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. UK adults 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.

What are electrolytes?

Dissolve table salt in water and it splits into sodium and chloride ions, each carrying a charge, and the water starts to conduct electricity. That is all the word means: a mineral that becomes charged in solution. 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 between about 135 and 145 mmol/L 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}

"The big three" are sodium, potassium and chloride, the three a blood test always reports; add magnesium and calcium for the five on most sachet labels; phosphate and bicarbonate complete the seven.

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 Weigh yourself before and after: each kilogram down is roughly a litre of sweat.

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 trial participants against 4% on placebo) and diarrhoea (12 to 16% against 9%) can dehydrate you and strain the kidneys.⁹ We answer the Mounjaro questions, including which electrolytes suit someone on tirzepatide, in do you need electrolytes on Mounjaro?

Very low-carbohydrate diets. "Keto flu" is real as an experience: a study of 448 forum posts found 54 symptoms, headache, fatigue, nausea and dizziness the commonest, peaking in the first week and resolving at a median of four and a half days, with electrolytes among 18 remedies users proposed, none tested in a trial.¹⁰

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.^{9, 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 and drinking 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) 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 contains magnesium, 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 (odds ratio 7.4), and the composition of what they drank was not associated.¹² Across five years of a 161 km ultramarathon, 15% of finishers 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 of exercise cramp rests mainly on anecdote, and fatigue of the muscle's own nerve control has the better experimental support, so sodium replacement is sensible for a heavy sweater in the heat without any 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 the 2020 Cochrane review of 195 trials, 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 and by nearly 6 mmHg in people with hypertension; 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, extra potassium lowered blood pressure up to a difference of about 30 mmol a day and raised it above about 80 mmol a day in people on blood-pressure drugs; the 492 mg in an Intense sachet 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.⁹ Too much shows as symptoms: thirst and puffy fingers for sodium; 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. They filter the whole blood volume many times a day and decide, under hormonal instruction, how much sodium, potassium, water and acid to keep: aldosterone holds sodium and sheds potassium, antidiuretic hormone holds water, and thirst tops the system up. That 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 or a diuretic alters the signals. High sodium or potassium on a blood test is a medical finding about this system, not something a drink corrects.²⁰ Of the seven, the one a UK diet most often falls short of is potassium, because it comes with potatoes, pulses, fruit and vegetables rather than packaged food.^{1, 6}

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

The NHS blood test called urea and electrolytes, or a renal panel, measures sodium, potassium, chloride and bicarbonate alongside urea and creatinine, the waste products that show how well the kidneys are filtering.²⁰ It is ordered to investigate symptoms, monitor medicines and check kidney function, and the GP interprets any result outside the range. A slightly low sodium on a U&E is not a shopping prompt, and a normal U&E does not mean an August marathon needs no plan.

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; 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. A sugar-free sachet carries too little carbohydrate to meet that definition and sits outside it, 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 salt arithmetic for ordinary days against training, illness and heat. Which drinks contain electrolytes? compares milk, coconut water, rehydration sachets, sports drinks and electrolyte sachets by sodium and potassium per serving. Signs you might be low on electrolytes covers symptoms and when it is a GP matter; electrolytes for runners carries the weigh-yourself method; rehydration sachets vs electrolyte drinks covers illness by age. For magnesium itself what is magnesium good for? is the hub of our vitamins and minerals guides.

Frequently asked questions

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

Sodium, potassium and chloride are the three every blood test reports; add magnesium and calcium for the five on most sachet labels; phosphate and bicarbonate make seven.²⁰

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 add half a kilogram of water to the scales for a day; that is fluid, not fat. A sugar-free sachet carries no sugar; sugared sports drinks carry 80 to 350 kcal a litre.⁵

Which fruit has the most electrolytes?

Dried dates, at about 727 mg of potassium per 100 g, then avocado at about 580 mg and banana at 330 mg; fruit carries almost no sodium, which is why fruit alone does not replace sweat.⁶

Which is better, a rehydration sachet or an electrolyte powder?

They do different jobs. A pharmacy rehydration sachet is a medicine for diarrhoea and vomiting, with glucose to carry the sodium in; an electrolyte powder is a food for sweaty or light-eating days.^{9, 11}

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

Sources

1. NHS. Vitamins and minerals: others. nhs.uk/conditions/vitamins-and-minerals/others.
2. NHS. Salt in your diet. nhs.uk/live-well/eat-well/food-types/salt-in-your-diet.
3. Public Health England. National Diet and Nutrition Survey: assessment of salt intake from urinary sodium in adults aged 19 to 64 in England, 2018 to 2019. 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. PubMed 26070030.
5. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. gov.uk.
6. Public Health England. McCance and Widdowson's Composition of Foods Integrated Dataset (CoFID), 2021. gov.uk.
7. NHS. Dehydration. nhs.uk/conditions/dehydration.
8. NHS. Diarrhoea and vomiting. nhs.uk/symptoms/diarrhoea-and-vomiting.
9. Summaries of Product Characteristics for tirzepatide (Mounjaro) and for a UK pharmacy oral rehydration sachet, via the MHRA products database. products.mhra.gov.uk.
10. Bostock ECS, et al. Consumer reports of "keto flu" associated with the ketogenic diet. Frontiers in Nutrition, 2020. PubMed 32232045.
11. Hahn S, Kim Y, Garner P. Reduced osmolarity oral rehydration solution for treating dehydration caused by acute diarrhoea in children. Cochrane Database of Systematic Reviews, 2002. PubMed 11869639.
12. Almond CS, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005. 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. 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. PubMed 26227507.
15. Schwellnus MP. Cause of exercise associated muscle cramps: altered neuromuscular control, dehydration or electrolyte depletion? British Journal of Sports Medicine, 2009. PubMed 18981039.
16. Graudal NA, Hubeck-Graudal T, Jurgens G. Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride. Cochrane Database of Systematic Reviews, 2020. PubMed 33314019.
17. EFSA Panel on Nutrition, Novel Foods and Food Allergens. Dietary reference values for sodium. EFSA Journal, 2019. PubMed 32626425.
18. Filippini T, et al. Potassium intake and blood pressure: a dose-response meta-analysis of randomized controlled trials. Journal of the American Heart Association, 2020. PubMed 32500831.
19. Sarnowski A, et al. Hyperkalemia in chronic kidney disease: links, risks and management. International Journal of Nephrology and Renovascular Disease, 2022. PubMed 35942480.
20. NHS. Blood tests, including urea and electrolytes. nhs.uk/tests-and-treatments/blood-tests.

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.