The Role of Fluids
The main function of sweat is to cool the body down. The average 70kg/154lb person running at a 6 minute mile pace will burn energy at the rate of about 1000kcal per hour. In a warm climate, this would equate to around 1.5 litres of sweat to remove this extra heat.
When you look at it from this point of view it’s easy to see how an athlete could lose as much as 3 to 4 litres of water per hour!
It only takes a 2 – 3% dehydration or 3lbs of sweat loss for a 68kg/150lb person for body function and exercise performance to be impaired.So, if you want to perform your best and avoid dehydration fatigue and the possibility of cramping from dehydration, you should have a strategy aimed at taking on the correct amount of fluid.
Because you’re burning energy, you also need to take in carbohydrate – however the processing of carbohydrate also requires water . . . about 3ml of water for every gram of carbohydrate you want to fix into the muscles as glycogen. So, in order to replenish your muscle glycogen reserve, you need more water and if you don’t replace it then it will be taken from the blood stream and accelerate dehydration.
A litre of sweat typically contains about 20mg Calcium, 50mg Magnesium, just over 1 gram of Sodium and 1.5 grams of Chloride . . . a good reason why hydrating is better with a sports drink containing these minerals than just with water.
In fact, the use of a sports drink with electrolyte minerals and carbohydrate has three key functions:
- Whereas an excess amount of water can dilute the blood plasma, possibly leading to potentially life-threatening conditions such as hyponatraemia (low sodium levels leading to swelling on the brain and confusion, seizure and even death) – by contrast, sports drinks will help maintain healthy plasma levels.
- Drinks containing electrolyte minerals promote thirst and lead to an increase in voluntary intake of fluid.
- There is evidence that when electrolyte minerals such as sodium and small amounts of glucose are present, the rate of fluid absorption from the small intestine into the rest of the body is improved. The same is true for hydration after the event.
Hydrating During The Race
There are 2 factors that determine how well you are able to get fluid to where it’s needed during a race – Gastric Emptying (how quickly the fluid leaves the stomach) and Intestinal Absorption (how well the small intestine is able to absorb fluid).
Studies have shown that a larger volume of fluid in the stomach as well as the presence of carbohydrate and electrolytes improves the uptake of fluid.
In regard to carbohydrate concentration, the optimal amount is around 2.5g per litre of water (about 4-5% glucose). Higher concentrations tend to slow the process down, however for endurance athletes the benefits of higher energy replacement may be more preferable.
Interestingly, fructose (sugar from fruits) tends to absorb more slowly and may slow down the uptake of water.