What Are Electrolytes and What Do They Do?

Electrolytes are minerals that carry an electric charge once they dissolve in water: sodium, potassium, chloride, magnesium, calcium, phosphate and bicarbonate. They hold water in the right compartments of the body, let nerves fire and muscles contract, and keep the blood's acidity steady, and the kidneys adjust them minute by minute. Adults in England eat about 8.4 g of salt a day against the NHS maximum of 6 g, so on an ordinary day the question is rarely how to get more sodium; the losses that matter come through sweat, at about 0.8 g of sodium per litre and roughly 1.2 litres an hour in exercise, through vomiting and diarrhoea, and when normal eating stops.^{1, 2, 3, 4} This hub sets out the arithmetic and links every guide in the series.

Sources checked on 30 October 2026.

What are electrolytes?

Dissolve table salt in water and it splits into charged sodium and chloride ions that let the water conduct electricity; that is all the word means. Sodium and chloride sit mostly outside the cells, potassium, magnesium and phosphate mostly inside, and the difference across each cell wall is the battery that nerves and muscles run on. Blood sodium is held in a narrow range whatever you eat, which is why "electrolyte" turns up in three unrelated places: the salt cellar, the "U&E" blood test, and the sachet in a runner's pocket.

What do the seven electrolytes do, and how much do you need?

Each has a job, a UK reference figure and a food source. The NHS figures are daily needs for adults; the Nutrient Reference Values (NRVs) are what a supplement label uses when it prints a percentage, and a nutrient carries an authorised claim only at 15% of its NRV or more per serving.^{1, 5, 21}

Where do electrolytes come from in a UK diet?

Sodium is the one nobody in Britain is short of by default. The 2018 to 2019 national sodium survey measured salt in the urine of nearly 600 adults in England and put the average at 8.4 g a day, 40% over the 6 g maximum: 9.2 g for men and 7.6 g for women.³ Three quarters of it is already inside bread, cereals, meat products and ready meals; white bread runs at 400 mg of sodium per 100 g, grilled back bacon at about 1,400 mg, Cheddar at about 720 mg.^{2, 6} Potassium is the opposite case: it lives in the foods a packaged diet skips. A boiled potato gives about 365 mg per 100 g, a banana 330 mg, dried dates 727 mg, and a 250 ml glass of semi-skimmed milk about 390 mg of potassium with 108 mg of sodium and 300 mg of calcium.⁶ Eggs, often asked about, are modest at 154 mg of sodium and 145 mg of potassium per 100 g.⁶

How do you lose electrolytes, and when does it matter?

Sweat. In 506 athletes across team and endurance sports, whole-body sweat sodium averaged 36 mmol/L, about 0.8 g per litre, with a range from 18 to 71 mmol/L, and sweat rate averaged 1.2 litres an hour, from 0.3 to 5.7.⁴

Vomiting and diarrhoea. Gut fluid is rich in sodium, potassium and chloride, and this is the one everyday situation where the NHS points you to a pharmacist for an oral rehydration sachet: if you or a child over five show signs of dehydration, or you are older or have a weakened immune system. Water and squash come first, fruit juice and fizzy drinks make diarrhoea worse, and vomiting that will not let fluids stay down, diarrhoea beyond seven days or dehydration signs that persist after sachets are matters for 111.^{7, 8}

GLP-1 medicines. The product information for tirzepatide (Mounjaro) tells patients to take precautions to avoid fluid depletion and electrolyte disturbances, because nausea (12 to 18% of participants in the type 2 diabetes trials against 4% on placebo) and diarrhoea (12 to 16% against 9%) can dehydrate you and strain the kidneys.⁹ The Mounjaro questions are answered in do you need electrolytes on Mounjaro?

Very low-carbohydrate diets. "Keto flu" is real as an experience: a study of 448 forum posts from 300 users found 54 symptoms, headache, fatigue, nausea and dizziness among the commonest, peaking in the first week; extra sodium and electrolytes led the 18 remedies users proposed, which the authors note are not evidence-based.¹⁰

The morning after drinking. What to eat and drink with a hangover reports what the trials and the NHS say.

What does an electrolyte sachet actually replace? The sodium arithmetic

Three yardsticks make any sachet legible. A litre of average sweat carries about 0.8 g of sodium, which is 2 g of salt.⁴ A UK pharmacy oral rehydration sachet, the kind sold for diarrhoea, dissolves 0.47 g of sodium chloride, 0.53 g of disodium hydrogen citrate, 0.30 g of potassium chloride and 3.56 g of glucose in 200 ml: about 300 mg of sodium (around 60 mmol/L) and 160 mg of potassium, the glucose there because it carries sodium and water across the gut wall.¹¹ And the NHS salt maximum is 6 g a day, 2,400 mg of sodium.²

Against those yardsticks, sachets fall into two sizes. A low-sodium sachet of around 300 mg of sodium (0.75 g of salt, an eighth of the NHS maximum) with some potassium, magnesium and calcium carries about the sodium of a pharmacy rehydration sachet without the glucose, or roughly a third of a litre of average sweat, which sizes it for a warm desk day, a gym hour, a light-eating day or the recovery days after a stomach bug once you are eating normally again. A high-sodium sachet of around 2,000 mg of sodium (5 g of salt, about 80% of the NHS daily maximum in one drink; about 4,000 mg a litre made up in 500 ml, against sweat at 400 to 1,600 mg a litre) carries roughly the sodium in two litres of average sweat, which is a long, hot or very sweaty session in an adult with no kidney or blood-pressure condition. Where a sachet's magnesium reaches 15% of the reference intake (56.25 mg a serving), magnesium contributes to electrolyte balance. Both kinds are food supplements, not oral rehydration solutions, and neither is for treating vomiting and diarrhoea.

Can you have too few? Hyponatraemia in long events

The documented danger in endurance sport is over-drinking, not under-salting. Of 488 non-elite runners who gave blood at the finish of the 2002 Boston Marathon, 13% had blood sodium at or below 135 mmol/L; the independent risk factors were gaining weight during the race (odds ratio 4.2), finishing in over four hours against under three and a half (odds ratio 7.4) and body-mass extremes, and the composition of what they drank was not associated.¹² Across five years of a 161 km ultramarathon, 15% of the 669 finishers tested were hyponatraemic and the authors called excessive sodium supplementation and drinking beyond thirst ill advised; the 2015 international consensus says drink to thirst.^{13, 14} An electrolyte drink is not a licence to drink more, and it does not prevent hyponatraemia. Cramp belongs here too: the salt-depletion theory rests mainly on anecdote, and altered nerve control of the muscle has the better support, so sodium replacement for a heavy sweater in the heat carries no promise that it stops cramp.¹⁵

Who should ask before taking electrolytes? Kidneys and blood pressure

Sodium raises blood pressure in proportion to how much you already have. In white participants in the 2020 Cochrane review, cutting sodium from about 4.7 g to 1.5 g a day lowered systolic pressure by about 1 mmHg in people with normal blood pressure (95 trials) and by nearly 6 mmHg in people with hypertension (88 trials); the European Food Safety Authority puts a safe and adequate adult intake at 2.0 g of sodium a day.^{16, 17} A high-sodium sachet counts towards your 6 g like any other salt, so anyone with high blood pressure, heart failure or kidney disease asks a pharmacist or GP first. Potassium is the other half. In a dose-response analysis of 32 trials, the fall in blood pressure from extra potassium weakened above a difference of about 30 mmol a day, and above about 80 mmol a day blood pressure rose in people already on blood-pressure drugs; 500 mg of potassium is about 13 mmol.¹⁸ The firmer caution is chronic kidney disease and the medicines that hold potassium back, an ACE inhibitor (ramipril, lisinopril), an angiotensin receptor blocker (losartan, candesartan), spironolactone or amiloride, where high blood potassium is common and can be dangerous.¹⁹ Pharmacy rehydration sachets carry their own line: not for self-treatment in kidney disease or diabetes, and under-twos only under a doctor.¹¹ Too much shows as symptoms: stomach pain, nausea and diarrhoea for potassium supplements; diarrhoea above about 400 mg a day of supplemental magnesium.¹

Which organ controls your electrolyte balance?

The kidneys. Under hormonal instruction they decide how much sodium, potassium and water to keep, which is why blood sodium barely moves between a salty takeaway and a day of salad, and why the rules change when the kidneys are impaired: in chronic kidney disease, falling potassium excretion can leave the body unable to hold potassium steady.¹⁹

Is the "U&E" blood test the same thing?

The NHS blood test called urea and electrolytes, or a renal panel, checks whether the amount of certain substances in your blood may be causing your symptoms; a GP, nurse or specialist decides when you need it.²⁰ A slightly low sodium on a U&E is a GP matter, not a shopping prompt.

What can an electrolyte drink say on its label?

Great Britain's health claims register is strict here. The only authorised "hydration" wording belongs to carbohydrate-electrolyte solutions, drinks providing 80 to 350 kcal per litre from carbohydrate, at least three quarters of it from fast sugars, with 20 to 50 mmol/L of sodium and an osmolality of 200 to 330 mOsm/kg; those may say they enhance the absorption of water during exercise and maintain endurance performance during prolonged exercise.⁵ Sodium itself has no authorised claim, and the Advertising Standards Authority (ruling A25-1315087, 18 March 2026) treated "to hydrate" in an advert for sugar-free electrolyte sachets as a health claim not on the register (how to read an electrolyte label has the detail). A sugar-free sachet sits outside that definition, so it speaks through its minerals: "magnesium contributes to electrolyte balance" is the one electrolyte-specific authorised line, available at 15% of the magnesium NRV or more, and potassium's claims need 300 mg a serving.⁵

The electrolyte guides in this series

Do you need to drink electrolytes every day? does the daily salt arithmetic. Which drinks contain electrolytes? compares drinks by sodium and potassium per serving. Signs you might be low on electrolytes covers symptoms; electrolytes for runners carries the weigh-yourself method; rehydration sachets vs electrolyte drinks covers illness by age. For long events: electrolytes for athletes, are high-sodium electrolytes better?, sugar-free electrolytes vs sports drinks and dehydration vs hyponatraemia. For magnesium, see what is magnesium good for?

What we don't know

• Whether salt loss causes exercise cramp: the support is mainly anecdote and case series, and four prospective cohort studies did not back it.¹⁵
• What very high potassium intakes do to blood pressure: the estimates above about 80 mmol a day rest on few trials.¹⁸
• Whether electrolytes ease "keto flu": the evidence is forum posts, not a trial.¹⁰

Frequently asked questions

What are the big three electrolytes, and what are the top five?

Sodium, potassium and chloride are the big three; add magnesium and calcium for the five on most sachet labels; phosphate and bicarbonate make seven.

Are there electrolytes without sugar?

Yes. Sugar-free sachets supply the minerals without the sugar; without glucose they do not use the glucose-sodium transport that makes a rehydration solution work in diarrhoea, so they are sized for sweaty, light-eating and recovery days once you are eating and drinking normally again, not for treating vomiting and diarrhoea.¹¹

Do electrolytes cause weight gain?

Sodium holds water, so a salty sachet can nudge the scales up with fluid, not fat.¹ A sugar-free sachet carries no sugar; a drink using the register's hydration wording must carry 80 to 350 kcal a litre.⁵

Which fruit has the most electrolytes?

Dried fruit leads on potassium: per 100 g, dried apricots about 1,880 mg, sultanas 1,060 mg, dried figs 970 mg and dried dates 727 mg; among fresh fruit, avocado gives about 580 mg and banana 330 mg. Fruit carries almost no sodium, which is why fruit alone does not replace sweat.⁶

How do you increase electrolytes quickly?

Eat and drink: a 250 ml glass of milk gives 108 mg of sodium and 390 mg of potassium, a salted meal more, and a sachet in water is the measured option.⁶ Dizziness on standing that does not settle, dark or scant urine, or fluids that will not stay down are 111, not another drink.^{7, 8}

Sources

1. NHS. Vitamins and minerals: others. Page last reviewed 3 August 2020 (review overdue). nhs.uk.
2. NHS. Salt in your diet. Page last reviewed 17 April 2023 (review overdue). nhs.uk.
3. Public Health England. National Diet and Nutrition Survey: salt intake from urinary sodium in adults aged 19 to 64 in England, 2018 to 2019. Official statistics, 24-hour urine collections from 596 adults. gov.uk.
4. Baker LB, et al. Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences, 2016. Retrospective analysis of athlete testing, 506 athletes; authors employed by a sports-drink maker's research institute. PubMed 26070030.
5. Great Britain nutrition and health claims (NHC) register. Updated 19 May 2026. gov.uk.
6. Public Health England. Composition of Foods Integrated Dataset (CoFID) 2021. Food composition dataset. gov.uk.
7. NHS. Dehydration. Page last reviewed 1 May 2026. nhs.uk.
8. NHS. Diarrhoea and vomiting. Page last reviewed 21 December 2023. nhs.uk.
9. Tirzepatide. Summary of Product Characteristics, sections 4.4 and 4.8. Date of revision of the text 29 August 2026. medicines.org.uk.
10. Bostock ECS, et al. Consumer reports of "keto flu" associated with the ketogenic diet. Frontiers in Nutrition, 2020. Qualitative study of 448 forum posts from 300 users; funding not stated on PubMed. PubMed 32232045.
11. Oral rehydration salts sachets (glucose, sodium chloride, potassium chloride, disodium hydrogen citrate). Summary of Product Characteristics, sections 2, 4.4 and 5.2. Date of revision of the text 30 June 2023. medicines.org.uk.
12. Almond CS, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005. Prospective cohort study, 488 runners; NIH-funded. PubMed 15829535.
13. Hoffman MD, et al. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine and Science in Sports and Exercise, 2013. Pooled analysis of five race years, 669 finishers tested; non-government research support. PubMed 23135369.
14. Hew-Butler T, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, 2015. British Journal of Sports Medicine, 2015. Expert consensus statement. PubMed 26227507.
15. Schwellnus MP. Cause of exercise associated muscle cramps. British Journal of Sports Medicine, 2009. Narrative review; non-government research support. PubMed 18981039.
16. Graudal NA, et al. Effects of low sodium diet versus high sodium diet on blood pressure. Cochrane Reviews, 2020. Systematic review of randomised trials, 195 included references; authors at public institutions, no industry funding. PubMed 33314019.
17. EFSA NDA Panel. Dietary reference values for sodium. EFSA Journal, 2019. Agency opinion. PubMed 32626425.
18. Filippini T, et al. Potassium intake and blood pressure: a dose-response meta-analysis. Journal of the American Heart Association, 2020. Meta-analysis, 32 randomised trials; non-government research support. PubMed 32500831.
19. Sarnowski A, et al. Hyperkalemia in chronic kidney disease: links, risks and management. International Journal of Nephrology and Renovascular Disease, 2022. Narrative review; one author declares drug-industry grants and fees. PubMed 35942480.
20. NHS. Blood tests. Page last reviewed 2 November 2023. nhs.uk.
21. Regulation (EU) No 1169/2011 as it applies in Great Britain, Annex XIII: nutrient reference values. legislation.gov.uk.

Spotted something out of date? Email support@wellgard.co.uk with the page address and the sentence.

This article is general information, not medical advice. If you have high blood pressure, heart failure or kidney disease, or take a diuretic, an ACE inhibitor, an angiotensin receptor blocker, spironolactone or amiloride, ask a pharmacist or your GP before using any electrolyte or rehydration product, and treat dehydration that does not settle as a reason to call 111.