An extreme new diet has been spreading on Instagram, promoted by people who claim to be wellness enthusiasts. Known as “dry fasting,” the regimen urges participants to restrict food and liquids, including water, for hours or days.

That’s a dangerous idea that is “not healthy in any way, shape or form,” says nutritionist and registered dietitian Leslie Bonci. Dry fasting, or any diet that limits hydration, can have harmful side effects.

“The biggest issue is the limitation on fluid,” Bonci told TODAY Health. “Hydration is absolutely essential. We know that there are so many health impacts to being marginally sub-hydrated, let alone saying, ‘Hey, let’s put ourselves on a massive fluid deficit!’ I just want to start smacking people!”

Sophie, an Instagram user who speaks candidly about her diet and regimen and asked only to be identified by her first name, says that she is a “fruititarian” and intermittent dry faster, typically going 13 to 18 hours a day without hydrating or eating. When she does consume food, most of her diet is based around fruit. She began dry-fasting just over a year ago, after a naturopath recommended the practice, and at the time underwent a 36-hour dry fast to start the regimen.

Hydration is absolutely essential.

“I changed my diet and transitioned to less and less water,” she told TODAY Health. “Nowadays I hydrate from fruits, coconut water and juices.”

Sophie said that she has not had any negative side effects from the fasting, but saw it as a “natural next step” in her diet and claims to feel healthier on this diet than she has before.

“I feel the best after having fruits,” Sophie said.

While there are some fruits with a high water content, such as cantaloupe and strawberries, registered dietitian Sarah Van Reit cautioned against using fruit as a primary source of hydration and said that a diet like Sophie’s is typically not sustainable.

“There is water in our fruits and vegetables, but, for example, a cup of grapes has about 100 milliliters of water in it,” said Van Reit, who frequently works as a nutrition counselor. “An adult body needs over 2,000 milliliters of water per day for just general healthy functioning, so to rely on fruit and vegetables to provide all the fluid that the body needs is not realistic.”