Fasted FASTED

Fasting Electrolyte Calculator

Baselines of 2,500 mg of sodium, 2,000 mg of potassium and 400 mg of magnesium a day, raised 15 percent on a longer fast and adjusted for the symptoms you tick — headache adds sodium, cramps add magnesium. It is a starting point, not a prescription, and it is the wrong page for you if you take a diuretic or a blood-pressure medicine.

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How this is calculated

The calculator starts from three daily baselines — 2,500 mg of sodium, 2,000 mg of potassium and 400 mg of magnesium — and then moves them for what you tell it. A longer fast raises all three by 15 percent. Heat or heavy sweating adds 500 mg of sodium and 500 mg of potassium; headache adds 500 mg of sodium and 50 mg of magnesium; cramps add 100 mg of magnesium and 200 mg of potassium; fatigue adds 300 mg of sodium; poor sleep adds 100 mg of magnesium; a racing heart adds 200 mg of sodium and 50 mg of magnesium. A short fast scales everything down to 70 percent and a moderate one to 85. The dose schedule then splits the day's sodium and potassium into three and the magnesium into two.

The baselines sit in the region of published adult reference intakes for these minerals; the adjustments do not come from any guideline, because none exists for fasting. They are this site's judgement about direction and rough size, built on one uncontroversial mechanism: insulin falls during a fast, the kidneys hold on to less sodium and water, and the headaches and cramps people blame on hunger on a long fast are often mineral loss instead.

That means this page is a starting point and not a prescription. Do not use it if you take a diuretic, a blood-pressure medicine, or anything that affects potassium; do not use it if you have a kidney or heart condition. Potassium in particular is the one to be careful with — too much of it is dangerous, and food is a safer way to get it than a supplement. If a symptom is severe or will not settle, stop the fast and eat.

Where this comes from

  1. 1. Dietary Reference Intakes for Sodium and Potassium — National Academies of Sciences, Engineering, and Medicine, 2019 (NCBI Bookshelf)

Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy. Method last checked 21 August 2026.

Electrolyte management is the difference between a comfortable fast and a miserable one. Most fasting side effects — headaches, cramps, fatigue, brain fog — are caused by electrolyte depletion rather than hunger itself.

When insulin drops during a fast, your kidneys flush sodium, which pulls potassium and magnesium with it. Replacing these minerals proactively keeps you feeling sharp and energised, making longer fasts far more sustainable and enjoyable.

Frequently Asked Questions

Can I take electrolytes while fasting without breaking my fast?
Yes. Pure electrolyte supplements containing sodium, potassium, and magnesium have zero calories and will not break your fast. Avoid electrolyte drinks that contain sugar, artificial sweeteners, or calories. Plain salt, potassium chloride, and magnesium citrate capsules are all safe during a fast.

Why do electrolytes drop during fasting?
When you fast, insulin levels drop significantly. Lower insulin causes your kidneys to excrete more sodium and water, which in turn pulls potassium and magnesium with it. This is why many people experience headaches, cramps, and fatigue during fasts — it is often electrolyte depletion, not hunger.

How much salt should I take while fasting?
For extended fasts over 18 hours, most adults need 2,000-3,000mg of sodium daily. Active individuals or those in hot climates may need up to 3,500mg. Start with a quarter teaspoon of salt in water in the morning and adjust based on how you feel.

What are the signs of electrolyte imbalance during fasting?
Common signs include headaches, muscle cramps, fatigue, dizziness, brain fog, heart palpitations, and nausea. If you experience any of these during a fast, try supplementing with electrolytes before ending your fast. Severe symptoms like irregular heartbeat or persistent vomiting mean you should break your fast and consult a doctor.

What are the best electrolyte supplements for fasting?
Look for supplements with no sugar, no artificial sweeteners, and no calories. Pure options include pink Himalayan salt or sea salt for sodium, potassium chloride (NoSalt or NuSalt brands), and magnesium citrate or glycinate capsules. Many people make their own "ketoade" by mixing these into water.

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