You need electrolytes, not just water

In trials of healthy adults, drinks with enough added sodium kept more of the fluid in the body for a few hours than plain water did, and at the sodium levels of most sports and electrolyte drinks the difference was small or absent. The kidneys and hormones hold blood sodium and potassium in range, and ordinary diets already supply sodium: the World Health Organization describes sodium deficiency as extremely unlikely in healthy people and puts average adult intake at 4,278 mg a day, more than double its recommended limit of 2,000 mg.

Electrolytes are the standard treatment in one setting, illness with diarrhoea, where water and salts are lost together. The World Health Organization treats diarrhoea with oral rehydration solution, clean water with salt and sugar. It names diarrhoea as the third leading cause of death in children aged one to 59 months, and lists persistent diarrhoea, blood in the stool and signs of dehydration as reasons to consult a health professional. After heavy sweating, a 2025 review of 20 small trials found that sugar-and-electrolyte drinks may rehydrate somewhat better than water, with certainty rated low to very low, and described water as an acceptable alternative where clean water is available. In endurance events lasting many hours, dangerously low blood sodium comes mainly from drinking beyond thirst, and sodium intake has not been shown to prevent it.

At rest, sodium content drove most of the difference. In the largest trial, published in 2016, 72 young men each drank a litre of still water and three of 12 other drinks, about 17 men per drink. Over the next four hours they passed 1,038 g of urine after an oral rehydration solution with 55 mmol of sodium per litre (about 1,265 mg of sodium in the litre), against 1,337 g after water, and whole and skimmed milk held fluid about as well as the rehydration solution. A sports drink with 21 mmol of sodium per litre was no different from water, and neither were cola, diet cola, tea, coffee or sparkling water. The trial was funded by the European Hydration Institute, which a 2012 paper it funded describes as having The Coca-Cola Company as a funding partner, and its cola, diet cola and sports drink were Coca-Cola products.

Two smaller trials at rest changed what was in the drink. In a 2019 trial of 12 young and 12 older adults drinking one litre, fluid retention rose with the drink's sodium in the young group, and the highest-sodium drink (60 mmol per litre) scored 1.24 on the beverage hydration index against 1.00 for water. Young adults were back in negative fluid balance within one to two hours whatever they drank. In a 2021 trial of 19 adults, a sugar-free electrolyte drink at about 22 mmol of sodium per litre, a typical sports-drink level, did not consistently retain more than water, while the same electrolytes with sugar or a dipeptide added did. The Coca-Cola Company funded the 2021 trial. The 2019 trial was funded partly by the National Institutes of Health and partly by an unrestricted gift from the maker of two of the drinks it tested. A 2017 trial of 49 healthy men (average age 25, fasting blood sugar in the normal range) had each drink distilled water on one day and a sports drink with 21 mmol of sodium per litre and 6% carbohydrate on another, about 1.9 litres in all over eight hours in an air-conditioned room with three meals, which came to about 930 mg of sodium and 116 g of sugar from the drink. They retained 17% of the fluid with the drink and 7% with water, and passed 1,862 mL of urine against 2,105 mL. The trial was supported by a research institute and by the company F&N Interflavine, and the drink carried the F&N name.

After exercise that removed 2.6% of body mass, 26 male athletes in a double-blind trial drank a volume equal to what they had lost. Over 3.5 hours they kept 58% of a water placebo, 74% of a sports drink with 18 mmol of sodium per litre and 77% of an oral rehydration solution with 45 mmol per litre. The trial was funded by a division of Abbott Nutrition. In the 2025 review, 11 studies compared 4% to 9% carbohydrate-electrolyte drinks with water on urine output: five found less urine with the drinks and six found no difference, and no difference was shown in net fluid balance or plasma volume. The reviewers noted that many of the trials were funded by the companies making the drinks.

Exercise-associated hyponatremia, a fall in blood sodium that has caused deaths in endurance events, is linked mainly to drinking beyond thirst while hormone signals keep the kidneys holding water. A 2017 review reported that in a 161 km ultramarathon, hyponatremia was unrelated to total sodium intake, and described a runner who developed it while overdrinking despite taking sodium supplements. The review's authors state that drinking according to thirst prevents it in temperate conditions for exercise lasting under 17 hours.

Adding electrolytes has its own trade-offs. The World Health Organization links high sodium intake to raised blood pressure and an estimated 1.7 million deaths in 2023. US dietary surveys show people consuming less potassium than recommended, and most potassium-containing supplements supply no more than 99 mg per serving, adding a mean of 87 mg a day among users. In people with chronic kidney disease, or taking ACE inhibitors or potassium-sparing diuretics, even potassium intake below the recommended level can cause hyperkalemia, which in severe cases causes life-threatening heart rhythms. The same fact sheet names type 1 diabetes, heart failure, adrenal insufficiency and liver disease as conditions in which hyperkalemia can occur. A prescription potassium chloride label also lists angiotensin receptor blockers, spironolactone and anti-inflammatory painkillers as raising potassium, and states that when excretion is impaired, potentially fatal hyperkalemia can result.