72 hour fast: benefits, risks and how to break it safely

A 72 hour fast can sound like an extreme test, but many people explore it for a practical reason: they want a clear metabolic shift, less perceived inflammation, and better control over hunger without constant snacking. In a recent conversation between an elite athlete and a nutrition educator, both landed on two useful ideas: fasting does not replace a well built baseline diet, and breaking a fast poorly can ruin the whole experience.

What changes in your body during a 72 hour fast

When you stop eating, your body does not shut down. It switches fuel sources and adjusts hormones to keep you functional.

From glycogen to ketones

In the first hours you use circulating glucose and liver glycogen. As time passes, your body increases fatty acid release and produces more ketone bodies. Many people notice that after the first day hunger becomes less loud and their mind feels steadier. This does not happen the same way for everyone, but the shift toward ketones explains part of the clarity some people report.

Longer fasts can also shift other variables:

Insulin drops and your ability to mobilize fat as fuel improves.
Perceived inflammation may improve, especially if you come from a diet high in sugar and ultra processed foods.
Satiety signaling can feel sharper when you eat again, but your brain still needs time to register fullness, so you want to reintroduce food slowly.

Potential benefits people usually look for

A longer fast is not magic, but it can offer benefits when you use it with intention and not as an ego challenge.

Habit reset: for three days you remove constant decisions and temptations, then you can return to a cleaner routine.
Metabolic flexibility: you practice moving from carbohydrates to fat without panicking when you do not eat every few hours.
Ketosis and the brain: in contact sports and other settings with high neurological load, people discuss ketones for their anti inflammatory effects and their role in oxidative stress buffering. This is not a license to take hits, but it helps explain why some athletes consider it.
A better relationship with hunger: you learn to separate true hunger from craving and habit.

Risks and who should avoid it

Here is the direct part. A 72 hour fast is not for everyone, and it should not be your first option.

Avoid it, or do it only with professional supervision, if any of these apply:

You have a history of disordered eating.
You are pregnant, breastfeeding, or still growing.
You take medications that affect blood glucose or blood pressure, or you have a chronic condition that requires adjustments.
You have diabetes, especially if you use insulin or sulfonylureas.
You are underweight, frail, or you have a history of fainting.

Even if you are healthy, watch for red flags: persistent dizziness, confusion, palpitations, marked weakness, or pain. Break the fast if those show up.

How to prepare for a 72 hour fast step by step

Preparation matters as much as the fast itself. If you improvise, you suffer more and learn less.

Before the fast

Pick three days with low social and work demands. If you plan to train hard, reconsider.
Reduce alcohol and ultra processed foods in the week before. Starting from a high sugar diet makes day one harder.
Prioritize protein, vegetables, and foods rich in potassium and magnesium the day before. Your body benefits from starting with micronutrients topped up.
Sleep well. Poor sleep increases hunger and lowers self control.

During the fast

A water fast usually means water, salt, and if you tolerate it, black coffee or tea with no calories. The most common failure point is hydration with electrolytes.

Drink water regularly. Do not wait until you feel thirsty.
Add salt if you notice headache or apathy, unless your clinician has told you not to.
Walk and keep movement easy. Avoid very intense sessions if you feel weak.
Keep your mind busy. Hunger comes in waves and often drops if you spend fifteen minutes doing something else.

How to break the fast without overdoing it

Refeeding determines whether you feel light or miserable. The typical mistake is eating fast and eating a lot, because your body takes time to tell you that you have had enough.

A simple strategy:

Start with a small, easy to digest serving, for example soup, plain yogurt, eggs, or fish with cooked vegetables.
Wait twenty minutes before you take more. Give your satiety signal time.
Avoid a sugar spike at the start. If you begin with sweets you can trigger cravings and digestive discomfort.
For the first full meal, return to a protein first base, with fruit or potatoes if you train and tolerate them well.

A practical plan to start without going to extremes

If you want fasting benefits, you do not need to jump straight to 72 hours. Build tolerance.

Week 1: 12 hour overnight fast and a breakfast with no added sugar.
Week 2: 14 to 16 hours on some days, with an earlier dinner.
Week 3: a 24 hour fast if you feel stable and not obsessive.
Week 4: decide whether 48 to 72 hours adds something real to your life. If it only creates stress, it is not worth it.

Conclusion

A 72 hour fast can be a tool, not an identity. Use it to learn: improve your baseline diet, plan hydration, and break the fast calmly. That turns a hard challenge into a practice with purpose.