Making sense of sports drinks

In a recent post about nutrition for endurance sports, I referred to isotonic sports drinks as a way to stay fuelled and hydrated during prolonged activity. You may have even seen these drinks listed in the grocery store aisles. But how many of us know what isotonic sports drinks are? 

There are two main categories of sports drinks available on the market: fluid replacement drinks and carbohydrate (energy) replacement drinks. Fluid replacement drinks are dilute solutions of electrolytes and sugars, most commonly glucose, sucrose, fructose and glucose polymers (maltodextrins). The purpose of these drinks is to replace fluid faster than plain water while helping to maintain blood sugar levels and spare glycogen. Within this category are isotonic and hypotonic drinks.

Carbohydrate drinks provide more carbohydrate per 100 mL than fluid replacement drinks. Carbohydrate is mainly in the form of glucose polymers (maltodextrins). The purpose is to provide larger amounts of carbohydrate at the same or lower osmolality than the same concentration of glucose. Ready-to-drink brands are generally isotonic. Powders could be either hypotonic or isotonic.

Osmolality refers to the concentration of a solution.

What is the difference between hypotonic, isotonic and hypotonic drinks?

Hypotonic – low osmolality (contains fewer particles) than the body’s own fluids. Because it is more dilute, it is absorbed faster than plain water. Generally, hypotonic drinks contain less than 4 g carbohydrate/100 mL (4%).

Isotonic – a typical “sports drink” – has the same osmolality as the body’s fluids and is therefore absorbed as fast as or faster than plain water. Most commercial brands contain between 4 and 8 g carbohydrate/100 mL (4-8%). In theory isotonic drinks provide the ideal compromise between rehydration and refuelling.

Hypertonic – Carbonated drinks are more concentrated which means they have a higher osmolality than body fluids. These drinks are absorbed more slowly than plain water. A hypertonic drink usually contains more than 8 grams carbohydrate/100 mL (8%).

The more concentrated drinks (hypertonic) tend to slow down the stomach emptying and therefore reduce the speed of fluid replacement. However, if glycogen replenishment is the goal, this drink formulation may be your best option.

Which should you choose for endurance sport?

A sports drink would benefit your performance during any moderate or high intensity event lasting longer than one hour. Numerous studies have shown that isotonic sports drinks promote both hydration and normal blood sugar levels, and enhance performance during intense and/or prolonged exercise. If you are exercising longer than 2 hours or sweating heavily, you should consider a sports drink that also contains sodium.

It seems, based on various studies of both cyclists (in time trials) and endurance runners, that a concentration of 5.5 – 6% is the ideal range for peak performance in high intensity, long-duration activities. 

By way of comparison, some of the more popular carbonated "sports drinks" on the market such as Rockstar contain upwards of 13% carbohydrate concentration. Clearly hypertonic. One scoop of Heed powder mixed with 500 mL of water contains 5.4% carbohydrate, within the ideal range. (If your water bottle is 750 ml, you will have to add a scoop and a half to achieve the ideal concentration.) Be aware that many of these drinks also contain food colouring agents which can be detrimental to your health.