A new fad diet making the rounds on wellness influencer Instagram won't actually help you lose weight. And it could cause dehydration, urinary tract infections, kidney stones, organ failure - even death.
It's called "dry fasting." It goes beyond what most of us would consider fasting - abstaining from solid food or liquid calories - and requires consuming no water or liquids of any kind for many hours or even days at a time.
Instagram and other social media sites have provided a glossy new platform for extremely dubious health and nutrition claims. Posts about dry fasting often tout the need to "heal" or "rest" or "reset" your kidneys, or "boost" their filtration. In practice, what dry fasting will do is make you look a bit more toned, because your body is using up the water in your cells for energy.
Even more dubious claims suggest that dry fasting forces your body to burn toxins, or fat, or inflammation, or tumors. It does not. When you stop feeding your body calories, it breaks down muscle and fat. The toxic byproducts of that breakdown process build up in your system, requiring extra hydration to flush them out.
In other words, if you're abstaining from food, your body needs more water, not less.
Experts agree: There is no dietary or nutritional reason to go on a "dry fast."
"I don't recommend it at all," said Dr. Pauline Yi, a physician at UCLA Health Beverly Hills who regularly treats patients in their late teens and early 20s. She said intermittent fasting and other fasting-type diets are a popular topic with patients, and she has no problem with people trying them out.
"But I also tell them when you're fasting you have to drink water," she said. "You cannot go without hydration."
The majority of the human body is water. Your individual water consumption needs depend on your height, weight, health and the climate, but generally speaking, Yi said people should be consuming at least 68 ounces - almost nine cups - of water every day.
Cary Kreutzer, an associate professor at USC's schools of gerontology and medicine whose area of expertise includes nutrition and diet, says digestive systems aren't meant to have extended "breaks." She likened making your kidneys go without water to letting your car's engine run out of oil. "You can basically burn out some parts of the car that you're going to have to get replaced," she said. "You don't want those replacement parts to include your vital organs."
Another unintended consequence of dry fasting: It sets your body in water-conservation mode.
"Your body likes homeostasis," said Yi, the physician. "If you're going to cut back on water, your body will produce hormones and chemicals to hold onto any water."
So while you might gain a very short-term benefit by looking a tiny bit more toned while you're severely dehydrated (body-builders have been known to dry fast before competitions for that reason), once you consume liquid again, your body rebounds and desperately hangs on to even more water than before. It's like yo-yo dieting in fast motion.
Valter Longo has studied starvation, fasting and calorie restriction in humans for nearly 30 years. He's currently the director of the Longevity Institute at USC and a professor of gerontology. He developed the Fasting-Mimicking Diet, or FMD, a fasting-type diet with small prepackaged meals intended to provide the health and longevity benefits of a five-day fast without requiring a doctor's supervision. Fasting-type diets have grown in popularity in recent years for a simple reason, he said: "Because they work."
But he said he's not aware of any reputable studies about the effects of dry fasting, and said he wouldn't even consider putting one together, also for a simple reason: It's incredibly dangerous.