If you thought the ketogenic diet's focus on eating an extremely limited amount of carbohydrates was restrictive, take a look at a trend known as the "keto egg fast."

The short-term diet, also known as the egg fast, calls for eating only eggs and healthy fats.

It is used by keto dieters who want to do everything from jump-start their weight loss to shock their bodies or change their eating habits.

The fast first gained prominence around three years ago and has become more well-known as the ketogenic, or keto, diet has become more popular. The keto diet, described as “Atkins on steroids” for its focus on foods high in fat and low in carbohydrates, was one of the most-Googled diets in the United States in 2019.

Here are some fast facts to know about the keto egg fast.

What are the 'rules' of the keto egg fast?

The recommendations for how to do the fast vary, but there are some overall guidelines that apply.

Dieters are to only eat eggs and healthy fats for the duration of the fast, which typically lasts three to five days.

For every egg eaten, a tablespoon of fat must be consumed with it. The fats on the fast can be anything from butter to cheese, though it's recommended to choose healthy fats like olive oil.

The daily total on the fast is at least six eggs and six tablespoons of fat.

You can also choose to regulate the timing of your meals. That would look like eating one egg within 30 minutes of waking up, eating an egg "meal" every three to five hours, and eating three hours before bed.

How is it supposed to work?

The theory is that you will be in a calorie deficit and ketosis on the diet.

Eggs are featured in the fast because they are a low-calorie, complete source of protein that can keep you from feeling hungry.

Going into ketosis -- whereby your body is so low on carbohydrates that it starts burning fat for fuel -- is the basis for the keto diet. Ketosis is also what the body does when fasting.

Proponents of the keto diet say it is the best way to lose weight without feeling hungry and that it increases energy levels -- and the same is true for the egg fast.

In addition to losing weight, the egg fast is seen as a way to change eating habits, according to Matt Barrot, who, with his wife Megha, runs the website Keto Connect.

"Part of the methodology behind it is the palatability of the food," he told "Good Morning America." "If you’re hungry you’ll eat eggs. If you’re not hungry you won’t eat an egg."

"It helps to align yourself with your actual hunger instead of the emotional hunger," Barrot explained.

What does it feel like to be on the keto egg fast?

The Barrots undertook the keto egg fast two years ago after receiving a high number of inquiries about it on their website.

They planned to do it for five days but stopped at four because "it really sucked," according to Megha Barrot who, like her husband, has followed the keto diet for around five years.

"It's not something we really recommend," she told "GMA." "Once you do it you’re going to go back to your normal way of eating and whatever water weight you lost is going to come back."

Matt Barrot said he found the fast did "its job" of keeping his appetite satisfied and making it easy to identify habitual eating versus true hunger.

He still, though, recommends looking at keto or any eating plan as a lifestyle change instead of a diet or a quick fix.

"What really matters is making lasting changes over time and not being perfect from Day One," he said. "Doing the egg fast would be a really dramatic change and doing something dramatic may lead you to relapse."

What do the experts say?

First, it should be said that although the keto diet is among the most-search diets online, it consistently ranks low on U.S. News & World Report's annual ranking of best diets because of its restrictions.

"The experts who rate diets for U.S. News say there’s simply no need to cut out entire food groups, such as dairy or whole grains, and doing so could put you at risk of nutritional deficiencies," Angela Haupt, managing editor of health at U.S. News & World Report, told "GMA" in January when the 2020 rankings were released. "It also makes a diet particularly difficult to follow, which lowers the odds of lasting success."

Similarly, two experts "GMA" spoke with said they would not recommend the egg fast diet to their patients. However, they also pointed out that because the fast is short-term, it is not likely to cause any long-term damage.

"My first reaction was, 'Oh boy, another fad out there,'" said Dr. Elizabeth Bradley, medical director for the Cleveland Clinic Center for Functional Medicine, adding that it reminded her of the cabbage diet. "My impression is that it's not necessarily the healthiest thing, but it’s also probably not going to kill you because it’s only three to five days."

Bradley, who runs a functional ketogenic program at the Cleveland Clinic, called eggs the "the epitome of a perfect protein," but said they are best eaten as part of a balanced diet, not combined solely with fats.

She advises her patients to consume a diverse mix of healthy vegetables and healthy proteins and fats like avocados and olive oil. Instead of one egg and one fat at every meal as in the fast, Bradley recommends patients on the keto diet consume at each meal a palmful of protein, two tablespoons of fat, and two cups of vegetables.

"We could put someone quickly into ketosis with MCT oil," she said. "You do not have to have an egg diet to do it."

PHOTO: Eggs boiling in pot of hot water.
STOCK PHOTO/Getty Images
Eggs boiling in pot of hot water.

Bradley also pointed out that any weight loss that may come as a result of the egg fast is likely water weight that will reappear once the fast is over.

Dr. Nieca Goldberg, a cardiologist and medical director of the Joan H. Tisch Center for Women's Health at NYU Langone Health, also raised concerns about the short-term nature of the egg fast.

"When you’re looking for weight loss, it’s quick to jump to a new program offering dramatic changes, but people should be looking at the long-term and what they can do to lose weight long-term," she said. "Probably people will find the [egg fast] diet to be pretty boring and that’s a lot of why people fall off diet programs."

Goldberg said any person should check with their doctor before starting on a diet like the egg fast. People who have high cholesterol or chronic health conditions like diabetes, high blood pressure and heart disease need to be especially cautious, according to Goldberg.

"From a cardiology point of view, a healthy diet is one that eliminates processed foods, because they often have a lot of salt and sugar, and increases the intake of fruits and vegetables and whole grains," she said. "It's low in meats and sweets."