Experts say the keto diet isn't sustainable, so why is it so popular?

The keto diet calls for banning carbs to make your body burn fat for fuel. (Shutterstock / CNN)

America is in the midst of a keto craze. The trendy diet -- which bans carbs to make your body burn fat for fuel -- has kicked Weight Watchers' derrière on the stock market, captured the endorsement of celebrities such as Kourtney Kardashian and Halle Berry, and deluged the internet with recipes and copious social media chatter about pounds lost.

Now the popular diet even has a day named after it. The Vitamin Shoppe, which wants to sell you a ton of keto-based products, has named the first Sunday of this new decade "National Keto Day."

"What on Earth justifies granting a day to memorialize a fad diet?" said Dr. David Katz, founding director of the Yale University Prevention Research Center. "The grapefruit diet surely warrants its own day too!"

Katz is no fan of keto, or any other diet that restricts entire food groups, calling them unhealthy and unsustainable.

"Losing weight fast by using a severely restricted, silly, unbalanced diet inevitably leads to even faster weight regain," said Katz, who is the president of the True Health Initiative, a non-profit organization dedicated to health promotion and disease prevention.

"Absent ketosis, keto is just a false label for some kind of diet that presumably restricts added sugar and refined carbohydrate -- which, frankly, any good diet does," Katz said.

Katz's low opinion of keto is echoed by many nutritional specialists across the country. Katz joined 24 other top names in the field to rank 35 popular weight loss programs for 2020 recently put out by U.S. News and World Report.

The popular keto diet flunked, coming in next to last -- which it has done for several years now. Only the highly restrictive protein-only Dukan Diet ranks lower.

"Most health professionals are concerned that the degree of carb restriction requires someone to cut out many of the foods that have been consistently recommended as being healthy: fruits, beans/legumes and whole intact grains," said Stanford professor Christopher Gardner, who conducts research on low-carb diets at Stanford Prevention Research Center.

With such negative reviews, just how did keto capture such a faithful following? Experts say it's because its legions of fans are focusing on the short-term benefits of fast weight loss, without factoring in possible long-term risks.

What is keto?

Keto is short for ketosis, a metabolic state that occurs when your liver begins to use stored fat to produce ketones for energy. The liver is programmed to do that when your body loses access to its preferred fuel -- carbohydrates -- and thinks it's starving.

The diet has actually been around since the 1920s, when a doctor stumbled on it as a way of controlling seizures in children with epilepsy who didn't respond to other treatment methods.

"It was recognized long ago that denying the brain access to glucose, and converting to ketone-based metabolism, dampens brain electrical activity," Katz said. "But why on Earth would you want to dampen brain electrical activity unless you had refractory (unmanageable) epilepsy?"

Creating ketosis is not as simple as it seems. Your liver is only forced into producing ketones when carb intake is drastically slashed. In the keto diet, you limit your intake of carbs to only 20 to 50 a day, the lower the better. To put that into perspective, a medium banana or apple is around 27 carbs, the full day's allowance.

It can take several days to weeks before your body fully transitions into burning fat. In the meantime, it will scream for carbs, and (speaking from personal experience) will punish you by sending a zombie to suck out your brains, a vampire to drain your blood and a giant troll to jump up and down on your body.

The feeling of fatigue and malaise is so bad that keto-lovers have christened the experience "keto-flu."

You'll also have "keto-breath," a wonderfully metallic smell similar to nail polish remover emanating from your mouth. Other than urination, that's the only way ketones can escape your body.

Drinking water might help with dragon-breath. You'll also need to drink a lot of water to try to counter constipation and other gastric-grumblings due to the lack of fiber from fruits and starchy veggies.

Once all that passes, keto-lovers maintain, you'll have more energy, a more focused brain, and best of all, very little hunger.

But those effects only last if you stay in ketosis. Cheat a bit, and your body scrambles to go back to what nature intended.

Therefore low-carb diets like keto rely heavily on fats to fill you up. At least 70% of the keto diet will be made up of fat -- some say it's more like 90%. Of course you can get all that fat from healthy unsaturated fats such as avocados, tofu, almonds, walnuts, seeds and olive oil.

But just in case you can't eat that many avocados, the diet also allows those not-so-good-for-your-arteries saturated fats like lard, butter, palm and coconut oils as well as whole-fat milk, cheese and mayonnaise.

And here's a twist: You can't rely too much on lean protein to accomplish ketosis. Eat more protein than an average 20% of your daily calories and your body will use that, and not fat, for fuel. Bye bye, ketosis.

Therefore protein sources for ketosis reply on "skin-on poultry, fattier parts like chicken thighs, rib-eye steaks, grass-fed ground beef, fattier fish like salmon, beef brisket or pork shoulder," according to U.S. News, as well as -- get ready America -- bacon!

Yessss. That's why this is a popular diet right? Like the dog in the 1980s commercial, we as a nation collectively jump up and down for bacon.

'Dirty' vs 'clean' eating

Of course the lure of all-the-bacon-or-fat-you-can-eat was arguably behind the initial success of the Atkins diet that exploded into popularity in the '90s. It was followed by more low-carb options such as South Beach, Paelo, Whole30 and Zone, among others.

Yet critics say those initially popular plans have struggled to keep the public's interest as dieters have succeeded in losing some weight, only to fail to keep it off over the long term.

Atkins has rebranded, offering different levels of carb restriction they call "Atkins 20" and "Atkins 40." Colette Heimowitz, Atkins vice president for Nutrition Communication & Education, told CNN the company's approach allows for more flexibility than keto "as we encourage people to incorporate foods back into their meals and find their carb tolerance level."

Keto appears to be undergoing the same process, with some promoting "clean" keto, which focuses on using all those avocados, nuts and seeds for fat sources, instead of "dirty" keto, in which folks take the buns off their fast food burger and chow down.

Clean keto advocates admit that it takes a good deal of effort to research food items and plan and prep meals, so "unsurprisingly, many a keto eater takes the easy way out, eating a diet centered around foods like bacon, cheese, butter, and packaged foods," according to an article on the Vitamin Shoppe's Keto HQ.

And that's the crux of the problem for nutritionists.

"Most people who claim to eat 'Paleo' use that banner to justify eating any kind of meat they like, notably, bacon, burgers and pepperoni," Katz said. "There was no paleolithic pepperoni!

"No doubt, the same is going on with keto -- people invoke the label to eat the foods they want to eat, notably processed meat," he said. "I suspect a very tiny minority of those attempting to eat keto are either eating clean or are in ketosis."

What do the studies say

Then there's the issue of varying health claims for keto and other low carb diets.

"The ketogenic diet is designed to be a short-term diet, and there are a number of studies and trials demonstrating its effectiveness," said chiropractor Josh Axe, a spokesperson for the Vitamin Shoppe, in statement.

"When done correctly, it can be a great tool used to treat and prevent several chronic conditions while also supporting overall health," said Axe, who is the author of "The Keto Diet: Your 30-day Plan to Lose Weight, Balance Hormones and Reverse Disease."

An Atkins spokesperson pointed to a two-year study by a health group selling ketosis diet interventions and told CNN in a statement that "today's science" shows "people can improve health markers pertaining to weight loss, cardiovascular disease and metabolic syndrome" when they control carbs.

Not exactly accurate, according to Gardner and Katz.

"There's very little research, and to the best of my knowledge, all of it is linked to a company marketing the keto diet," Katz said.

"The bottom line is that despite its current popularity, we have very few studies that can support or refute its impact on health," Gardner said.

The National Lipid Association Nutrition and Lifestyle Task Force reviewed all the available evidence in 2019 and found low and very-low carb diets "are not superior to other dietary approaches for weight loss," and in some cases even raised cholesterol levels.

In addition, they found "three separate observational studies, including a large prospective cohort study with long-term follow-up," showed an association between very low-carb diets and "all-cause mortality."

So far, at least, it appears science has found the benefits of low-carb diets are fleeting.

"What the early studies have shown is that there are early benefits in terms of weight loss and glucose control," Gardner said. "But in the few studies that have gone on for 12 months, the benefit in comparison to other diet approaches diminishes and is no longer statistically significant."

Which is why nutritionists fail to see the benefit of subjecting your body to the stresses of a low-carb diet just to lose a bit of weight, gain it back, and then start all over again.

"To achieve and maintain a healthy body weight, or optimize diabetes or heart disease risk factors, we should not be focusing on a 'diet', " said Alice Lichtenstein, director and senior scientist at Tuft's University's Cardiovascular Nutrition Laboratory.

"We should be focusing on dietary patterns, making changes in current practices that can be sustained lifelong."