Isotonic sports drink: what it is and how to make one
An isotonic drink has about the same concentration as your blood. That is the sweet spot between getting fluid and energy through your stomach quickly and not upsetting it.
Three kinds of drink
The difference is how much carbohydrate sits in the liquid.
- Hypotonic, under 4 percent: leaves the stomach fastest and is absorbed very quickly. Best when it is extremely hot and fluid matters more than energy.
- Isotonic, 4 to 8 percent: the same concentration as blood. The best balance of fast emptying, fluid and energy, and the standard for most sport. Up to about 60 g of carbohydrate per liter.
- Hypertonic, over 8 percent: pulls fluid into the gut and slows the stomach. It can give a sloshing stomach, cramps and diarrhea. Not a good drink to quench your thirst in a hard effort.
How to make one
A percentage is grams per 100 ml. A 6 percent drink has 60 g of carbohydrate per liter, so a 500 ml bottle holds 30 g. Use a powder with glucose or maltodextrin plus fructose and follow the label. If your drink already contains sodium, extra electrolyte tablets are usually unnecessary.
When a stronger drink can work
The 8 percent limit applies to ordinary drinks with a single sugar. A mix of glucose and fructose (2 to 1 or 1 to 0.8) uses more than one route through the gut and can be absorbed faster, but a very concentrated drink still slows the stomach. Hydrogel drinks use alginate or pectin to form a gel in the stomach and can go up to about 14 percent without the same delay.
How much to drink
Most athletes drink between 500 ml and 1 liter per hour, depending on sweat loss and temperature. Never go beyond 1.0 to 1.2 liters per hour: too much plain fluid can dilute sodium in the blood and is dangerous. In hot weather, a strong drink can work against you, so do not rely on one bottle for all your carbohydrate.
Do the sums for high intake
To take in 90 g per hour from a 6 percent drink alone you would need 1.5 liters every hour. That is above the safe ceiling. For high intake you need gels, a stronger mix, or both. See our guide on energy gels.
General information for healthy adults, not medical advice. If you have diabetes, a gut condition, an eating disorder, or take medication, talk to your doctor or a sports dietitian first.
Common questions
How much carbohydrate is in an isotonic drink?
Between 4 and 8 g per 100 ml, up to about 60 g per liter.
Can I drink only sports drink during a long session?
Not if you need a lot of carbohydrate per hour. At a high intake the volume gets too large. Combine drink with gels.
Do I need electrolyte tablets as well?
Often not. If your sports drink or gels already contain sodium, extra tablets are usually unnecessary. They can help in long, hot sessions with lots of sweat.
Want this planned around your own week?
The 2fuel app turns every training session into kcal, carbs per hour and protein, then into meals. With an AI coach or a sports dietitian. In beta now.