Drinks which contain a small amount of carbohydrate and sodium replace the fluid lost during exercise more quickly than plain water alone.

Most sports drinks are formulated to contain some carbohydrate (6-8% or, put another way, 60-80g per litre) and some sodium (usually about 20mmol/litre, or about 0.5g per litre). The type of carbohyrdate varies from drink to drink and some drinks contain a mixture of different sources of carbohydrate. Recent research indicates that drinks which contain up to 60g of glucoses or corn syrup solids (types of carbohydrate) per litre, or up to 80g of sucrose or maltodextrin (also types of carbohydrate) per litle will be rapidly absorbed.

Check your sports drink. It will probably contain at least one of the following types of carbohydrate: glucose, glucose syrup, sucrose, fructose, Maltodrextrin or glucose polymer.

The types of carbohydrate in the drink does not appear to be of great importance, although fructose alone is less effective because it is absorbed more slowly than other types of carbohydrate.

The important thing is that the drink is the right concentration. Fizzy drinks, cola or concentrated fruit juice will all inhibit water absorption because they are too concentrated.

Sports drinks such as Isostar have been formulated to include the correct proportions of solids to liquids (usually 60-80g carbohydrate per litre). These drinks are often reported to be isotonic, which means they contain a similar number of particles to your body fluids. The term can, however, be misleading since individual differences will influence whether a drink is truly isotonic.

The use of sports drinks during a workout and over a competition period will help to ensure optimal rehydration. These drinks are more effective than using plain water, juices or squashes.

The carbohydrates in these drinks also provide additional energy. which is an advantage during long workouts (over one hour).

The sports drinks which are currently popular among swimmers include Isostar, Gatorade, Hydra-fuel and Pro-Long. All these products contain some carbohyrdrates and sodium and are suitable fluid replacement beverages. The drink you choose will depend upon many factors, including taste, cost and availability.

If you cannot afford to buy these commercial preparations, you can make up your own drink. A home-made drink is unlikely to be as effective as the commercial alternatives, such as Isostar which have been specially formulated. However, these may still be more effective than drinking plain water.

• Mix 40-60g table sugar in a litre of water and add a pinch (0.5g) of salt.

• One part orange juice to two parts water (ie: one-third of a bottle filled with juice, and two-thirds water) and add a pinch of salt.

The main problem with this formulation is that the juice will contain fructose and potassium, which may alter the effectiveness of the drink. However, this drink may taste better than the first suggestion!

Dilute a small quantity of squash / cordial to contain approximately 60-70g carbohydrate/litre (you will need to check the label because squash drinks vary in their carbohydrate content).