Rehydration

Rehydration is the deliberate replacement of fluids and electrolytes lost during exercise, heat exposure or illness to restore the body to its normal hydrated state. It is not simply drinking until thirst passes but a strategy to fully replace water and sodium deficits.

Rehydration principles

After exercise, blood volume and intracellular water are reduced because sweat rates during sport can reach 0.5–2 litres per hour and body mass losses of more than 2 % impair performance and cognitive function. To restore euhydration, athletes should aim to replace fluid losses within a few hours. A practical guide is to drink about 1.25–1.5 litres of fluid for every kilogram of body mass lost, because some of the intake will be lost through urine. Including sodium in post‑exercise drinks helps retain fluid and stimulates thirst, while small amounts of carbohydrate aid muscle glycogen resynthesis. Water alone may not fully restore hydration because it increases urine output; drinks containing electrolytes or salty foods improve fluid retention.

Practical examples

Rehydration strategies often begin by weighing yourself before and after training to gauge fluid deficit. For example, if an athlete loses 1 kg during a long run, they can plan to consume roughly 1.5 litres of fluid over the next few hours, divided into multiple servings. Sports drinks or rehydration solutions with 20–50 mmol of sodium per litre enhance fluid retention, while milk or broth can also be effective. Consuming water with meals and snacks that provide salt helps replace electrolytes. Avoid relying on caffeinated or alcoholic beverages for rehydration because they can increase urine production.

Rehydration restores fluid balance and supports recovery after exercise or heat stress. Using body weight to estimate losses and choosing fluids with adequate sodium and some carbohydrate ensures that you return to euhydration without overdrinking.

Related Terms: Hydration, Prehydration, Euhydration, Beverage Hydration Index, Hypohydration