, a registered dietitian based in Philadelphia. “Intermittent fasting can be an easy and effective way to lose weight. Depending on the type of IF practice, it means you will only eat during certain days of the week or during a specific feeding window of time.”
The number of calories individuals need on a daily basis varies widely, depending on such factors as their size and how much they exercise, she says. According to the Food and Drug Administration, males ages 26 to 45 who exercise moderately need 2,600 calories a day and women ages 26 to 50 who exercise moderately need 2,000 calories a day.
It’s important to keep in mind that intermittent fasting is a concept, not a specific diet.
Can You Eat While Intermittent Fasting?
Also known as “flipping the metabolic switch,” intermittent fasting aims to utilize ketone bodies (a byproduct of burning fat that are generated in the liver) as a source of cellular energy instead of the body’s usual, more efficient process of using dietary glucose, says Dr. Holly F. Lofton, clinical associate professor of surgery and medicine and director of the NYU Langone Weight Management Program in New York City.
The typical American dietary pattern of eating three meals and two snacks a day causes a constant source of dietary glucose from consumed food, she explains. With this type of meal pattern, blood ketone levels remain low, and therefore, adipose (fatty) tissue stores are not really used as a source of energy. That type of fat-burning only happens when a person is asleep or doesn’t have access to food. It’s a little similar — and less extreme — to the concept behind the keto diet.
Once one has little or no caloric intake for eight to 12 hours, triglycerides are broken down, creating a flurry of fatty acids and glycerols. “The liver then converts fatty acids to ketones to be used as an energy source for the brain and other tissues,” Lofton says.
Types of Intermittent Fasting
For many people, a regular and healthy eating pattern entails consuming three well-balanced meals a day: breakfast, lunch and dinner, Jones says. However, intermittent fasting approaches meal planning a little differently.
One of the more popular approaches to intermittent fasting is called time-restricted eating. This approach calls for eating only during an eight-hour window, and fasting for the remaining 16 hours of the day. The fasting period is the “rest” period to allow the body to metabolize the calories that were consumed during the feeding phase, Lofton says.
“If you eat 1,200 calories over eight hours, for example, you will burn those during the 16-hour fast and this will force the body to use some fat (energy reserves) for energy,” says Lofton.
Another popular approach is the 5:2 plan, in which you follow a normal, healthy meal pattern for five days a week. The other two days a week, you consume only one meal of between 500 and 700 calories each day.
A healthy meal pattern typically means eating foods high in fiber, getting 30% or less of your calories from carbohydrates and consuming moderate amounts of fat, Lofton says. “Calorie intake varies widely for individuals, so I would rather not oversimplify it by saying there is a gender-based goal. Five hundred to 700 calories a day on fasting days is a caloric deficit for most healthy individuals.”
Does Intermittent Fasting Work?
Blood sugar control is particularly important in shielding you from Type 2 diabetes, accounting for 95% of diabetes cases. People with Type 2 diabetes have the ability to produce some of their own insulin. However, their bodies can’t use the insulin they produce to completely control blood sugar levels, which is known as insulin resistance.
Unhealthy habits — such as eating too many fatty, high-calorie and nutrient-poor foods, carrying too much weight and not exercising much — are risk factors for developing Type 2 diabetes.
Recent studies suggest the health benefits of intermittent fasting include:
— A review of research published in the journal Nutrients in February 2022 suggests that intermittent fasting has positive effects on a range of conditions, including obesity, Type 2 diabetes and hypertension.
— A review of research published in the journal Clinical Diabetes and Endocrinology in 2021 suggests that “intermittent fasting is an effective non-medicinal treatment option” for Type 2 diabetes. More research is needed to accurately describe the effects of intermittent fasting on weight loss, and doctors should “consider educating themselves regarding the benefits of intermittent fasting.” Patients with diabetes should consult with their doctor before beginning an intermittent fasting regimen to ensure appropriate oversight, researchers wrote.
— Research published in 2021 in the Annual Review of Nutrition suggests that may intermittent fasting may “benefit cardiometabolic health by decreasing blood pressure, insulin resistance, and oxidative stress.” Cardiometabolic health is defined by an array of factors, including an individual’s blood pressure, blood sugar, cholesterol, heart disease risk and weight.
— A study published in the New England Journal of Medicine in 2019 suggests that preclinical and clinical trials have shown that intermittent fasting has broad-spectrum benefits for many health conditions, such as obesity, diabetes, cardiovascular disease, cancers and neurologic disorders. Clinical research has focused primarily on overweight young and middle-aged adults, the study says.
Whatever method of intermittent fasting you choose, it’s important to apply the same fundamental nutrition principles to intermittent fasting as to other healthy eating plans, says Ryan Maciel, a registered dietitian and head dietitian and performance coach with Catalyst Fitness & Performance in Cambridge, Massachusetts.
“In fact,” Maciel says, “these (principles) may be even more critical since you are going for more extended periods without food, which can result in overeating for some people” during the periods when you can eat on the plan.
Intermittent Fasting Foods
If you’re on an intermittent fasting regimen, there are certain foods that are good to consume, Jones says. While there is no specific IF diet, the general principles of healthy eating still apply.
The principles of a healthy eating program include:
— Consume minimally processed foods most of the time.
— Create flavorful, delicious meals that you enjoy.
— Eat your meals slowly and mindfully, until you’re satisfied.
Intermittent fasting diets don’t mandate specific menus. However, if you’re adhering to good eating principles, there are certain types of foods that are best to consume and a few you should limit.
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Foods to Eat on an Intermittent Fasting Diet
The foods you should be sure to eat on an intermittent fasting diet come from the following food groups:
— Lean proteins.
Eating lean protein keeps you feeling full longer than consuming other foods, which is especially important when fasting, and will help you maintain or build muscle, Maciel says. Some types of protein, including red meat like bacon, sausage and hamburger meat, are typically high in fat and contain LDL cholesterol, the “bad” kind that is associated with an increased risk of heart disease.
Examples of lean, healthy protein sources include:
— Chicken breast.
— Beans and legumes, like lentils.
— Fish and shellfish.
— Tofu and tempeh.
As with any eating regimen, it’s important to consume highly nutritious foods while intermittent fasting. Fruits and vegetables are typically packed with vitamins, minerals, phytonutrients (plant nutrients) and fiber. These vitamins, minerals and nutrients can help lower cholesterol levels, control blood sugar levels and maintain bowel health. Another plus: fruits and vegetables are low in calories.
The government’s 2020-25 Dietary Guidelines for Americans recommend that for a 2,000-calorie-a-day diet, most people should eat about 2 cups of fruit daily.
Examples of healthy fruits you should look to consume when intermittent fasting include:
Vegetables can be an important part of an intermittent fasting regimen. Research shows that a diet rich in leafy greens may reduce your risk of heart disease, Type 2 diabetes, cancer, cognitive decline and more. The government’s 2020-25 Dietary Guidelines for Americans recommend that for a 2,000-calorie-a-day diet, most people should eat 2.5 cups of vegetables on a daily basis.
Affordable veggies that can work on an intermittent fasting protocol include:
— Green beans.
Leafy greens are also an excellent choice, as they supply lots of nutrients and fiber. Look to add these options to your diet:
— Collard greens.
Foods to Limit on an Intermittent Fasting Diet
There are certain foods that aren’t as good to consume as part of an intermittent fasting regimen. You should limit foods that are calorie-dense and contain high amounts of added sugars, heart-unhealthy saturated fat or salt.
“They won’t fill you up after a fast, and can even make you hungrier,” Maciel says. “They also provide little to no nutrients.”
To maintain a healthy intermittent eating regimen, limit these foods:
— Snack chips.
— Pretzels and crackers.
You should also avoid foods that are high in added sugar. Sugar that comes in processed foods and drinks is devoid of nutrition and amounts to sweet, empty calories, which is not what you’re seeking if you’re fasting intermittently, Maciel says. “They’ll make you hungry since the sugar metabolizes super-fast.”
Examples of sugary foods you should limit if you’re engaging in intermittent fasting include:
— Fruit drinks.
— Highly sweetened coffee and teas.
— Sugary cereals with little fiber and granola.
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Update 10/19/22: This story was previously published and has been updated with new information.