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Is there a cheap and effective sports drink I can mix myself? During exercise when should I start drinking and how much should I drink?
This answer is brought to you by many of the Australian nutrition professionals who regularly contribute to a nutrition email discussion group.
The major consideration for any hydration beverage is the provision and retention of water in the body. If you are exercising only moderately vigorously, and in mild weather, the form in which the water is provided is less important than for high activity/hot weather: plain water, flavoured water, commercially-available sports drink or home-made sports drink (made according to the criteria below) are all adequate. However, there is good evidence that a cooled, flavoured sports drink of suitable composition encourages drinking and so may be better than plain water as a means of preventing dehydration, even for low-moderate physical activity in mild conditions.
For more vigorous and/or sustained exercise, particularly in the heat, the evidence is even stronger that the addition of carbohydrate and sodium (eg as salt) to water will assist significantly in preventing dehydration. A satisfactory drink can be made cheaply if it conforms to the following criteria:
The concentration of the carbohydrate (glucose, sugar or other similar carbohydrate, such as glucose polymers) should be in the range 4-8% (that is, between 40g and 80g carbohydrate per litre (L) of water). Greater than about 8% carbohydrate in your sports drink will slow down the rate at which the fluid empties from the stomach into the intestine and will therefore interfere with rehydration.
Sodium content should be 20-30mmol per litre (that is ~1-2g of table salt per litre).
Any flavouring added (eg fruit flavoured cordial concentrate) does not take the carbohydrate level above ~8%.
With the availability of a multitude of pre-mixed, flavoured sports drinks, or powders to which you simply add water, it is only the very dedicated (or money-conscious) sports person who is likely to bother with mixing their own sports drink. Always check the label of any drink you buy to make sure that it does not provide more than 8% carbohydrate (8g per 100mL) and that it contains no more than ~30mmol of sodium per L).
Having bought or mixed your drink, when should you start drinking and how much should you drink?
The sense of thirst is slow to react to dehydration. To avoid 'voluntary dehydration' (ie, dehydrating without knowing about it) you should drink before you become thirsty. It is also a good idea to drink before the activity - eg, about 300-500mL of your sports drink (or as much as you can comfortably drink up to this amount) about 15 minutes before the activity begins.
While exercising, you should drink regularly to replace the fluid lost as sweat. Depending on how fast you are sweating, you may need to drink about 1L per hour or even more. This is best consumed in the form of frequent, small drinks (eg ~200mL every 10-15 minutes). In cases of extremely high activity at very high temperatures, sweat rates can be huge - Alberto Salazar was reported to be sweating at 3.7L/hour during the 1984 LA Olympic marathon. The stomach cannot cope with fluid coming in at that rate (and will even struggle to empty 2L/hour). So if you are sweating extremely profusely, the best you can do is drink as much as you can tolerate.
Despite drinking during the activity it is possible (even probable in very hot weather) that you will have undergone some dehydration. Body water will be effectively replenished after the activity by drinking the same formulation as that recommended for consumption during the activity. If there is carbohydrate present in the drink during this rehydration phase, this carbohydrate will assist not only with rehydration but also with restoring the muscles' reserves of 'glycogen' - the form in which glucose is stored in the muscle. Again, about 4-8% carbohydrate and a small quantity of sodium (eg 1-2g of salt per L of drink) are recommended. Glycogen replenishment is most effective if carbohydrate is taken very soon after exercise (ie, within ~20 minutes).
You can estimate how much you need to drink by measuring your weight before and after the activity in exactly the same clothes for each weighing. Most of the weight lost will be body water (lost as sweat). The amount of weight lost is the minimum amount of fluid that needs to be consumed (remembering that 1 L of fluid weighs ~ 1 kg).