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Practical Guide

Electrolytes During a Fast: What to Take and When

The electrolyte advice aimed at fasters is mostly written for people fasting two, three or five days and read by people fasting sixteen hours. The honest answer is that the length of the fast decides almost everything, and that there is no published dosing guideline for electrolytes in intermittent fasting at all.

By Sarah Patel

The short version

Up to about 24 hours — salted food around the fast is usually enough. 36 hours or more — sodium is the one worth thinking about. Potassium — not a do-it-yourself supplement. Magnesium — stay under the published supplement limit. On medication or with a medical condition — talk to a doctor or registered dietitian before you fast or supplement.

What "electrolytes" means here

Sodium, potassium and magnesium are minerals that carry an electrical charge in body fluids. They matter for nerve signalling, muscle contraction and heart rhythm, which is why the internet cares about them and why getting them wrong is not just a comfort issue.

Everything below comes from published reference values and one large clinical study. Where I am giving my own judgment rather than a published figure, I say so.

Why a fast can lower sodium

In a 2019 observational study in PLoS One, Wilhelmi de Toledo and colleagues followed 1,422 people through medically supervised fasts of 4 to 21 days. The authors describe increased sodium excretion in the urine in the first phase of fasting, linked to ketones being excreted, which eases later as the kidney handles ammonium differently.

The numbers are reassuring for a supervised group and still instructive:

  • Average blood sodium fell slightly, from 140.1 to 138.7 mmol/L, and stayed in the normal range for the group.
  • Potassium, calcium and magnesium were within the normal range at the start and end of the fast.
  • Six people developed mild low sodium during the fast; the lowest reading was 127 mmol/L. It was described as non-serious and was corrected with sodium chloride.
  • Participants were not fasting on water alone. They had about 250 ml of juice at noon and 250 ml of vegetable soup in the evening (roughly 200 to 250 kcal a day), drank about 3 litres of fluid, and were examined by physicians two to three times a week with daily nursing supervision.

That last point is the caveat. The study shows that fasting under supervision, with soup and juice and regular checks, was well tolerated. It does not tell you what happens on plain water, unsupervised, in someone taking a blood pressure tablet.

What to do, by fast length

This table is my editorial framework built on the sources at the bottom of the page. It is not a published dosing protocol.

Fast lengthReasonable approachNotes
12–18 hoursNothing extraSalt your meals normally. Water, and coffee or tea if you like them.
20–24 hoursUsually nothing extraIf you feel lightheaded, a glass of water with a little salt, or a cup of broth, is a low-risk experiment for a healthy person.
36–48 hoursSodium is the one to plan forBroth or lightly salted water. Food-based sources for potassium and magnesium in the meals either side of the fast.
72 hours or moreAsk a clinician firstBlood tests and supervision, as in the study above. Do not build a supplement stack from a blog post, including this one.

Sodium

Sodium is the electrolyte most likely to matter, because it is the one the study describes being lost early.

Two published reference points help you think about scale. The National Academies' 2019 report sets an Adequate Intake of 1,500 mg of sodium a day for adults aged 14 to 50, and a Chronic Disease Risk Reduction level of 2,300 mg, above which cutting intake is expected to lower chronic disease risk. The FDA describes 2,300 mg as about one teaspoon of table salt.

Those figures describe ordinary eating, not fasting, and 2,300 mg is a level to stay under across the whole day, not a target to hit. If you add salt during a fast, count it as part of that day's total.

Check before you add salt

If you take medicine for blood pressure or heart failure, take a diuretic, have kidney disease, or have been told to limit sodium, do not add salt to a fast on your own. The right amount for you can be very different from the general figures above.

Potassium: leave the pills alone

Potassium is where fasting advice online turns risky. Two facts from the NIH Office of Dietary Supplements explain why.

  • Over-the-counter potassium supplements usually contain no more than 99 mg per serving, because the FDA has ruled that some oral potassium chloride drug products above that amount are not safe, having been associated with small-bowel lesions.
  • Too much potassium in the blood, called hyperkalemia, can cause abnormal and dangerous heart rhythms. Common causes include ACE inhibitors, angiotensin receptor blockers (ARBs), potassium-sparing diuretics such as spironolactone, triamterene and amiloride, and impaired kidney function.

A "potassium salt" product or a powder with a big potassium number is not a harmless alternative to table salt for someone in those groups. In the study above potassium fell during fasting but stayed in the normal range for the group; that is not the same as a reason for you to add a supplement. Get potassium from food in your eating window, and ask a doctor before going further.

Magnesium: mind the ceiling

The NIH Office of Dietary Supplements lists an RDA of 310 to 320 mg a day for adult women and 400 to 420 mg for adult men. It sets a tolerable upper limit of 350 mg a day for magnesium from supplements, because high supplement doses can cause diarrhea, nausea and cramping. Magnesium from food is not counted toward that limit, because the kidneys clear the excess.

Diarrhea is the reason this matters for fasters: it costs you fluid and minerals, which is the opposite of what you were trying to achieve. If you use a magnesium supplement, check the label for the elemental amount and keep the total below 350 mg unless a professional has told you otherwise. In the fasting study, magnesium stayed stable.

When to take them

There is no published research that says one clock time is best, so treat the following as practical judgment.

  1. Put most of your mineral intake in the eating window. Salted food, vegetables, dairy, beans, nuts and fish supply sodium, potassium and magnesium together in food form, which is the safest way to take potassium in particular.
  2. If you salt a longer fast, spread it out. A small amount in broth or water once or twice across the day is easier to tolerate than one large salty drink, and it keeps you from overshooting.
  3. Do not make up for a fast in one sitting. A very salty, very large first meal is the same mistake as any oversized first meal. See how to break a fast for the sequence.

Does an electrolyte drink break the fast?

Plain salted water contains no calories. Many commercial electrolyte drinks and powders contain sugar or other calorie sources, and whether that counts as breaking your fast depends on your goal. If you are fasting for a metabolic reason, read the label and decide in advance. If you are fasting for religious reasons, the rules of your tradition apply.

Symptoms that are not a salt problem

Headache, tiredness and light dizziness are common when fasting, and they are non-specific. They can also be signs of something else, so a pinch of salt is a reasonable first step only for a healthy person with mild symptoms.

Stop the fast and get medical help if you have

Fainting, confusion, a racing or irregular heartbeat, persistent vomiting, or muscle weakness that is getting worse. Do not try to solve these with more supplements.

Who should talk to a professional first

Speak to a doctor or registered dietitian before fasting, and certainly before supplementing, if any of these apply:

  • You take blood pressure medication, a diuretic, or medicine for heart failure.
  • You take insulin or another diabetes medication.
  • You have kidney disease, heart disease, or any condition that affects fluid and mineral balance.
  • You are pregnant or breastfeeding, or have a history of disordered eating.
  • You plan to fast for three days or more. The study I cited excluded people with advanced kidney, liver or cerebrovascular insufficiency, cachexia, anorexia nervosa, dementia, pregnancy and lactation, and it was run in a clinic.

The refeeding risk after long fasts also involves electrolytes. Reintroducing food after prolonged starvation can push potassium, phosphate and magnesium into cells and drop their blood levels. NICE guidance on nutrition support in adults lists risk factors such as little or no intake for more than five days, low baseline potassium, phosphate or magnesium, a history of alcohol misuse, and medicines including insulin and diuretics. That is a medical-supervision topic, not a home-remedy topic.

Related reading

Sources

Sarah Patel writes on fasting protocols and adherence. This article is general information and not medical advice, and nobody involved in writing it is your doctor. If you take medication or have a medical condition, talk to a doctor or registered dietitian before fasting or taking any electrolyte supplement.