Why Trying a Water Fast Is So Risky

Fasting diets, including a water fast, alternate-day fasting or time-restricted eating, have become buzzy in the weight-loss world. But despite the popularity of various fasting methods, there's no quick fix when it comes to weight loss.

You may lose precious muscle weight on a water fast. (Image: Ingram Publishing/Ingram Publishing/Getty Images)

Water fasting, or fasting of any kind, is not usually recommended without supervision from a health professional. Fasting can slow the metabolism, leaving you hangry all day. Ditch the fad diets and talk to your doctor about the best ways to lose fat for your body instead.

What Is Water Fasting and Is It For You?

Water fasting is pretty much what it sounds like: On a water fast, you don't eat any food or drink any beverages except water. The routine can last anywhere from 24 to 72 hours, but they've also been attempted in other iterations, too.

Despite the recent popularity of different forms (like alternating between fasting and feeding days, according to Harvard Health Publishing), fasting isn't a casual or safe fat loss method. Doctors don't typically recommend fasting as a weight loss technique. Water fasting in particular has many negative potential side effects including kidney issues, dizziness or nausea and, for people with diabetes, low blood sugar, according to the Mayo Clinic.

Water fasting can have a negative affect on kidney function, according to a small 2018 study published in AE Press. The researchers found that after people followed a water fast for 11 days, they had excess uric acid in their blood, which can lead to kidney stones or gout.

For many, water fasting is a form of crash diet, throwing your body into an extreme and unexpected calorie deficit, which can lead to dizziness or lightheadedness, according to Harvard Health Publishing. This seemingly non-severe side effect can quickly grow dangerous, causing you to faint or vomit.

A water fast can be especially risky if you have diabetes. Fasting can lead to an extreme drop in blood sugar or hypoglycemia, according to a 2013 study published in Postgraduate Medicine. People with diabetes who experience hypoglycemia need immediate medical attention.

Unless a doctor recommends water fasting — and doctors rarely endorse such extreme techniques for weight loss — it's safe to assume that this dieting technique is not for you. And that's especially true if you have diabetes, liver or kidney issues or have experienced disordered eating in the past.

The Truth Behind Fasting and Fat Loss

While most people know that weight loss occurs when you burn more calories than you consume, the science of slimming down is a little more complex. Your metabolism is the process by which your body burns calories in order to keep you breathing, keep your blood circulating — basically, keep you alive, according to the Mayo Clinic. The speed of your metabolism is largely determined by your body composition: The more muscle and less fat you've got, the speedier your metabolism.

Although fasting means you consume fewer calories overall, your body also starts to burn those calories more slowly. Crash diets, like fasts, can cause your body to burn muscle for energy instead of carbs or fat, which leads to a slower metabolism, according to the UK's National Health Service.

Fasting has also been shown to have little benefit over standard calorie-restricted dieting, according to a July 2017 study published in JAMA. After comparing people who followed an alternate-day fast to those who followed a standard calorie-restricted diet, researchers found that avoiding food altogether had no advantage in weight loss or weight maintenance.

Much of the long-term research on fasting has been done in rodents; more research is needed to replicate the results in humans, according to the Harvard T.H. Chan School of Public Health.

Are Moderate Fasting Methods Actually Better?

While water fasting isn't the answer for shedding fat or extra pounds, other fasting techniques may help lower a person's risk of obesity, hypertension and asthma, among other conditions, according to a 2013 article published in Cell Metabolism.

Time-restricted feeding is a popular, more sustainable way to get the benefits of fasting for people who are curious to try it, according to Harvard's School of Public Health. To try it, dieters create a seven- or eight-hour window where they eat normally and don't eat anything else for the remainder of the day.

Others might try the 24-hour fast, which involves abstaining from food for an entire day, followed by regular eating for the remainder of the week, according to Harvard. Some have further amended this strategy: They'll eat normally for five days a week and restrict the other two to 400 to 500 calories.

Although these are more moderate forms of fasting and have shown some health benefits, it's still always best to discuss your diet plans with a medical professional.

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