The latest diet craze among celebrities promises weight loss without sacrificing bacon.

The keto diet, short for "ketogenic," involves eating a high amount of fat, a moderate amount of protein and very few carbs — even fruit is off the table. As with any fad diet, adherents tout weight loss, increased energy and greater mental clarity among the benefits. But is the keto diet all it's cracked up to be?

Not precisely, nutritionists and dietitians say. Low-carb diets like the keto do appear to lead to some short-term weight loss, but they're not significantly more effective than any other commercial or self-help diet. And they don't appear to improve athletic performance. [Diet and Weight Loss: The Best Ways to Eat]

"Depending on your approach, [keto diets] can contribute to significant lean body mass loss along with fat loss," said Melinda Manore, a professor of nutrition at Oregon State University. (Typically, dieters want to shed only fat, not lean body mass, which includes muscle.) And as with other fad diets, people typically regain the weight once they go off the diet.

The keto diet was originally designed not for weight loss, but for epilepsy. In the 1920s, doctors realized that keeping their patients on low-carb diets forced their bodies to use fat as the first-line source of fuel, instead of the usual glucose. When only fat is available for the body to burn, the body converts the fats into fatty acids, and then into compounds called ketones, which can be taken up and used to fuel the body's cells.

For reasons not entirely understood even today, fueling the body on primarily ketones reduces seizures. However, with the development of anti-seizure medications, few people with epilepsy rely on ketogenic diets today, according to a 2008 paper in the journal Current Treatment Options in Neurology, but some people who don't respond to medications can still benefit.

For weight loss, today's keto diets are the descendants of low-carb diets like the Atkins diet, which peaked in popularity in the early 2000s. Both types of diets reject carbs in favor of meatier meals. There is no single blueprint for the keto diet, but plans usually call for eating fewer than 50 grams of carbohydrates a day. (Wheat bread contains about 16 grams of carbohydrates per ounce, according to the USDA.) Celebrity adherents to the diet include Halle Berry and Kourtney Kardashian.

A keto diet forces the body into a state called ketosis, meaning that the body's cells depend largely on ketones for energy. It's not entirely clear why that leads to weight loss, said Jo Ann Carson, a professor of clinical nutrition at the University of Texas Southwest Medical Center and the chair of the American Heart Association's (AHA) Nutrition Committee, but ketosis seems to blunt the appetite and may affect hormones like insulin that regulate hunger. Fats and proteins may also keep people fuller than carbohydrates, leading to lower calorie intake overall, Carson told Live Science.

Still, studies of low-carb diets don't paint them in a particularly revolutionary light. When researchers pit branded diets head-to-head in studies, they find that no particular diet, be it low-carb or low-fat, stands out as a winner.  

In one head-to-head comparison published in the Journal of the American Medical Association in 2014, researchers analyzed 48 separate diet experiments in which participants were randomly assigned to one of several popular diets. The diets included the low-carb Atkins, South Beach and Zone diets as well as low-fat diets like the Ornish diet and portion-control diets like Jenny Craig and Weight Watchers.

They found that any diet resulted in more weight loss than no diet at all after six months. Low-fat and low-carb diets were pretty much indistinguishable, with low-carb dieters losing 19 pounds (8.73 kilograms), on average, and low-fat dieters losing an average of 17.6 pounds (7.99 kg), both compared to non-dieters. At 12 months, the benefits showed signs of leveling off for both types of diets, with both low-fat and low-carb dieters reporting being 16 pounds (7.27 kg) lighter, on average, than non-dieters.

"Weight loss differences between individual named diets were small," the researchers concluded. "This supports the practice of recommending any diet that a patient will adhere to in order to lose weight."

Another analysis of popular diets published in the Annals of Internal Medicine in April 2015 found the Atkins diet to result in more weight loss than simply educating people on portion control, but also noted that most of the studies of this low-carb diet involved registered dieticians helping participants make food choices, rather than the self-directed process by which most people pick up the diets. That's true of many diet studies, the researchers noted, so study results likely look rosier than weight loss in the real world. 

Finally, a direct comparison of low-fat and low-carb dieting, published in February in the journal JAMA, found that over a year, there was no statistically significant difference in the amount of weight dropped. Low-fat dieters lost 11.7 pounds (5.3 kg), on average, and low-carb dieters lost 13 pounds (6 kg), on average.

Keto diets "can help us lose weight, but compared to other diet strategies, they're not more helpful," said Melissa Majumdar, a dietitian at the Brigham and Women's Center for Metabolic and Bariatric Surgery, and a spokesperson for the Academy of Nutrition and Dietetics. Much of the weight lost in the initial stages of a keto diet is water weight, because carbohydrate stores in the body carry water molecules with them, Majumdar told Live Science. That can move the scale an exciting amount initially, but weight loss inevitably slows with time.

Unfortunately, keto diets are probably more prone than many others to end with weight regain because they can be hard to stick to in the long run, Carson said. And being in ketosis for more than a few weeks might not be best for overall health, she said. [The Best Way to Lose Weight Safely]

What's more, "there is concern about [levels of] saturated fat and cholesterol" in the diet, she said. Some of this is genetically determined: The extent to which dietary fats and cholesterol translate to increased levels of blood cholesterol (which are associated with heart disease) is partly individual, Carson said.

"It probably is particularly important if you're going to try something like a ketogenic diet is to really check with your physician [and] get lab work done to make sure you're not doing extreme harm," she said.

Ketogenic diets also tend to cause more calcium to be lost in the urine, Carson said, which can lead to a decrease in bone density over time and increase the risk of osteoporosis.

"Another overall issue is that when you're following a ketogenic diet, you're probably not eating as much regular fruits and vegetables as a heart-healthy diet pattern would have," Carson said.

The AHA recommends eating a variety of fruits, vegetables, whole grains, low-fat dairy, skinless poultry and non-fried fish, nuts and legumes, and limiting red meat, saturated fat and sweets for a heart-healthy diet.

"I do not promote the ketogenic diet for patients, because it's generally not sustainable, and anytime we're taking out whole food groups we're missing whole nutrients," Majumdar said. In the case of keto diets, putting the kibosh on fruits, many veggies and whole grains means that people don't end up consuming much fiber.

The AHA also recommends at least 150 minutes of moderate exercise a week for heart health. Weight loss and maintenance might take a lot more exercise than that, Carson said, which could be a problem for those on ketogenic diets.

Regarding ketones as fuel, Carson said, "It's harder to have energy to be physically active."

Original article on Live Science.