Rehydration during bouts of activity is not just important, it is essential. However, there are caveats to this – water alone may not replenish the necessary minerals required for survival. For example, it has been noted that mountaineers who drink melted snow are more likely to suffer from dehydration irrespective of the amount of water consumed, as snow does not contain the necessary ions and electrolytes to feed the body.

Dehydration is defined as the point when you lose 2% of your body weight. The American College of Sports Medicine (ACSM) makes the following recommendations regarding water intake when performing exercise:

1) By measuring body weight over a series of mornings to establish a baseline for weight, it is easy during a prolonged training session to monitor weight loss and take in water and nutrients accordingly as replacements.

2) Take in fluids several hours before training to allow absorption into the body. The ACSM also recommends beverages with sodium.

3) The body weight comparison is important because overhydration (also known as water intoxication) can be just as deadly.

4) Fluid intake after sport is meant to replace fluid and electrolyte losses. Whilst 1 litre of water is the accepted norm, the ACSM recommends 1.5 litres per kilo lost together with low sodium snacks when performing high intensity exercise.

So, what are the problems with overhydration?

The kidneys are unable to process excessive amounts, which leads to water retention in the body. This causes an imbalance between water and sodium levels in the blood which can lead to liver and kidney problems and also congestive heart failure.

The solution is simple; wherever possible, monitor your weight shifts during training and respond accordingly. If you are severely overhydrated, a reduction in fluid intake and an increase in diuretics to increase urine flow should do the trick. While it’s important to be aware of overhydration, dehydration is far more common for athletes.

Is water the only fluid you should consume?

If you are exercising for less than hour then water is fine, but for longer periods of exercise, consuming carbohydrates is essential. Most sports drinks are generally fine, but you can make up your own drink by mixing 5 tablespoons of  table salt with each litre you drink. You may wish to add some squash, since salty water doesn’t taste great by itself! It is recommended that you take 4 or 5 sips every ten minutes or so if possible. If not, then 7 or 8 sips every fifteen minutes will encourage a rapid movement of fluid in the small intestine.

Fluid intake is essential, but it can be taken in many forms. In addition to water, the NHS recommends you consume semi-skimmed milk, diluted squash and diluted fruit juice for hydration purposes. A sweet drink can replace lost sugars and a salt drink can replace essential minerals.

You don’t have to stick to liquids however – many food stuffs have a high percentage of water: tomatoes, broccoli and lettuce with over 90% water, oatmeal with 84% water and low fat milk at 90% water. These represent a range of foods that are excellent for keeping you hydrated.

The real test, as mentioned, is the colour of your urine. A light, straw-like colour is optimal. The darker your urine, the more likely it is that you are dehydrated.

[photo by greenfinger]