How much should I drink?
The easiest way to calculate your fluid requirements during exercise is to weigh yourself before and after training sessions.
For example, if you weigh 70kg before training and 69.3kg after a 60 min run, you have lost 0.7kg of body weight during the run, which is the equivalent of 0.7L of body fluid. This loss is 1% of your body weight. Although this level of fluid loss is unlikely to affect your performance and health, it is important to replace fluid losses as soon as possible after recovery. The general rule of thumb for recovery is every 1kg of weight lost in exercise should be replaced by 1-1.5L of fluid.
Consider that you are taking part in a longer event (i.e. a marathon). You weigh 70kg before the run and 67.9kg after a 3-hour run, therefore losing 2.1kg, which is equivalent to 3% of body weight. A 3% loss in body fluids is likely to put extra strain on your cardiovascular system and negatively affect performance. In this case it is recommended that you consume fluids during the run to minimise body fluid loss which will reduce cardiovascular strain and help preserve optimal performance.
You should also be cautious not to overhydrate before or during exercise. By drinking too much water, the weight of the fluid that you are carrying around can be detrimental to your running performance. In rare cases, large volumes of low sodium drinks taken during prolonged exercise can cause hyponatremia (sodium concentration in the blood is diluted). This is associated with symptoms such as confusion, headaches, fatigue and, in the most serious cases, coma. Those most at risk of hyponatremia are small athletes who may not sweat a lot and are drinking large volumes of water or salt free drinks. This is another reason to be aware of your personal fluid requirements.
What should I drink?
According to ACSM guidelines, water is the most reasonable choice of drink for exercise lasting less than 60 minutes.
For running events longer than 10 km, it is important to consider the availability of carbohydrate, which is an important fuel for exercising muscle. Therefore, it can be useful to consume fluids that contain carbohydrates before, during and after exercise as a simple and effective way of topping up your fuel stores.
Sweating causes the loss of electrolytes and water from the body. Most runners will benefit from including sodium and potassium-rich foods in their diet. Electrolyte supplementation during exercise can be required for heavy and ‘salty’ sweaters to maintain sodium balance. If you notice that white patches dry into your dark tee-shirt after training, this is a good indication that you are a ‘salty’ sweater, thus you may benefit from consuming drinks that contain electrolytes during longer sessions.