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Water fasting is the practice of abstaining from all food and caloric beverages for a set period while only consuming water. It has gained traction as both a health trend and a potential therapeutic strategy. While it may sound extreme to some, advocates point to benefits ranging from fat loss and improved insulin sensitivity to cellular repair and longevity. But how much of this is grounded in science, and is it suitable for the average fitness enthusiast?

This article takes a clear-eyed look at water fasting, pulling from peer-reviewed medical research and expert commentary to deliver a balanced view of the pros, cons, and common myths surrounding the practice.

What Is Water Fasting?

Water fasting is a form of complete caloric restriction where individuals consume only water for a predetermined duration, typically ranging from 24 hours to several days. It differs from intermittent fasting, which allows for eating within certain time windows, and from calorie-restricted diets that limit food intake but don’t eliminate it entirely.

While water fasting has ancient roots in spiritual and religious practices, it has gained scientific attention for its possible effects on metabolism, cellular health, and chronic disease prevention. Especially in the Influencer world, which is why we wanted to share the unbiased facts.

What Happens to the Body During a Fast?

During water fasting, the body undergoes a metabolic shift:

1. Glycogen Depletion: In the first 24 hours, the body uses stored glycogen (carbohydrates stored in the liver and muscles) for energy.
2. Ketosis: Once glycogen is depleted, the body begins breaking down fat stores into ketones, a process known as ketosis.
3. Autophagy: Around 24 to 72 hours into fasting, autophagy may be triggered, a cellular "clean-up" process where cells recycle damaged components - Mizushima & Levine, 2010.

This transition can lead to physical and mental changes including improved mental clarity, energy dips or surges, and noticeable fat loss.

Potential Benefits of Water Fasting

1. Improved Insulin Sensitivity

Several studies suggest fasting improves insulin sensitivity and helps regulate blood glucose - Patterson & Sears, 2017. In layman's this means your body's cells are responding to insulin more effectively, which helps regulate blood sugar levels. This could benefit those at risk for type 2 diabetes. The effects of this during a fast may not be applicable as you will not be having insulin spikes from your diet.

The improved insulin sensitivity following a water fast can last anywhere from several days to a few weeks (quite a range), depending on various factors including:

• Length of the fast: Longer fasts (e.g., 3+ days) tend to have more prolonged effects on insulin sensitivity than shorter intermittent fasts.
• Post-fast diet: If you return to a diet high in refined carbs and sugars, insulin sensitivity can diminish rapidly. A healthy, balanced diet can help sustain the benefits longer.
• Individual metabolic health: People with insulin resistance, pre-diabetes, or metabolic syndrome may see more significant improvements, but the duration of the benefit varies by individual.

In general, insulin sensitivity starts to decline again once regular eating resumes, especially if it includes high glycemic foods. However, the metabolic improvements from even a short water fast can persist for several days to a week or more if supported by a low-sugar, whole-foods diet and physical activity.

So, while fasting creates a temporary window of improved insulin action, long-term benefits depend heavily on post-fast lifestyle choices.

2. Fat Loss

Water fasting often leads to weight loss, though some of this may be muscle (especially as the fast duration increases) and water weight. A short-term fast can create a caloric deficit without complex dieting strategies.

The main worry is, do you regain this straight after the fast ends? Not necessarily, but it depends heavily on what you do after the fast:

• Water weight: Some of the initial weight lost, especially in the first 1–3 days, is mostly water, due to depleted glycogen stores. This water weight is typically regained quickly once you start eating again.
• Fat loss: If the fast lasts long enough (typically beyond 24–48 hours), your body begins to tap into fat stores. This fat loss can be maintained if you transition to a healthy, calorie-controlled diet afterward.
• Muscle loss: Longer fasts (3+ days) may lead to some muscle breakdown, especially without resistance training. This won’t come back automatically unless you work to rebuild it through strength training and adequate protein intake.
• Rebound weight gain: If you return to overeating or a highly processed, high-carb diet after the fast, you may regain the lost weight, possibly more (known as "yo-yo" weight cycling). But if you follow up with a mindful eating plan, the weight loss, especially the fat loss, can be sustained.

In summary you don’t automatically regain all the weight, but keeping it off depends on your post-fast eating and activity habits.

3. Cellular Repair and Longevity

Autophagy has been linked to aging and disease prevention. Fasting may promote cellular regeneration and reduce inflammation, it's essentially a cellular process where the cell breaks down and recycles its own components - Madeo et al., 2019.

What your body might accomplish in 2–3 days of water fasting (in terms of cellular cleanup) could take several weeks or longer to achieve through regular, non-fasting conditions.

4. Cardiovascular Health

Some evidence suggests fasting can lower blood pressure, cholesterol, and triglyceride levels - Goldhamer et al, 2002. Check out our blood pressure basics article!

Risks and Downsides

1. Nutrient Deficiencies

Extended fasts can lead to shortages of essential vitamins and minerals. Without food intake, micronutrient status must be monitored.

2. Muscle Loss

The body may begin to break down muscle tissue for energy, increasing after 48 hours but particularly in fasts lasting longer than 72 hours.

3. Metabolic Slowdown

Prolonged caloric restriction can reduce resting metabolic rate, making future weight maintenance harder.

4. Dizziness, Fatigue, and Brain Fog

Side effects like lightheadedness and decreased concentration are common, especially in the initial days of fasting.

5. Medical Risks

People with certain medical conditions, such as diabetes, cardiovascular disease, or eating disorders, should avoid water fasting unless under medical supervision.

Who Should Avoid Water Fasting?

• Individuals with a history of eating disorders
• Pregnant or breastfeeding women
• People with chronic medical conditions or on medication
• Anyone under 18

Always consult a healthcare provider before attempting a prolonged fast.

Fact vs. Fiction

• Water fasting detoxifies the body - 🚫 Fiction - The liver and kidneys already detox the body efficiently.
• Fasting burns only fat - ❌ Fiction - Some muscle loss is possible, especially in long fasts.
• Autophagy improves health - ✅ Fact - Supported by numerous cellular biology studies.
• Water fasting improves insulin sensitivity - ✅ Fact - Backed by human and animal studies.

Is It Worth It for Fitness Enthusiasts?

For the average gym-goer, short-term water fasting (24-48 hours) may offer some metabolic benefits, including improved insulin sensitivity and fat mobilisation. However, for most, these benefits can be achieved more sustainably through controlled & balanced diet's, intermittent fasting, proper nutrition, and regular training.

Long-term or frequent fasting carries more risks, particularly muscle loss and slowed metabolism, which can undermine fitness goals. If weight loss or metabolic reset is the aim, intermittent fasting or cyclical fasting may be safer and more practical options.

Conclusion

Water fasting is a powerful physiological tool that may yield real benefits, especially when practiced under the right conditions. But it isn’t a silver bullet, and it comes with risks. The best results come when fasting is strategic, short-term, and medically guided.

For those who are healthy, well-informed, and supported, it can be a rewarding personal challenge. For others, there may be safer and equally effective paths to better health.

The final note is that fasting does "reset" the metabolic system in many ways, but the long-term benefit depends entirely on what happens afterward. If you follow up with a heart-healthy diet (e.g., whole-food, plant-based or Mediterranean-style), these improvements can be sustained or even enhanced. If not, they can fade within days to weeks.

Tempered Strength’s Stance

Now for the opinionated part.

While there are clear benefits to water fasting, even when separating fact from myth, the way most people approach diets and experiments often isn’t structured or accurate enough to achieve the expected results, with a yo-yo effect happening straight after.

Many treat water fasting like a silver bullet, but given how it might affect you, it probably isn’t worth it for most. We recommend focusing on dialling in your baseline nutrition and training plan, and building strong habits there before attempting anything extreme. A water fast won't fix an unhealthy lifestyle.

Always consult a doctor before attempting a water fast.

References

We try to cite only large group studies and clinical papers whenevr possible. Below is a list of sources used to collate data for this article.

1. Efficacy and safety of prolonged water fasting: a narrative review of human trials
2. The evidence base behind modern fasting guidelines
3. Autophagy: cellular and molecular mechanisms
4. Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting