Fasting promises a total mind and body transformation. But are the hunger pangs and fatigue worth it? 

Few wellness fads have gripped the public’s imagination recently quite like intermittent fasting. And since skipping breakfast or cutting out late-night snacks can help people manage their weight and improve metabolic health, some are taking the approach to an extreme: replacing food with water, sometimes for days on end. 

Prolonged water fasts are popping up in fitness circles and across social media as people try to slim down and reset the gut – fast. UFC CEO Dana White completed a brutal 86-hour water fast last year, and celebrities like Hugh Jackman and Jennifer Lopez rave about prolonged fasting. 

Based on the science, are water fasts actually a useful tactic for improving health or a dangerous short-term fix?

According to Danielle Crumble Smith, a registered dietitian based in the United States, the trend of prolonged water fasting is “extremely concerning.”

 “While fasting can offer health benefits when done correctly and safely, extreme practices like prolonged water fasting require careful consideration and professional guidance,” she says, adding that every individual is unique, so what works for one person could be harmful to another.

“The allure of dramatic results shared via social media should not override the importance of personal safety and health."

What is water fasting?

While fasting has become a mainstay of modern wellness, it’s nothing new. It’s been used for spiritual, mental and physiological purposes for centuries. 

Certain types of fasting are especially common: intermittent fasting (IF) or time-restricted eating (TRE) involves restricting meals to specific “eating windows,” often lasting about 8 hours. Others do the “5:2,” eating normally on certain days, then limiting calories or skipping meals entirely for a day or two each week.

Some people swear by alternate-day fasting, see-sawing between days of eating and fasting. 

The latest buzzy tactic is prolonged water fasting: skipping meals for periods of 24 hours to multiple days, and relying on water or water-based beverages like tea or coffee during periods without food.

What are the potential benefits of water fasting?

In the short term, fasting can be uncomfortable, producing hunger pangs, physical weakness, irritability, and fatigue. But a growing body of data suggests it may be worth pushing through these initial side effects, as some forms (primarily intermittent fasting and caloric restriction) can trigger a range of feel-good effects, if done safely.

#1. Metabolic Health

During a fast, insulin levels drop, the body shifts from burning glucose to fat for energy, and the production of ketones goes up.

In turn, this metabolic cascade can lead to increased insulin sensitivity, better cholesterol and triglyceride levels, and lower blood pressure. 

#2. Weight Loss

Many people fast to drop weight, a health claim that’s backed up by a solid body of data. For people who struggle to stick to a particular diet, intermittent fasting can be a simple way to create a calorie deficit and improve body composition. 

#3. Gut Health

Some claim that fasting can “reset” the gut, give the digestive system a break, and overhaul the gut microbiome—but there haven’t been enough long-term studies to support this potential benefit. In theory, fasting might decrease intestinal permeability, reduce gut inflammation, and modify the gut microbiota, ultimately benefiting conditions like irritable bowel syndrome (IBS), Crumble Smith explains. More investigation is needed to confirm this claim.

#4. Longevity

Recent studies, mostly in animals, suggest that fasting and calorie restriction may help people live longer, healthier. Fasting can kickstart autophagy (the body’s cellular cleanup process), decrease oxidative stress, and improve genomic stability—all of which combine to induce this longevity effect, Crumble Smith explains. Still, more long-term studies on humans are needed to unravel fasting’s effect on longevity.

Beyond these physiological shifts, some say that fasting can be a “spiritual detox” and spark mental clarity. These effects are highly variable and subjective, and haven’t been found consistently in clinical trials.

Social media forums are packed with dramatic before and after photos, as well as a diverse range of testimonials. Some fasters describe astonishing effects like shifted brain chemistry, reduced cravings, and improved digestion. While others recall headaches, chills, brain fog, and fatigue. 

What are the potential risks of water fasting?

Despite the range of positive side effects, skipping food for extended periods and guzzling water isn’t risk free. Prolonged water fasting can morph quickly into disordered eating and isn’t recommended for anyone with a history of eating disorders, chronic conditions like diabetes, kidney disease, heart conditions, people taking GLP-1s or other blood-sugar lowering medications, older people, children, or those who may be pregnant.

Crumble Smith is hesitant to recommend prolonged fasting, especially for women under a lot of stress. Fasting can over-tax the body, especially when there are other chronic stressors present such as poor sleep, autoimmune conditions, or hormone imbalances.

Prolonged fasting can also lead to dangerous side effects like electrolyte and fluid imbalances, hyponatremia (low sodium levels), nutrient deficiencies, muscle loss, and cognitive or physical impairments like fatigue, decreased focus, and nausea.

The verdict: is water fasting worth it?

Ultimately, it’s always best to approach health trends with “caution and informed perspective,” Crumble Smith advises.

So, if you do want to fast, try less extreme forms like intermittent fasting or time-restricted eating rather than jumping into the deep end with a water fast. These types of eating patterns can provide the same benefit as a prolonged fast while minimising risk.  

“If you’re going from ‘zero to 100,’ first start by reducing processed foods and simple sugars while incorporating more vegetables, fruits, lean proteins, healthy fats, and fibre-rich ‘real foods,’” Crumble Smith says. This can benefit insulin sensitivity, cardiovascular health, and weight loss in a far more sustainable way than a water fast.

Remember, no fast will create an instant transformation, no matter what anyone says. Be wary of advice or products that promise quick fixes or miraculous results, Crumble Smith says.

“Good health typically comes from making consistent, sustainable lifestyle changes, not from ‘magic’ supplements or radical diets.”

If you do want to try fasting, Crumble Smith advises the following steps:

#1. Talk to your health provider

This is especially important if you have any health conditions or are taking medication.

#2. Start small

Start with intermittent fasting or time-restricted eating, rather than a multi-day water fast. Pay attention to how your body reacts, so you can ensure you're doing more good than harm.

#3. Stay Hydrated

Maintain adequate hydration but don’t go overboard. Some experts suggest about 2-3 liters of mineralized water a day while fasting.

#4. Listen to Your Body

If you experience symptoms like dizziness, confusion, fainting, or extreme weakness, end the fast immediately and seek medical advice.

#5. Plan for a Safe Environment

During a prolonged fast, physical capabilities might be reduced. Ensure you’re in a safe environment where you can rest and don’t have to perform hazardous tasks like driving. Take it easy on exercise so you don’t heighten the risk of electrolyte imbalances or nutrient deficiencies. 

Educate Yourself

Learn the science of fasting and the difference between evidence-based practices and anecdotal experiences shared on social media. Working with a registered dietitian is a great way to distinguish the truth from hype.

Nutrient Replenishment

After completing a fast, reintroduce food gradually and focus on nutrient-dense foods to replenish any deficiencies and support recovery.