Sports drinks contain sodium, and the reason is frequently misunderstood as replacing what is lost in sweat. The concentrations used are far below sweat levels, because the sodium is doing something else.
Sodium drives water absorption in the intestine
Water crosses the intestinal wall passively, following particles that are actively transported. Sodium and glucose are moved together by a shared transporter, and water follows them.
A drink containing both is therefore absorbed faster than plain water, because it supplies the particles whose movement pulls water across the wall.
This mechanism is the basis for oral rehydration formulations generally, and sports drinks are a variation on the same principle adapted for exercise. The clinical version prioritises absorption over taste and fuel.
Retention matters as much as absorption
Water absorbed without sodium dilutes the blood slightly, which suppresses the hormone that conserves water and prompts the kidneys to excrete the excess.
Fluid taken with sodium keeps blood concentration closer to normal, so less of it is lost to urine and more remains in circulation.
This is why rehydration after a session is more effective when fluid is taken with food or with a drink containing salt than when plain water is used alone. A salted meal achieves much of what a formulated drink does.
Concentration is limited by taste and by osmolality
A drink matching sweat sodium concentration would taste strongly salty and be unpleasant to consume in the volumes required during exercise. Intake falls when a drink is unpalatable, which defeats the purpose.
There is a physiological limit as well. A drink more concentrated than blood draws water into the intestine initially, delaying absorption and increasing the chance of discomfort.
Formulations therefore sit below blood concentration, trading complete replacement for a rate of delivery that the gut tolerates during activity. The compromise favours getting fluid in over matching what was lost.
Carbohydrate content serves two purposes at once
The sugars in a sports drink provide fuel and simultaneously drive the transport mechanism that carries sodium and water across the intestinal wall.
Concentration is a balance, since more carbohydrate delivers more fuel but slows the rate at which the stomach empties, which limits how quickly fluid arrives. Products aimed at hot conditions are usually more dilute for this reason.
The right choice depends on the situation
For short sessions, plain water is adequate because neither fuel nor sodium is limiting. The formulation matters as duration, heat and sweat losses increase. Below about an hour the choice makes little practical difference.
Athletes with high sweat sodium losses may need more than a standard drink provides, and that assessment belongs with a qualified sports dietitian or medical practitioner. Visible salt on skin and clothing is the usual first clue.
