What ‘dry fasting’ is and why for your health you should not do it

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 tumours. 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 two litres — 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.

Dry fasting is not the same thing as intermittent fasting, which has become a popular fad diet in recent years. There are different variations of intermittent fasting, but most people start with 16 hours of fasting followed by eight hours of eating. Martin Berkhan created the “LeanGains” 16:8 intermittent fasting guide and is widely credited with popularising the diet. On his website, leangains.com, Berkhan writes that during the 16-hour fasting window, coffee, calorie-free sweeteners, diet soda, sugar-free gum and up to a teaspoon of milk in a cup of coffee won’t break the fast.

The subreddit for fasting, r/fasting, has an “Introduction to Intermittent Fasting” guide that contains the following tips for surviving the fasting portion of your day:

• Drink lots of cold water

• Always carry water, a canteen, a bottle, or keep a full glass within sight

• Water, water, water, water

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.

“For sure, the body needs to reset, but there are safe ways of doing that, and dry fasting is not one of them,” Longo said. “We require water.”

His work has also involved looking at how cultures and religions have engaged with starvation and fasting throughout human history, and says he hasn’t heard of any that involved extended fasting without water. The closest is Ramadan, during which observers go without food or water during daylight hours — but at most, that lasts for 16 hours, and it’s preceded and followed by extensive hydration.

If someone tries dry fasting for a full day, Longo said, they risk side effects like developing kidney stones. Longer than that, and you start risking your life.

Some proponents of “dry fasting” eschew water but recommend hydrating with fresh fruits and vegetables. Hydrating with fruit is certainly better than not hydrating at all. An orange has about a half-cup of water in it; to get to the recommended two litres of water a day, you’d have to eat around 17 oranges. That’s a lot of peeling.

So, in conclusion: Dry fasting puts you at risk of kidney stones or organ failure. There are no known, proven long-term benefits to doing it. Though different types of fasts and fasting diets can be beneficial, there is no medical evidence to suggest you need to stop consuming water for any period of time, or that water from fruit is better for you than filtered drinking water. Do not take medical advice from a photo of a person in a sarong.

Please drink some water.

By Jessica Roy