Fitness influencers swear by the 'carnivore diet'—it's 'basically a terrible idea,' doctor says

Beef, butter, bacon and eggs — that's what some influencers swear by for the "carnivore diet." The diet beefs up on meat and minimizes or cuts out fruit and vegetables entirely.

On TikTok, people can be seen eating bowls of steak and 12 scrambled eggs all in one day — and some even snack on a stick of butter, biting off a piece the way one would a carrot.

The diet, similar in style to the Atkins and keto diets, goes by many names: carnivore diet, lion diet, high-fat diet and animal-based diet. Devout followers of the lifestyle boast that their skin is clearer than it's ever been, their gut is healthier and they're in the best shape of their lives.

"One of the best things that's happened since I quit the vegan diet and went carnivore is that my body odor just disappeared," TikToker @steakandbuttergal said in one of her videos. "I don't use any soap, I don't use any deodorant and I smell amazing."

Here's what experts have to say about the safety and sustainability of the carnivore diet.

Weight loss is one of the huge benefits that people who follow the carnivore diet claim they've experienced since adding more animal-based products to their diet. This is likely because the eating pattern also cuts down on carbs, says Dr. Walter Willett, a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health.

"It's possible that some people who have been eating a lot of refined starch and sugar may get better in the short run," with the carnivore diet, Willett says. "But this sounds like a diet that is going to be very unhealthy in the long run."

With a diet of just beef, butter, bacon and eggs, people won't get enough fiber, carotenoids and polyphenols which are rich in fruits and vegetables.

Getting fiber in your diet is vital for gut health and can lower your chances of developing depression and breast cancer. Carotenoids have cancer-fighting properties, and polyphenols have properties that can protect against the development of health conditions like diabetes, heart disease and cancer.

The foods that are prominent in carnivore diets also contain high amounts of saturated fat and cholesterol, Willett adds.

In a 2012 study published in the Archives of Internal Medicine, Harvard researchers found that of more than 100,000 men and women, "People in the study who ate the most red meat tended to die younger, and to die more often from cardiovascular disease and cancer," according to Harvard Health Publishing.

Despite the multitude of studies that connect red meat consumption and heart disease, some people just don't agree that consuming red meat often is bad for your heart.

"This is the the mainstream messaging that we hear about red meat. It's essentially been blamed for all kinds of human health catastrophes, from cardiovascular disease to colon cancer," says Dr. Georgia Ede, a Harvard-trained, board-certified psychiatrist who specializes in nutritional psychiatry.

"They're based almost entirely on a type of research method called nutrition epidemiology, which is just untested theories, essentially, guesswork, about how red meat might be affecting us, that have never been tested in clinical trials and been found to be supported," Ede says. "Then the rest of the very little additional evidence that does come from experimental studies, that comes from very strange animal studies."

To better understand how food intake may lead to disease, researchers have study participants write down or complete surveys about what they've eaten, which are all self-reported.

Some believe that this is a flawed way of coming to conclusions about how foods impact health, but experts have yet to land on a better alternative.

'If you're eating that kind of meal, you're helping bring down another tree'

But even if people are really weary about the way in which nutritional studies are conducted, what can't be denied are the effects of meat production on the climate.

To this, Ede says: "Industrialized food production, whether it's plants or animals, is really very harmful for the planet."

And while this is true, there is a clear difference between how much the production of plant foods is impacting the environment versus animal-based products. The emissions of global greenhouse gases, like methane, from the production of animal-based foods are double that of the production of plant-based foods.

"In addition to the direct health effects that are going to be quite adverse," Willett says. "There's also the issue of justice that basically the Global North, Europe [and] the United States, cause most of the problems with climate change that we have today, and this sort of perpetuates that."

"You can think [that] if you're eating that kind of meal, you're helping bring down another tree on the other side," he adds. "Sounds like basically a terrible idea."

Want to make extra money outside of your day job? Sign up for CNBC's new online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories. Register today and save 50% with discount code EARLYBIRD.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.