Summer is coming, and with its heat, problems linked to hydration occur more frequently. 

With temperatures over 30°C, a rider can lose as much as a litre of fluid an hour through sweating and respiration, especially on climbs. 

Since the quantity of liquid absorbed in the stomach and intestines can be inferior to fluid loss, a hard training program or a race can easily bring about a state of hypohydration. 

Be sure to check body weight after training and compare it to pre-training morning weight taken before breakfast. Weight loss above 1% of body weight can indicate hypohydration. 

Remember that full fluid replenishment can take between 4 and 24 hours—a rather long period during which the athlete can continue to lose liquids through urination, especially if only water is taken as a replenishment. 

Addition of sodium (50-100 mM/l or 3-6 g/l) to water can inhibit the diuretic effect and facilitate fluid retention. 

Loss of sodium through persperation amounts to about 1.2 - 5g/l. If one loses 2 liters of sweat (approx. 2 kg of body weight), from 2.5 to 10 grams of sodium will also be lost, depending if you are acclimatized to heat or not; the acclimatization usually requires 1 to 2 weeks and allows you to expel a less sodium-loaded sweat. 

 

Caffeine and alcohol are firmly discouraged, due to their diuretic effects. Remember also that when you are dehydrated by 3%-4% of body weight, gastric functions (and thus fluid absorption and retention) are compromised. Rehydration is therefore essential also in order to allow the athlete to have a proper food intake.

It is therefore necessary to reintegrate about 150% of body weight lost during the 12 hours following activity by taking an appropriate quantity of water with: 

- 3-5 g/l of salt (NaCl) 
- 2% glucose (to facilitate absorbtion) 

If you lost 2kg during the training or racing period, you should take in about 3 litres of extra fluids in the 24 hours following the effort.