If you’ve been hearing a lot more about intermittent fasting lately and wondered what it’s all about, you’re not alone. This approach to eating has recently gained traction among dieters and some experts, who say it’s a safe and effective way to lose weight and potentially improve your overall health. Supporters also claim it’s easier to adhere to than other diets and offers more flexibility than traditional calorie-restricted diets.
But what exactly is it?
“Intermittent fasting, in general, means having a period of time without eating,” explains Megan Wroe, wellness manager and registered dietitian at St. Jude Medical Center in Southern California. With intermittent fasting, the focus is less on how many calories or macronutrients you’re consuming, and more on when you’re eating them.
Lisa Jones, a registered dietitian based in Philadelphia, says that intermittent fasting, or IF, “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 Theory Behind Intermittent Fasting
Also known as “flipping the metabolic switch,” intermittent fasting aims to utilize ketone bodies (a byproduct of burning fat that is 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 three meals and two snacks a day creates 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 — than the concept behind the keto diet.
Once you’ve had little or no caloric intake for eight to 12 hours, your body begins breaking down triglycerides (a type of fat) for energy, creating a flurry of fatty acids and glycerols (a sugar-alcohol compound).
“The liver then converts fatty acids to ketones to be used as an energy source for the brain and other tissues,” Lofton says. In other words, your body is now burning fat, which can potentially lead to weight loss.
Does Intermittent Fasting Work?
Samantha Cochrane, a registered dietitian nutritionist with the Ohio State University Wexner Medical Center in Columbus, says that “many people take part in intermittent fasting for the purpose of weight loss. It generally works for weight loss due to the decrease in calories from missed meal opportunities during times of fasting.”
Blood sugar control is particularly important in shielding you from Type 2 diabetes, which accounts 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. This 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 highlighting the possible health benefits of intermittent fasting include:
— A review of research published in the journalNutrients in February 2022 suggests intermittent fasting has positive effects on a range of conditions, including obesity, Type 2 diabetes and hypertension.
— A review of research published in the journalClinical 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 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.
Risks of Intermittent Fasting
Intermittent fasting, however, isn’t a foolproof approach to weight loss or health gains.
“Our bodies are built to survive through periods of fasting, so for the average healthy person, there shouldn’t be any serious concerns for safety. However, long term, I worry about effects to metabolism, just like I do with any diet that can heavily restrict calories. When calories are restricted from eating this way, there is potential for a lower metabolic rate long term, making it harder to maintain weight loss/continue to lose weight,” Cochrane says.
Wroe adds that there are certain people who simply should not fast, “such as those who are pregnant or lactating, those with a history of disordered eating, children and those with any form of malnourishment.”
And if you’re taking any medications to manage your blood sugar levels, be sure to check with your physician before starting intermittent fasting, as you could develop hypoglycemia (low blood sugar) or hypotension (low blood pressure). These “could lead to dizziness, headache or extreme fatigue. Some may also experience changes in their bowels or even mood swings,” Wroe says.
When advising people with diabetes on intermittent fasting, Cochrane notes that she usually recommends that they keep the dating period to 10 to 12 hours overnight to keep blood sugar steadier and more stable.
Types of Intermittent Fasting
There are several ways to approach intermittent fasting, Wroe says, “so there’s no one method or agreed-upon definition.” It’s important to keep in mind that intermittent fasting is a concept, not a specific diet. Common approaches include:
— Time-restricted eating.
— 5:2 plan.
— Alternate-day fasting.
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 explains.
“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,” Lofton says.
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.”
With alternate-day fasting, you’ll eat normally for 24 hours, then consume nothing for the next 24 hours, Wroe says. This pattern continues every other day indefinitely until the desired results are achieved.
What to Eat
While intermittent fasting forbids eating during certain time periods, you do need to eat during your feeding window. What exactly that intake should look like, however, is a matter of some debate, Wroe says. “There is a lot of research happening on eating more keto vs. Mediterranean, as well as plans that allow very specific eating even within the fasting window, such as the fasting-mimicking diet,” she explains.
You also have to consider your overall calorie intake. In order to lose weight, you have to create a calorie deficit by reducing your overall calorie intake or burning more through exercise. IF can help make that simpler by limiting the time during which you’re eating, but you have to be careful not to overeat during the feeding window; otherwise, you could end up stuck at the same weight or even gaining weight.
The number of calories a person needs each day varies widely, depending on such factors as your size and how much you exercise. According to the Food and Drug Administration, males aged 26 to 45 who exercise moderately need 2,600 calories a day, and women aged 26 to 50 who exercise moderately need 2,000 calories a day.
In addition, while IF doesn’t tell you exactly what foods to eat, you should aim to keep your meals as healthy as possible, no matter which pattern of IF you’re following. Ryan Maciel, a registered dietitian and head dietitian and performance coach with Catalyst Fitness & Performance in Cambridge, Massachusetts, says that it’s important to apply the same fundamental nutrition principles to intermittent fasting as to other healthy eating plans.
“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.
These principles include:
— Consuming minimally processed foods most of the time.
— Creating flavorful, delicious meals that you enjoy.
— Eating 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. The foods you should be sure to eat on an intermittent fasting diet come from the following food groups:
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 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 during 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.
Examples of healthy fruits you should look to consume during 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 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.
Cochrane adds that while there’s little in the way of a specific eating plan when it comes to IF, “hunger for the first meal after a fast can be extremely high and can result in larger portions and more convenient choices.” This could lead to reaching for the first available and possibly less healthy food.
“My suggestion is to avoid this situation by setting yourself up for success by meal planning and prepping to have more nutritious choices available to you when you break the fast,” she says. She also recommends “listening to your body to better gauge when to start and break a fast. If you start to get very hungry at 10 a.m., try setting yourself up so that your fasting window ends more around that time.”
And if you find that 16 hours is just too long to wait or you find that you’re “absolutely ravenous after 16 hours, resulting in binge eating or more easy or convenient foods, then only fast 10 to 12 hours and see if this is more sustainable.”
Foods to Limit on an Intermittent Fasting Diet
While there are plenty of healthy foods you should include in your diet when you’re intermittent fasting, there are also some you should seek to limit, such as calorie-dense foods or those that 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 snack foods such as 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.
If you’re interested in trying intermittent fasting, Wroe recommends checking with your doctor first to see whether any health conditions you have or medications you may be taking could be affected. “If someone is wanting to try intermittent fasting, I also coach them first on knowing their body so they can assess if they are feeling hypoglycemic (having low blood sugar), for instance, as well as what kinds of foods will be most nourishing after they have fasted.”
Cochrane notes that while IF works for some people, at the end of the day, “it’s just another diet with usually temporary results that can have some undesirable long-term consequences.” She says that most of the research that has looked into intermittent fasting has only examined “short-term results and might only be done on men. I highly encourage people considering this to think about if this is something that makes sense for your lifestyle forever. For some, it might be, but for most it’s not sustainable. If it’s not sustainable, skip it!”
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Update 01/13/23: This story was previously published at an earlier date and has been updated with new information.