Electrolytes for Hydration Support

If you need to replace real fluid losses, an electrolyte drink with enough sodium is likely to hydrate you better than the same volume of plain water. In 72 men, 1 L of oral rehydration solution (ORS) left 1,038 g of urine over 4 hours versus 1,337 g after water, and its beverage hydration index (BHI, fluid kept relative to water) was 1.54 at 2 hours [1]. After exercise-induced dehydration, how much of a drink people kept rose directly with its sodium content [2].

The catch is the dose of sodium. A standard sports drink was no different from water in the same 72-person trial [1], and a 2021 trial found electrolytes at sports-drink levels did not consistently beat water on their own [3]. These are short laboratory studies of fluid balance, mostly in young healthy adults, not trials of health or performance outcomes, so the verdict is likely rather than strong.

Thinking about electrolytes after a night of drinking? Our article on electrolyte drinks for hangovers covers what the research shows.

Plain water drunk in volume dilutes blood sodium, and the kidneys respond by releasing more urine. A drink that carries sodium keeps plasma sodium and blood volume steadier, so less of the fluid is lost. In one trial, plasma volume after rehydration rose more with 52 and 100 mmol/L sodium drinks than with a near-sodium-free drink [2].

Volume and sodium work together. Drinking 150-200% of sweat losses only improved balance when the drink had enough sodium; with a 23 mmol/L drink, the extra fluid mainly increased urine output [4].

Carbohydrate and amino acids can add to retention: at rest, a carbohydrate-electrolyte drink and a dipeptide-electrolyte drink both beat water, although electrolytes contributed the largest share of the effect [3]. In young adults, fluid kept rose with drink sodium whether the drink was glucose- or amino-acid-based [5].

Better fluid balance over a few hours does not by itself mean better performance or health. Salt capsules during a 3-hour cycling time trial in cool conditions changed plasma volume but not finishing time [6].

The trials that showed a clear benefit used either oral rehydration solution at rest [1] or drinks with 52-100 mmol/L sodium after exercise, given at 1.5 times the fluid lost [2]. That is about 1,200-2,300 mg of sodium per litre, more than most sports drinks; one study's commercial sports-style drinks ranged from 20 to 60 mmol/L [5]. Check the label, because electrolyte products vary widely.

The American College of Sports Medicine advises estimating sweat losses by weighing before and after exercise, preventing losses above 2% of body weight during exercise, and replacing any fluid and electrolyte deficit afterwards [7]. Sweat rate and sweat sodium vary a lot between people, so personal testing beats a one-size rule [8].

In the largest trial, 72 hydrated men drank 1 L of one of 13 drinks. Oral rehydration solution, full-fat milk and skimmed milk all left less urine than water over 4 hours (P < 0.001); a sports drink, colas, tea, coffee, lager, orange juice and sparkling water did not differ from water [1].

A trial in 12 young and 12 older adults found that fluid retention rose with drink sodium in the young, with the 60 mmol/L drink scoring 1.24 against water's 1.00 (P = 0.01); in older adults a 30 mmol/L drink scored highest (1.20) [5]. In 18 trained adults, an electrolyte tablet in water modestly improved fluid balance from 2 hours, and doubling it added nothing [9].

Six men dehydrated by 1.9% of body mass drank 1.5 times their losses as drinks with 2, 26, 52 or 100 mmol/L sodium. By the end they were 689 mL and 359 mL short on the two lowest-sodium drinks, but back in balance on 52 mmol/L (-2 mL) and 100 mmol/L (+98 mL) [2].

In 12 men dehydrated by about 2%, drinking more than sweat losses improved balance only with a 61 mmol/L sodium drink; with 23 mmol/L, much of the extra simply came out as urine [4].

Electrolytes alone at sports-drink levels did not consistently beat water in 19 young adults, though they contributed the largest share of any drink's advantage [3]. Salt capsules did not improve a 72 km cycling time trial in cool conditions [6]. Every trial here is small, short and in healthy adults, and none measured health outcomes such as kidney stones, heat illness or hospital visits.

At the volumes studied, electrolyte drinks were well tolerated; one trial found no change in heart rate or blood pressure [9]. The bigger risks are overdoing sodium as a daily habit and drinking too much fluid. US guidance sets an adequate intake of 1,500 mg of sodium a day for adults and advises reducing intake above 2,300 mg [10]; a litre of a 100 mmol/L drink alone contains about 2,300 mg.

Drinking beyond thirst during long exercise causes exercise-associated hyponatremia, and electrolyte drinks don't prevent it: in Boston Marathon runners, choosing sports drinks over water made no difference [11].

Most interactions come from the potassium and sodium. Potassium can accumulate when the kidneys or medicines limit its excretion [12], and high-sodium products can work against sodium restriction prescribed for blood pressure or heart failure.