Isotonic, hypotonic or hypertonic drink: which one, and when?
by AUA Hydrate
Isotonic, hypotonic, hypertonic: these words appear on bottles in the sports aisle as a badge of seriousness. Yet they describe just one thing, how concentrated a drink is compared with your blood, and that is what decides how quickly the water you drink reaches your body. Here is how to tell them apart, when an isotonic drink really helps, and when it only gives you sugar.
⏱ The essentials in 20 seconds
- Isotonic = as concentrated as blood (about 290 mOsm/kg). Hypotonic: less. Hypertonic: more.
- An isotonic drink only helps beyond one hour of exercise: its 6 to 8% sugar is fuel.
- Below that, and in everyday life, a hypotonic drink (water, sugar-free electrolytes) hydrates just as well, without 120 to 160 kcal per half-litre.
- Soft drinks, juices and energy drinks are hypertonic: during exercise, they slow hydration down.
Isotonic, hypotonic, hypertonic: a question of concentration
It all comes down to osmolality: the number of dissolved particles (sugars, mineral salts) in a kilo of water. For blood, it sits between 285 and 295 mOsm/kg. A drink is called:
- 🟡 hypotonic when it is less concentrated than blood: water, sugar-free electrolyte drinks, heavily diluted juice;
- 🟠 isotonic when it is roughly as concentrated (about 270 to 330 mOsm/kg): most sports drinks, with 6 to 8 g of sugar per 100 ml;
- 🔴 hypertonic when it is more concentrated: soft drinks, fruit juices, energy drinks.
Osmolality of some drinks (mOsm/kg)
Why concentration changes everything
A drink does nothing for your muscles while it sits in your stomach: it first has to leave it, then be absorbed by the gut. Two mechanisms come into play.
The more concentrated a drink, the longer it stays in the stomach. Up to about 8% sugar, gastric emptying is not slowed; beyond that, it is. A hypertonic drink even does the opposite of what you expect: once in the gut, it first draws water out of the body to dilute itself, before giving it back. Hence the bloating when you drink a soft drink while running.
Sugar and sodium travel together. In the gut, glucose and sodium use the same transporter, which pulls water along with them. That is why the European Food Safety Authority (EFSA) recognises that a drink combining carbohydrates and electrolytes enhances water absorption during physical exercise, under precise conditions: 80 to 350 kcal per litre, 460 to 1,150 mg of sodium per litre and an osmolality of 200 to 330 mOsm/kg, in other words from slightly hypotonic to isotonic.
When an isotonic drink is really useful
The sugar in an isotonic drink does not hydrate: it is fuel. Muscle carbohydrate stores run down during long exercise, and the American College of Sports Medicine recommends taking in 30 to 60 g per hour beyond one hour of exercise, up to 90 g for events lasting more than two and a half hours, combining glucose and fructose.
- 🚴 a bike ride, demanding hike or run of more than an hour to an hour and a half;
- 🏃 competition: half marathon, trail race, all-day tournament;
- ☀️ long exercise in strong heat, when you sweat a litre or more per hour (see heatwaves).
In these cases, 500 ml of a 6% drink provides 30 g of carbohydrate, the lower end of the hourly range. For longer efforts, add a banana, dried fruit or a gel taken with water.
When it is pointless
For a 45-minute gym session, an easy jog, a day at the office or a walk, your reserves are more than enough. The isotonic drink then only brings calories: 30 to 40 g of sugar per half-litre, or 120 to 160 kcal, about the same as a bread roll. What you actually lose is water and sodium, and a hypotonic drink replaces both just as well. The details, moment by moment: what to drink before, during and after exercise.
Another surprise: “isotonic” does not mean the same thing from one brand to the next. Analysing the sports drinks sold in Switzerland, Mettler and Weibel (2018) measured osmolalities from about 100 to over 300 mOsm/kg. The most telling figure remains the sugar content on the back of the bottle: 6 to 8 g per 100 ml for long exercise.
| Type | Sugar per 100 ml | Examples | The right moment |
|---|---|---|---|
| Hypotonic | 0 to 4 g | water, sugar-free electrolytes | every day, exercise under an hour, heat |
| Isotonic | 6 to 8 g | sports drinks | exercise over an hour, competition |
| Hypertonic | 10 g or more | soft drinks, juices, energy drinks | after exercise to refill stores, never instead of water during it |
So where does AUA fit in?
Let's be clear: an AUA stick in 500 ml of water is not an isotonic drink. Sugar-free and at 2 kcal, it is clearly hypotonic, and with 184 mg of sodium per half-litre it stays below the 460 mg per litre that EFSA sets for sports drinks. That is a choice: the stick is designed for all the times you sweat without needing sugar, at the gym, on a half-day hike, in a heatwave, in an overheated office, with sodium, potassium, magnesium and trace elements, and no calories.
For a long outing, you need extra fuel. Two options: keep your bottle as it is and eat (banana, bar, gel), or transform it.
💡 The AUA tip: for an outing of more than an hour and a half, pour 500 ml of water, one stick, 30 g of white sugar and 0.5 g of salt (1/12 of a teaspoon) into your bottle. You get 6% sugar, about 380 mg of sodium and a concentration close to that of blood. Choose white sugar over honey or glucose syrup: weight for weight, their glucose and fructose count almost double towards concentration.
Prefer to start from scratch? Our homemade electrolyte recipes are measured to the gram. And if you are unsure about your hydration, here are the signs of dehydration that never lie.
For all the times sugar is too much
Choose from our three flavours: strawberry, peach, lemon.
Frequently asked questions
Do isotonic drinks make you gain weight?
Not in themselves, but half a litre provides 120 to 160 kcal. Drunk without exercising, it is a slightly less sugary soft drink.
Do you need an isotonic drink to run 10 km?
Under an hour, no: water or a hypotonic drink is enough. Beyond that, or in strong heat, it becomes useful.
What is the difference with an energy drink?
An energy drink contains caffeine and about 10 to 11 g of sugar per 100 ml: it is hypertonic and not designed to hydrate you during exercise. On caffeine, see coffee, tea, alcohol.
Does hypotonic mean less effective?
No. A slightly hypotonic drink is absorbed at least as quickly as an isotonic one. What it lacks is sugar, which only helps during long exercise (see why water alone is not enough).
