Do sports drinks hydrate better than water?

• Default serving500 mlSoft drink
• Counts toward goal525 mlhydration factor 1.05
• Calories125 kcal25 kcal per 100 ml

"Isotonic" is a statement about your stomach

The word describes concentration, not benefit. An isotonic drink is mixed so its osmolality is close to that of blood, near 290 milliosmoles per kilogram. That lets it leave the stomach and cross the gut wall quickly. Make it more concentrated and it sits there, drawing water into your intestine. That is what a warm flat cola does to a runner at kilometre 30. The formula exists so that a runner can swallow fuel at speed.

The carbohydrate load follows from the same target: about 6 g per 100 ml. That is where the app's 25 calories per 100 ml comes from. A 500 ml bottle is 125 calories and roughly 30 g of carbohydrate. That is genuinely useful in the second hour of hard exercise, and a soft drink on a desk.

Level with water, well behind a rehydration solution

The Beverage Hydration Index trial included a commercial sports drink among its thirteen beverages and measured urine output over four hours. It came out alongside water. The two drinks that clearly held fluid better were milk and oral rehydration solution, and the reason is salt. A rehydration solution carries several times the sodium of a sports drink. It is engineered for illness and severe fluid loss, not for taste over an hour of running.

So if you are recovering from a stomach bug, a sports drink is the wrong tool and a pharmacy rehydration sachet is the right one. If you are drinking one at your desk because it says hydration on the bottle, plain water does the same job for 125 fewer calories.

Why the app still gives it 1.05

500 ml logged becomes 525 ml toward the goal. The 5% reflects two things. The sugar and the modest sodium slow gastric emptying slightly compared to water. And people finish a flavoured drink more readily than a plain one, so the bottle actually gets emptied. It is a small bonus, not a claim of superiority. The place a sports drink genuinely wins is long, sweaty sessions where the carbohydrate and the sodium are both the point. There the app's larger 500 ml default serving matches what you would really drink.

Hydration factors follow the Beverage Hydration Index: Maughan RJ, Watson P, Cordery PA, et al. “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.” American Journal of Clinical Nutrition, 2016;103(3):717-723. Read the paper on PubMed. In WaterChamp an isotonic drink counts for slightly more than plain water.