Electrolytes during a fast

The single most useful thing almost nobody leads with. Most of what people call "fasting feeling bad" in the first week is a salt deficit, and it has a close-to-comically-simple fix — for most people.

Before anything else: if you have high blood pressure, kidney disease, heart failure, or you take blood-pressure medication, do not add sodium on the strength of a web page. That includes you if your blood pressure is controlled by medication — a number that reads normal because a drug is holding it there still means this is about you. For most people fasting, the risk is too little salt. For some people, added salt is genuinely dangerous, and only your doctor knows which you are.

Why you feel bad in week one

When insulin falls, your kidneys let go of sodium, and water goes with it. The headache, fatigue, irritability, foggy head, and lightheadedness-on-standing that most people hit in the first week are usually that — not a sign your body is telling you to stop. You're low on salt.

Sodium

For most people, the fix is a quarter to half a teaspoon of ordinary salt in a glass of water when the flat, foggy, headachy feeling arrives. Ordinary table salt is fine. There is no version of this that requires a branded electrolyte powder.

Potassium and magnesium — from food, carefully

Get these from food where you can. Potassium supplementation is the one to be careful with: taken as pills it carries a real risk to heart rhythm (hyperkalemia), especially alongside certain medications or kidney issues. Food does not have that problem. Leafy greens, avocado, and the like on your eating window do more than a pill, more safely. Magnesium from food (nuts, seeds, greens) similarly; if you supplement magnesium, the usual gentle forms and modest doses.

Potassium pills are not a casual purchase. If you're considering supplementing potassium, that is a conversation with a doctor or pharmacist, not a web page — particularly if you take any heart or blood-pressure medication.

The whole thing, honestly

For most people, fasting-day electrolytes are: enough salt, food-sourced potassium and magnesium, and water. That's it. It is salt, food, and water. Anything sold to you as more than that is being sold to you.

This is the substance of Chapter 10 of Fasting, Honestly (coming soon), reproduced here because it keeps people out of trouble. It is educational content, not medical advice.