Most of the unpleasant symptoms people blame on hunger — the headaches, the fatigue, the lightheadedness, the brain fog around hour 20 — are not hunger. They are sodium depletion. Add electrolytes to your fast and the entire experience becomes easier, longer, and safer.
This guide explains what each electrolyte does, how much you need at different fast lengths, what to use (and what to avoid), and how to know when you’ve overdone it.
Table of Contents
Why Fasting Drains Electrolytes
When you eat, you take in roughly 3–4 grams of sodium per day on a typical Western diet, plus potassium and magnesium from food. When you stop eating, that intake stops too. Meanwhile, two physiological shifts accelerate electrolyte loss.
First, insulin drops. Insulin signals the kidneys to retain sodium. When insulin falls — which is the entire point of fasting — the kidneys excrete more sodium. This is the dominant mechanism behind “keto flu” and the equivalent “fasting headache” that hits most new fasters around hour 18–24.
Second, glycogen breakdown releases water. Each gram of stored glycogen holds about 3 grams of water. As glycogen depletes over the first 24–48 hours of a fast, you can lose 1–3 kilograms of water weight, and the dissolved electrolytes go with it.
The combined effect is rapid: by hour 24 of a water-only fast, sodium losses can be substantial. By day 3, potassium and magnesium become relevant too. None of this is a problem if you replace what you lose. All of it becomes a problem if you don’t.
Sodium: The One That Matters Most
Sodium is the most-depleted electrolyte during fasting and the one that most affects how you feel. The mainstream public-health message — “eat less salt” — is calibrated for people eating ultra-processed food three times a day. It does not apply during a fast.
Symptoms of low sodium during a fast
- Headache, particularly a dull pressure across the forehead or temples
- Fatigue out of proportion to fast length
- Lightheadedness on standing
- Brain fog and difficulty concentrating
- Muscle weakness
- Mild nausea
- Cravings for salt specifically (not food generally)
If you experience these and add 1–2 grams of sodium to water (about ½ teaspoon of salt), they typically resolve within 20–40 minutes. That diagnostic test is itself useful: if salt fixes it, it was sodium. If salt doesn’t fix it, look elsewhere.
How much sodium
- Fasts under 18 hours: Usually unnecessary if your previous meal was salted normally.
- 18–36 hour fasts: 1–2 grams of added sodium across the fast (split into 2–3 doses).
- 36–72 hour fasts: 2–4 grams per day.
- Beyond 72 hours: 3–5 grams per day, plus increasing attention to potassium and magnesium.
Conversion: 1 teaspoon of salt is approximately 6 grams of salt = 2.3 grams of sodium. So ½ teaspoon ≈ 1.2 g sodium.
Sources of sodium for fasting
- Plain table salt or sea salt in water. Cheap, effective, and works.
- Pink Himalayan salt. No meaningful nutritional advantage over regular salt despite marketing claims, but if you prefer the taste, fine.
- Plain bouillon or bone broth. Bone broth is debated — it contains some calories and protein and may technically end an autophagy fast — but for general fasting comfort, it’s effective and palatable. A standard cup contains roughly 800–1200 mg sodium.
- Pickle juice. Highly concentrated sodium plus some vinegar. Polarising taste; effective.
Potassium: The Quiet One
Potassium loss during fasting is slower and less symptomatic than sodium loss, but on multi-day fasts it becomes the limiting factor for cardiac stability. Severe potassium depletion (hypokalaemia) causes muscle weakness, irregular heart rhythm, and in extreme cases cardiac arrest.
How much potassium
- Fasts under 24 hours: Generally not needed.
- 24–72 hours: 1–2 grams (1000–2000 mg) per day.
- Beyond 72 hours: 2–3 grams per day, ideally with medical supervision.
Sources
- No-salt salt (potassium chloride). Sold under brands like NuSalt or Lo-Salt. About 530 mg potassium per ¼ teaspoon. Bitter taste in water; tolerable diluted.
- Cream of tartar (potassium bitartrate). About 495 mg potassium per teaspoon. Tart but workable in citrusy water.
- Coconut water. Contains potassium and some natural sugar. Will end a strict fast but is fine for general comfort fasting.
Caution: don’t megadose potassium
Unlike sodium, potassium has a narrow safe range. Acute high doses can be dangerous, particularly for anyone on ACE inhibitors, ARBs, potassium-sparing diuretics, or with kidney disease. Spread potassium intake across the day. Don’t take more than 1 gram in a single dose.
Magnesium: For Cramps and Sleep
Magnesium depletion during fasting is gradual — most people don’t notice it on fasts under 48 hours. Beyond that, the classic symptoms are muscle cramps (especially calf cramps at night), restless sleep, and twitching eyelids.
How much magnesium
- Fasts under 48 hours: Optional.
- 48–72 hours: 200–400 mg elemental magnesium.
- Beyond 72 hours: 300–500 mg daily, split into 2 doses.
Forms of magnesium
The form matters. The cheap forms cause GI upset; the better-absorbed forms cost more.
- Magnesium glycinate / bisglycinate. Best tolerated. Calming. Good for evening dose.
- Magnesium citrate. Well absorbed but can be laxative at higher doses (sometimes useful, sometimes not).
- Magnesium malate. Energising; better for daytime.
- Magnesium oxide. Cheap, poorly absorbed, and reliably causes diarrhoea. Skip it.
- Topical / Epsom salt baths. Absorption claims are exaggerated, but a warm Epsom bath is genuinely relaxing on a long fast — worth doing for the comfort, not relied on for replacement.
How Much You Need by Fast Length
A summary, daily targets, for healthy adults without contraindications:
What to Actually Drink
Option 1: DIY mix (cheapest)
In 1 litre of water, dissolve:
- ½ teaspoon plain salt (~1.2 g sodium)
- ¼ teaspoon no-salt / potassium chloride (~500 mg potassium)
- Optional: squeeze of lemon for taste
Sip across 4–6 hours. Repeat as needed. Magnesium taken separately as a capsule with water.
Option 2: Commercial fasting electrolyte powders
Several brands now sell fasting-specific electrolyte mixes (Redmond Re-Lyte, LMNT, Keto Chow, others). They typically contain ~1 g sodium, 200 mg potassium, and 60 mg magnesium per serving, with no calories or sweeteners that meaningfully affect fasting. Convenient but cost 10–30× the DIY version.
Option 3: Plain bouillon or broth
A cup of clear bouillon contains roughly 800–1000 mg sodium. Acceptable for general fasting comfort. Bone broth adds protein (3–9 g per cup) and may end a strict autophagy fast; for most purposes, fine.
Option 4: Sports drinks (not recommended)
Most contain 30+ grams of sugar per bottle and only 100–200 mg sodium. The sugar will end your fast immediately. Sugar-free versions are closer to acceptable but are calibrated for endurance athletes losing electrolytes through sweat, not fasters losing them through urine. The DIY mix above is better targeted.
Do Electrolytes Break a Fast?
Plain electrolyte mineral salts — sodium chloride, potassium chloride, magnesium glycinate — contain no calories and do not raise insulin meaningfully. They do not break a fast by any practical definition.
The complications are in the additives:
- Sugar or honey: Breaks any fast.
- Stevia, monk fruit, sucralose, erythritol: Don’t raise insulin much, but they may stimulate the cephalic phase response (your brain anticipates food). For a strict autophagy fast, avoid. For general fasting, fine.
- Citric acid, natural flavours, “0 calorie” vitamin additions: Generally negligible.
Practical rule: if your goal is weight management or insulin sensitivity, almost any electrolyte source is fine. If your goal is maximum autophagy, stick to plain mineral salts in water and skip flavoured products.
Signs You’ve Overdone It
Electrolyte excess is rare in healthy adults — kidneys handle excess sodium and magnesium well — but it does happen, particularly with potassium.
Too much sodium
- Excessive thirst
- Mild swelling in fingers or ankles
- Elevated blood pressure (relevant if you have hypertension)
Too much potassium
- Tingling or numbness in fingers and lips
- Muscle weakness
- Irregular heartbeat (this is a stop-now sign)
Too much magnesium
- Loose stools or diarrhoea — the most common dose-limiting symptom
- Reduce dose or switch to glycinate form
If you have kidney disease or take potassium-affecting medication (ACE inhibitors, ARBs, potassium-sparing diuretics, NSAIDs at high dose), get electrolyte protocols cleared by your doctor before starting.
Frequently Asked Questions
I get a headache around hour 20 of every fast. What do I do?
Almost certainly sodium. Try ½ teaspoon of salt in a cup of warm water at hour 16, before the headache starts. If it prevents the headache, you’ve diagnosed the problem.
Can I just drink mineral water?
Most mineral waters contain modest amounts of magnesium and calcium but very little sodium — far less than you need. Useful as part of hydration, not sufficient on its own.
What about “snake juice”?
Snake juice is a popular DIY recipe from extended-fasting communities: water + salt + no-salt (potassium) + magnesium + a touch of food-grade lite salt. It’s essentially the same as Option 1 above with magnesium added. Effective; nothing magical about the brand name.
Can I take electrolyte capsules instead of mixing drinks?
Yes. Capsules are convenient and often gentler on taste preferences. Total daily dose is what matters, not delivery method. Just split across the day rather than taking a whole day’s worth at once.
Will adding salt make me retain water and ruin my weight loss?
Short-term scale weight may not drop as fast — sodium does cause some water retention. But the metric that matters is fat loss, not scale weight. Adequate sodium also lets you sustain longer fasts without misery, which produces more fat loss over weeks. The trade is worth it.
I have high blood pressure. Should I still add salt?
Talk to your doctor. People with salt-sensitive hypertension may need to be more conservative. People without, who are otherwise healthy and fasting, generally tolerate added sodium fine. Some people’s blood pressure actually drops on fasting, which can compound with antihypertensive medication — another reason for medical input.
The Short Version
If you take only one thing from this guide: sodium fixes most fasting symptoms. Half a teaspoon of salt in water, taken before symptoms appear, prevents the majority of fasting headaches, fatigue, and brain fog that drive new fasters to break their fast prematurely.
Potassium and magnesium become important on multi-day fasts. For everyday 16:8 or even OMAD, sodium alone is usually enough.
Electrolytes are not optional on extended fasts. They are the single most effective tool for making fasting sustainable.