3-Day Fasting: What Actually Happens During a 72-Hour Fast

Fasting for 16 or even 24 hours has become routine for many people, but going 72 hours without solid food? That’s a real challenge. More and more people are curious about the so-called 3-day fast, often described as a ‘reset’ for the body. But what really happens during a 72-hour fast, and who is it actually suitable for? If you’re considering trying it yourself, it’s best to talk to a doctor first to make sure this approach is safe for your body.

What Happens in the Body During a 72-Hour Fast?

After 12–24 hours: Glycogen stores in the liver and muscles are depleted, and the body increasingly switches to burning fat for energy.
After 24–48 hours: Ketone levels in the blood begin to rise. The metabolism shifts more strongly toward fat as the primary fuel source.
After around 48 hours: Autophagy (the body’s natural “cell-cleaning” process) is activated more strongly. Old cell components and proteins are broken down and recycled.
After 72 hours: Studies show early signs of immune system regeneration. Levels of IGF-1 (a growth factor) decrease, and stem cell activity may be stimulated.

Most of these effects have been observed mainly in animal studies and smaller human trials. There are promising signs of potential benefits, but the research is still far from conclusive.

Possible Benefits of a 72-Hour Fast

Boosted Autophagy: Cells clear out waste products more efficiently.
Improved Insulin Sensitivity: Longer fasting periods may support short-term metabolic health.
Immune System “Reset”: Research (including studies by Prof. Valter Longo, USC) suggests that prolonged fasting can reduce old immune cells and stimulate the production of new ones after refeeding.
Mental Clarity & Mindfulness: Many people report feeling more focused and calm, especially from day two onward.

Risks and Side Effects

A 72-hour fast is not a beginner-friendly approach. It may cause fatigue, headaches, dizziness, or electrolyte imbalances. People who are underweight, have a history of eating disorders, live with chronic illnesses, or are pregnant or breastfeeding should avoid extended fasting or seek medical guidance first.

Tips for a Safe 72-Hour Fast

Preparation: Reduce meal sizes and lower carb intake a few days in advance.
Stay Hydrated: Drink plenty of water, herbal tea, or clear broth (for electrolytes).
Watch Your Electrolytes: Magnesium, potassium, and salt can help prevent dizziness or circulation issues.
Gentle Movement: Light walks are fine, but avoid intense workouts.
Break the Fast Slowly: After 72 hours, start with light foods such as soup or cooked vegetables.

Who Should Avoid a 72-Hour Fast?

People with diabetes (type 1 or 2) or other metabolic conditions without medical supervision
Those who are underweight, have eating disorders, or chronic illnesses
Pregnant women, breastfeeding mothers, children, and adolescents

👉 When in doubt: always consult a healthcare professional first.

Conclusion

A 72-hour fast can trigger profound metabolic processes, from autophagy and ketosis to possible effects on the immune system. However, it’s an intense intervention in your usual metabolism and not suitable for everyone. If you decide to try it, preparation, hydration, electrolyte balance, and listening to your body are essential.

👉 Important: Many of these effects have been observed mainly in animal studies or small human trials. They should therefore be considered potential benefits, not guaranteed outcomes. Long-term effects and risks are still not fully understood.

Sources

Longo V. D. et al. (2014): Fasting-mimicking diet and immune system regeneration. Cell Stem Cell. doi.org/10.1016/j.stem.2014.02.014
National Institute on Aging (2023): Intermittent and prolonged fasting research. nia.nih.gov
Harvard Health (2022): Extended fasting – what you should know. health.harvard.edu
USC News (2015): Prolonged fasting triggers stem cell regeneration. news.usc.edu
Johns Hopkins Medicine (2024): Intermittent and extended fasting – risks and benefits. hopkinsmedicine.org