Celebrities, social media fuel high-fat keto diet fad

A lot of celebrities—from Kourtney Kardashian to LeBron James and Tim Tebow to Halle Berry—have adopted the keto diet. If you haven't heard about yet you likely will soon.

"It's really limiting carbohydrates in the diet and it's a high-fat, high-protein diet," nutritionist Rachel Lustgarten said. "But the focus is really on fat."

The keto, short for ketogenic, diet isn't new, she said. It has been around for years as a way to treat epilepsy. But now it is all the rage on Instagram, with millions of posts about it each day.

The diet is touted as the ultimate reset. It relies heavily on high-fat foods (such as milk, butter, heavy cream, and cheese), lots of protein, and virtually no carbs. Some people claim it has helped them lose up to 60 pounds.

So how does it work?

"When our body doesn't have carbohydrates to use for fuel, it's then converting fat for energy," Lustgarten said.

The diet deliberately places the body in a state of ketosis, where fat is released from your cells and turned into ketones, which the body uses for energy. It sounds great in theory, but Lustgarten warned of the long-term implications.

"People are losing a lot of weight really quickly, and a lot of that comes from water weight," she said. "And there is some of yo-yoing that happens because when someone stops following this very rigid diet and goes back to their regular way of eating, that's when we see weight regain—and that can be really detrimental to health.

And another downside to the keto diet might be getting what is called the "keto flu," at least in the beginning. That is when you are nauseous, you have bad breath, your sweat smells, your urine smells, and you have a really bad headache. But for most people, the keto flu usually passes just as soon as the body detoxes.