If you exercise hard enough you will start to sweat, which is your body’s way of cooling itself, and it is not unusual to lose a litre of fluid during an hour of intense exercise. Your capacity to exercise, or even function, reduces as you lose fluid, and losing just 2% of your bodyweight can impair your function by as much as 15%. To maintain your hydration levels you need to replace the fluid lost through sweating. As well as losing fluid when you exercise, you are using up vital energy stores so you might also want to top-up your carbohydrate levels.

Consuming carbohydrates at the same time as water can slow down the rate at which your drink empties from your stomach and becomes available for absorption through your small intestine - so you need to get the concentration of carbohydrate (sugar) right. If your main aim is to replace lost fluids then an isotonic drink is the most appropriate. An isotonic drink contains 4-8% sugar and is absorbed at about the same rate as plain water, with the advantage of an energy boost.

There are plenty of commercially produced sports drinks available but they are not essential and do not give you any advantage. A good guide for making your own sports drink is fruit juice with water in the ratio of 1:1, or squash with water in the ratio of 1:4, e.g. 200 ml squash and 800 ml water. Be sure to use squash made with sugar rather than sugar-free.

It is generally not necessary to add electrolytes to your sports drink as your body has its own efficient mechanisms for regulating electrolyte balance and will cope with normal sweat losses. The only electrolyte that may be of benefit is sodium to enhance absorption of your drink. You only need a small amount of sodium each day though for your body to function efficiently and you can get adequate intake from dietary sources.  If you want sodium in your sports drink then add 1-1.5 g of salt per litre of liquid, not so much as you taste it so try just a little first.

The amount of fluid you need to drink during and after exercise depends on how much you sweat, and everyone is different. As a rough guide you are likely to need one litre of fluid for every hour of exercise. To find your individual requirement, weigh yourself before and after your exercise session (near naked is best) and the difference is the amount of fluid you have lost. One kilogram of bodyweight lost equals one litre of fluid. For every litre of fluid you have lost (sweated) aim to replace it with one and a half litres of fluid. This ensures your body has sufficient water for all its metabolic functions.

As well as rehydrating during and after exercise, ensure you are properly hydrated before you begin your session. As a guide, drinking half a litre of water two hours before your exercise session and a further half a litre 15 minutes before you start will ensure you are well hydrated.

Thirst is not a good indicator of hydration levels. If you are thirsty you are already dehydrated. An excellent indicator of hydration status is your urine - on a daily basis this should be pale in colour and without a strong smell.