Electrolytes During Fasting: The Practical Guide

Why Fasting Changes Electrolyte Needs

On a typical Western diet, many adults take in roughly 3–4 grams of sodium a day, along with potassium and magnesium from food. When you stop eating, that intake stops too. At the same time, two changes in the body tend to increase losses.

First, insulin falls. Insulin encourages the kidneys to hold on to sodium. When insulin levels drop during a fast, the kidneys excrete more sodium and water. This is thought to be a major reason for the “keto flu” and for the headaches some people get around the 18–24 hour mark.

Second, glycogen stores release water. Each gram of stored glycogen is bound to roughly 3 grams of water. As glycogen is used up over the first day or two of a fast, some of that water — and the minerals dissolved in it — is lost. This is why the scale often drops quickly at the start of a fast, and why much of that early drop is water rather than fat.

On short daily fasts these changes are small and your meals easily make up the difference. On longer fasts they add up, which is why electrolytes matter more as fasts get longer.

Sodium: Usually the Most Noticeable

Sodium is typically the electrolyte whose loss people notice first during a fast. General public-health advice to limit salt is aimed at everyday eating, where most sodium comes from processed food. During a longer fast you are eating no food at all, so your needs can look different — but that is not a reason to ignore salt limits if you have high blood pressure or another condition affected by sodium.

Possible signs of low sodium during a fast

• Headache, often a dull pressure across the forehead or temples
• Fatigue that feels out of proportion to the length of the fast
• Lightheadedness when standing up
• Brain fog and difficulty concentrating
• Muscle weakness
• Mild nausea
• Craving salt specifically, rather than food in general

Many people find that a small amount of salt in water — for example ¼–½ teaspoon (about 0.6–1.2 g sodium) — eases these symptoms. If it doesn’t, the cause may be something else, such as low blood sugar, dehydration, or caffeine withdrawal. Severe or worsening symptoms, fainting, confusion, or chest pain are reasons to stop the fast and get medical help.

General sodium ranges

• Fasts under 18 hours: Usually unnecessary if your meals are normally salted.
• 18–36 hour fasts: Some people add around 1–2 g of sodium across the fast, split into 2–3 doses.
• Multi-day fasts: Ranges of roughly 2–4 g per day are commonly used by healthy adults, taken in several small doses. Fasts of this length are best done with medical guidance.

Conversion: 1 teaspoon of table salt is about 6 g of salt, which contains about 2.3 g of sodium. So ½ teaspoon provides about 1.2 g of sodium. Salt is only about 40% sodium, so always check whether a label or guide is talking about “salt” or “sodium”.

Sources of sodium while fasting

• Plain table salt or sea salt dissolved in water. Cheap and effective.
• Pink or specialty salts. No meaningful nutritional advantage over regular salt; the trace minerals are present in tiny amounts. Use them if you prefer the taste.
• Clear bouillon or broth. A cup often contains roughly 700–1,000 mg of sodium (check the label). Bone broth also contains some protein and calories, so it technically ends a strict fast, but many people use it on longer fasts for comfort. See our bone broth guide.
• Pickle juice. High in sodium, with some vinegar. An acquired taste, but it works for some people.

Potassium: Handle With Care

Potassium losses during fasting tend to be slower and less noticeable than sodium losses, but they matter more on multi-day fasts. Potassium is essential for normal muscle and heart function. Both too little (hypokalaemia) and too much (hyperkalaemia) can cause muscle weakness and dangerous heart-rhythm problems — which is why potassium deserves more caution than sodium.

General potassium ranges

• Fasts under 24 hours: Generally not needed. Eat potassium-rich foods (vegetables, legumes, potatoes, fruit, dairy) during your eating window.
• 24–72 hour fasts: Some healthy adults use a modest amount, around 1–2 g per day, split into several small doses.
• Beyond 72 hours: Should be discussed with a healthcare provider. Blood tests may be needed to know what is appropriate for you.

Sources

• Potassium chloride salt substitutes (sold as “no-salt” or “lite salt”). Potassium content varies a lot between products — some are pure potassium chloride, others are blends with regular salt — so check the label. Pure potassium chloride provides roughly 600–700 mg of potassium per ¼ teaspoon. The taste is bitter in plain water but tolerable when diluted.
• Cream of tartar (potassium bitartrate). About 500 mg of potassium per teaspoon. Only use small amounts; large doses of cream of tartar have caused dangerously high potassium levels.
• Coconut water. Contains potassium but also natural sugar, so it ends a strict fast.

Magnesium: For Cramps and Sleep

Magnesium levels change gradually during fasting, and most people don’t notice anything on fasts under 48 hours. On longer fasts, some people report muscle cramps (especially night-time calf cramps), restless sleep, or eyelid twitching, and find that magnesium helps.

General magnesium ranges

• Fasts under 48 hours: Optional.
• Longer fasts: Around 200–350 mg of elemental magnesium per day from supplements is a common range, split into two doses if needed.

For context, the upper limit for magnesium from supplements set by US health authorities for adults is 350 mg per day; higher amounts mainly increase the risk of diarrhoea. People with kidney disease should not take magnesium supplements without medical advice.

Forms of magnesium

The form affects how well it is tolerated:

• Magnesium glycinate / bisglycinate. Usually well tolerated. Many people take it in the evening.
• Magnesium citrate. Well absorbed, but can have a laxative effect at higher doses.
• Magnesium malate. Another reasonably well-tolerated form.
• Magnesium oxide. Inexpensive but less well absorbed and more likely to cause loose stools.
• Epsom salt baths. Evidence that meaningful magnesium is absorbed through the skin is weak. A warm bath can be relaxing, but don’t rely on it for replacement.

General Ranges by Fast Length

A summary of daily ranges commonly used by healthy adults with no kidney, heart, or blood-pressure conditions. Start at the low end, and let symptoms — not the maximum number — guide you.

What to Actually Drink

Option 1: Simple homemade mix

In 1 litre of water, dissolve:

• ¼–½ teaspoon plain salt (about 0.6–1.2 g sodium)
• Optional, for multi-day fasts only: a small pinch (up to ⅛ teaspoon) of potassium chloride salt substitute — skip this if any of the cautions above apply
• Optional: a squeeze of lemon for taste

Sip it over several hours rather than drinking it all at once. If you use magnesium, take it separately as a supplement with water.

Option 2: Ready-made electrolyte powders

Many zero-sugar electrolyte powders and tablets are sold for fasting or low-carb diets. Their sodium, potassium, and magnesium content varies widely, so read the label and add up your total intake. Some contain sweeteners or flavourings (see below). They are convenient but cost considerably more than a homemade mix.

Option 3: Clear bouillon or broth

A cup of clear bouillon typically provides several hundred milligrams to around 1 g of sodium. Bone broth also adds a few grams of protein and some calories, so it ends a strict fast, but many people find it helpful on longer fasts.

Option 4: Regular sports drinks (not ideal)

Standard sports drinks usually contain a lot of sugar and relatively little sodium. The sugar ends a fast. Sugar-free versions are closer to suitable, but they are formulated for replacing sweat losses during exercise rather than for fasting.

Do Electrolytes Break a Fast?

Plain mineral salts — sodium chloride, potassium chloride, magnesium supplements — contain no calories and are not expected to raise insulin. For practical purposes, they do not break a fast.

The additives are what to watch:

• Sugar, honey, or maltodextrin: Break a fast.
• Non-nutritive sweeteners (stevia, monk fruit, sucralose, erythritol): Contain little or no energy and generally have small effects on insulin, although evidence on subtle effects is mixed. If you want a strict fast, plain salts are the simplest choice. See our sweeteners guide.
• Citric acid, natural flavours, added vitamins: Generally negligible.

Practical rule: if you want to keep things simple and strict, use plain mineral salts in water and skip flavoured products.

Signs You’ve Overdone It

Healthy kidneys handle modest excess sodium and magnesium well, but problems can occur — most importantly with potassium, and in anyone whose kidney function is reduced.

Too much sodium

• Excessive thirst
• Swelling in fingers or ankles
• Raised blood pressure (important if you have hypertension)

Too much potassium

• Tingling or numbness
• Muscle weakness
• Palpitations or an irregular heartbeat

High potassium can also cause no symptoms at all until it becomes dangerous. If you notice palpitations, an irregular heartbeat, chest discomfort, or sudden weakness, stop taking potassium and seek urgent medical care.

Too much magnesium

• Loose stools or diarrhoea — the most common sign. Reduce the dose or try a gentler form such as glycinate.
• Nausea, flushing, or unusual drowsiness — rare in people with healthy kidneys; seek medical advice.

If you have kidney disease or take medication that affects potassium or fluid balance (ACE inhibitors, ARBs, diuretics, regular high-dose NSAIDs, or others), talk to your doctor before using electrolyte supplements. Our fasting and medications guide covers more of these interactions.

Frequently Asked Questions