Intermittent fasting has undoubtedly garnered a lot of popularity over the years, driven largely by the health and fitness industry with its promise of a safe and effective way to a healthier life.
Not only does the diet trend promote weight loss, but many claim that this eating strategy offers a wealth of other health benefits as well, including increased metabolism, improved sleep and better focus and memory.
But what exactly is intermittent fasting?
— What Is Intermittent Fasting?
— Best Foods to Eat While Fasting
— Foods to Avoid While Fasting
— Does Intermittent Fasting Work for Weight Loss?
— Risks of Intermittent Fasting
— Is Intermittent Fasting Worth It?
What Is Intermittent Fasting?
Intermittent fasting is a dietary approach that alternates between periods of eating and fasting, focusing more on when you eat rather than what you eat.
Also known as “flipping the metabolic switch,” intermittent fasting aims to utilize ketones, 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.
The typical American dietary pattern of three meals and two snacks a day creates a constant source of dietary glucose from consumed food. However, with intermittent fasting, blood ketone levels remain low; therefore, adipose (fatty) tissue stores are not used as a source of energy. It’s a little similar — and less extreme — than the concept behind the keto diet.
During your fasting window, your body begins breaking down a type of fat known as triglycerides for energy.
“The liver then converts fatty acids to ketones to be used as an energy source for the brain and other tissues,” explains Dr. Holly F. Lofton, clinical associate professor of surgery and medicine and director of the NYU Langone Weight Management Program in New York City.
In other words, your body is now burning fat, which can potentially lead to weight loss.
Types of intermittent fasting diets
It’s important to keep in mind that intermittent fasting is a concept, not a specific diet. While there are several variations to intermittent fasting, four common approaches are:
— Time-restricted eating: 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. 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 for energy.
— 5:2 plan: Another popular approach is the 5:2 plan, in which you follow a normal, healthy meal pattern for five days a week. During the other two days a week, you consume only one meal that consists of 500 to 700 calories each day, which is a caloric deficit for most healthy individuals, Lofton says. 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.
— Alternate-day fasting: With alternate-day fasting, you’ll eat normally for 24 hours, then consume nothing for the next 24 hours. This on-off pattern continues every other day indefinitely until the desired results are achieved.
— Eat, stop, eat: This approach to intermittent fasting entails selecting one or two nonconsecutive days each week to fast for 24 hours. During the other non-fast days, you can eat as you normally would to meet your caloric and nutritional needs. For example, if you decide that your fasting window will be from 7 p.m. on Sunday through 7 p.m. on Monday, it’s important to consume a nutritious meal immediately before and after that time period to make sure you don’t exceed fasting for more than 24 hours.
Other versions of intermittent fasting include the fasting-mimicking diet, the ProLon diet and the more extreme Warrior diet.
[READ: Effective Ways to Lose Weight Without Exercise, According to Experts]
Best Foods to Eat While Fasting
Although intermittent fasting doesn’t tell you exactly what foods to eat, you should aim to follow a healthy, well-balanced diet, no matter what version of intermittent fasting you choose to follow.
Fasting principles include:
— Consuming minimally processed foods most of the time.
— Eating a balance of lean protein, vegetables, fruits, healthy carbs, whole grains and healthy fats.
— Creating flavorful, delicious meals that you enjoy.
— Eating your meals slowly and mindfully, until you’re satisfied.
“In fact, 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,” says Ryan Maciel, head dietitian and performance coach with Catalyst Fitness & Performance in Cambridge, Massachusetts.
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:
— Lean proteins.
— Fruits.
— Vegetables.
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. Some types of protein, including red meat like bacon, sausage and hamburger meat, are typically high in fat and raise LDL cholesterol, the “bad” kind associated with an increased risk of heart disease.
Examples of lean, healthy protein sources include:
— Chicken breast
— Ground turkey.
— Beans and legumes.
— Fish and seafood.
— Tofu and tempeh.
Fruits
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.
The government’s Dietary Guidelines for Americans recommend most people eat about 2 cups of fruit daily.
Examples of healthy fruits you should look to consume during intermittent fasting include:
— Apples.
— Avocados.
— Blueberries.
— Blackberries.
— Cherries.
— Strawberries.
— Watermelon.
Vegetables
Vegetables are an important part of an intermittent fasting regimen. The Dietary Guidelines for Americans recommend most people eat 2.5 cups of vegetables on a daily basis.
Vegetables to incorporate into your intermittent fasting protocol include:
— Broccoli.
— Tomatoes.
— Cauliflower.
— Green beans.
— Leafy greens, such as kale, spinach and chard.
Be warned: Fasting may leave people feeling extremely hungry before the first meal and can result in eating larger portions and/or 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,” says Samantha Cochrane,a registered dietitian nutritionist with the Ohio State University Wexner Medical Center in Columbus. “(Listen) 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 around that time.”
If you find that 16 hours is simply too long to wait, then only fast 10 to 12 hours and see if this is more sustainable.
In addition, it’s important to stay hydrated during your fast. Although you will not be able to consume anything during your fast, you can drink water, tea and coffee — without any added sugars or creamers.
[READ: Guide to Boosting Your Metabolism.]
Foods to Avoid While Fasting
While there are plenty of healthy foods you should include in your diet when you’re intermittent fasting, there are also some you should avoid.
As with any healthy, nutritious diet, it’s best to avoid highly processed foods loaded with 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.”
Specifically, sugar that comes in processed foods and drinks is devoid of nutrition and amounts to sweet, empty calories.
“They’ll make you hungry since the sugar metabolizes super fast,” Maciel says.
Examples of foods and drinks you should avoid while intermittent fasting include:
— Candy.
— Cookies.
— Chips.
— Fruit drinks.
— Soda.
— Sweetened coffee and tea.
— Sugary cereals.
[READ: Ways to Eat Well and Save Money at Home]
Does Intermittent Fasting Work for Weight Loss?
Yes, studies show that the primary benefit of intermittent fasting is weight loss.
According to research conducted by Krista Varady, a professor of kinesiology and nutrition at the University of Illinois, Chicago, people tend to lose anywhere between 3% and 7% of their body weight over six months, with most of the weight loss occurring in the initial three to six months. After that, it tends to plateau, as with most other diets. This typically translates to anywhere between 7 pounds and 25 pounds of weight loss for most people.
“The coolest part about these diets is that you don’t have to monitor your calories,” says Varady, whose research is focused on the efficacy of intermittent fasting for losing weight and reducing metabolic diseases in people with obesity. “You literally just watch the clock and limit your eating window to … whatever works for you. By doing that, you’re naturally cutting out calories.”
While intermittent fasting can help you establish a caloric deficit by limiting the time during which you’re eating, 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 more weight.
Although research shows a correlation between intermittent fasting and other health benefits, it does not necessarily mean there’s causation. The widely touted benefits of intermittent fasting are largely due to the downstream effect of reducing calories, which is an effective way to lose excess weight that leads to improvements in people’s overall health, particularly lower glucose, blood pressure and cholesterol levels, Varady says.
“If someone loses more than 5% of their body weight, that’s when they tend to see these benefits to their cardiovascular and diabetes risk factors,” Varady adds.
Risks of Intermittent Fasting
Intermittent fasting, however, isn’t a foolproof approach to weight loss or health gains.
Because the human body is designed to go for periods of time without access to food as a result of evolutionary adaptation, it is generally regarded as a safe practice, says Dr. Hooman M. Melamed, a board-certified orthopedic spine surgeon in Beverly Hills, California.
However, some people may experience side effects, including:
— Hunger.
— Constipation.
— Irritability.
— Headaches.
— Halitosis (bad breath).
There are also certain people who simply should not fast, including those who are pregnant or lactating, those with a history of disordered eating, children and those with any form of malnourishment.
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.
When advising people with diabetes on intermittent fasting, Cochrane usually recommends that they keep the fasting period to 10 to 12 hours overnight to keep blood sugar steadier and more stable.
Is Intermittent Fasting Worth It?
While a growing body of research suggests that intermittent fasting may be a safe and effective way to shed unwanted pounds and reap the downstream benefits of losing excess weight, it’s important to consider whether it’s sustainable in the long run.
“It’s just another diet with usually temporary results that can have some undesirable long-term consequences,” Cochrane says. “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!”
If you’re determined to try it, experts recommend easing your way into it.
“Start off with a four-hour window of not eating, then move up to six hours and then eight,” Melamed says.
Ultimately, it’s important to talk with your doctor first to see whether any health conditions you have or medications you may be taking could be affected.
More from U.S. News
How to Gain Muscle and Lose Fat at the Same Time
Exercises Trainers Would Never Do
Food Swaps to Lose Weight and Eat Healthier
Foods to Eat During Intermittent Fasting (and What to Avoid) originally appeared on usnews.com
Update 02/26/24: This story was previously published at an earlier date and has been updated with new information.
