Tonicity, Simply
These three words describe how concentrated a drink is, relative to the fluid in your blood. "Iso" means equal - an isotonic drink has roughly the same dissolved-particle concentration as blood. "Hypo" means less; "hyper" means more. The body absorbs fluid fastest when the concentration is around isotonic or slightly hypotonic.
Isotonic
The classic isotonic sports drink - Gatorade, Powerade, Staminade, SiS GO Isotonic - sits at around 6-8% carbohydrate by mass. This is the textbook profile that supports both fluid and fuel during sustained exercise. The carbohydrate also slightly slows gastric emptying compared to plain water, which is sometimes a feature (sustained energy) and sometimes a drawback (heavy feeling).
Hypotonic
A hypotonic drink is more dilute than blood. Lower-carbohydrate electrolyte drinks - Nuun tablets, Hydralyte, sugar-free daily-hydration products - fall here. They tend to be absorbed quickly, which is why oral-rehydration-style products are deliberately hypotonic. They're for fluid and minerals, not fuel.
Hypertonic
Hypertonic drinks have a higher concentration than blood. Many soft drinks and full-strength fruit juices fall here. In a sports context, hypertonic formulas are sometimes used post-session for rapid carbohydrate delivery, but during exercise they're slower to absorb and can sit heavily in the stomach.
Why It Matters
Tonicity is one of the reasons cyclists and runners often prefer different products - cyclists can tolerate higher carbohydrate concentrations because their stomach isn't getting jolted; runners typically prefer leaner drinks or alternate water and gels for the same reason. Sports Drinks for Runners covers this.
Sources
Authoritative references this article draws on. External links open in a new tab.
Frequently Asked
What's the difference between isotonic and hypotonic drinks?
Concentration relative to blood. Isotonic drinks match it; hypotonic drinks are more dilute. Practically, isotonic drinks deliver carbohydrate alongside fluid; hypotonic drinks prioritise fast absorption with less fuel.
Which type is best during exercise?
Isotonic drinks are the textbook in-session choice for sustained exercise needing both fuel and fluid. Hypotonic options suit hot or short sessions where the priority is fluid replacement without carbs. Hypertonic drinks tend to sit heavy mid-effort.
Is plain water isotonic?
No - plain water is hypotonic relative to blood. The body absorbs water from plain water just fine; the practical advantage of an isotonic drink is the combined carb + electrolyte profile, not absorption speed per se.
Why do soft drinks make you thirsty?
Most soft drinks are hypertonic - high sugar concentration relative to blood. The body actually pulls water into the digestive tract briefly to dilute them, which can give a temporary thirst sensation.
