Experts Explain What "Dry Fasting" Actually Does To Your Body

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As Instagram floods with people trying intermittent fasts in the hope of improving their health — despite the warnings of healthcare professionals about the dangers — dry fasting, in which people stay away from both food and liquid for up to a day, is becoming more widespread. However, experts say that doing a dry fast in the hope of health boosts isn't a good idea, to put it mildly.

"This is an unsafe diet fad that lacks credible scientific evidence to support any health benefits," Kristen Smith, M.S., R.D., a registered dietitian at Piedmont Healthcare and spokesperson for the Academy of Nutrition and Dietetics, tells Bustle.

Dry fasting involves staying away from any kind of food or drink for up to 24 hours. Claims about dry fasting's benefits range from cleansing the body of "toxins" to improving digestion and weight management. However, the risks of dehydration are real.

Hydration is an essential part of bodily functioning. "We know that proper hydration is key for physical and mental health," Max Lugavere, a science writer and author of Genius Foods, tells Bustle. Not drinking anything at all for 12 or 24 hours, even if an influencer says it makes her "glow," is an unsound poor idea for your body. "Restricting fluids can lead to severe dehydration, extreme fatigue, and even organ failure," Smith says. Even if you're eating while you're avoiding liquids, she says, it's still difficult to get adequate hydration. Water-heavy foods like fruits and vegetables simply don't offer enough water to help your body function, which is estimated at 2.7 liters per day for an adult woman.

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Cutting off all sources of water can be dangerous in multiple ways. Smith says that hydration plays a key role in maintaining optimal health because it regulates body temperature, protects organs, carries nutrients to cells, and helps flush out waste products. Any claims that dry fasting can help remove "toxins" are wrong: the liver requires hydration to function properly, so staying away from all liquids will impede your body's ability to process waste, not help it. Dehydration can also impair cognitive function and mental health, Lugavere says. A 2017 study of 50 people in Annals of Nutrition & Metabolism found that even mild dehydration for one day could cause mood to plummet.

Fasting from both food and liquids hurts your electrolyte levels. Electrolytes, including sodium and potassium, are found in both food and drinks, and help your organs work properly. When your body is low in them, it can cause muscle cramping, dizziness, brain fog, and exhaustion.

Dry fasting can also have an ongoing negative effect on relationships with food. "This diet could be extremely dangerous, especially for individuals who struggle with disordered eating or have chronic disease," Smith says. "It doesn’t teach habits that will help create a healthy relationship with food and isn't sustainable or recommended in the long haul." While the language around dry fasting can be tempting, the reality is very different, she says.

If you're tempted to do a dry fast, experts advise just saying no. Hydration is crucial to a healthy body and brain, and depriving yourself of water for any prolonged period is a very bad idea. "Skip getting your nutrition advice from influencers, and seek direction from a registered dietitian who is adequately trained on the science of nutrition," Smith says.


Max Lugavere, author of Genius Foods

Kristen Smith M.S., R.D., dietitian at Piedmont Healthcare and spokesperson for the Academy of Nutrition and Dietetics

Studies cited:

Pross, N. (2017) Effects of Dehydration on Brain Functioning: A Life-Span Perspective. Ann Nutr Metab. 70 Suppl 1:30-36. doi: 10.1159/000463060.

Trepanowski, J. F., & Bloomer, R. J. (2010). The impact of religious fasting on human health. Nutrition journal, 9, 57.