FLUID INTAKE
Fluid intake prior, during and after exercise.
In my previous article I discussed the importance of water intake. In this article I will be discussing fluid intake during exercise and explain the difference between hypotonic, isotonic and hypertonic drinks.
During exercise the body consumes energy and looses fluid and electrolytes through sweating. The energy comes from carbohydrates which is stored in the liver and muscles in the form of glucose. On average about 80g of glucose are stored in the liver at one time and about 350g in the muscle tissue. Carbohydrates deplete at a rate of about 3-4 grams a minute during exercise. This is the reason why prolong exercise cause a decrease in performance.
Electrolytes consist of minerals including calcium, magnesium, potassium, sodium and chloride. Their function is to control osmosis in the different body cells and maintain an acid balance for normal cellular function. Osmosis is the movement of molecules from a high concentration to an area of low concentration.
How can we replace these depleted stores? Firstly, let us look at the different types of energy drinks. They fall in three categories.
Hypotonic drinks: Consist of carbohydrate and electrolytes that is less in concentrations than the body fluid.
Isotonic drinks: Consist of carbohydrate and electrolytes that is similar in concentration to that of the body fluid. An example would be lucozade.
Hypertonic drinks: Consist mainly off carbohydrates which are higher in concentration than that of the body fluid.
Body fluid consists of intracellular and extra cellular fluid. Intra cellular fluid is the fluid within the body’s cells. Extra cellular fluid consists of fluid within the blood plasma and all the interstitial fluid in the rest of the body. It has an osmolality of 280-300 milli osmoles per kilogram. Osmolality refers to the concentration of particles dissolved in a fluid.
Hypotonic drinks have a lower osmolality than body fluid. Drinking this will lower the body fluid osmolality and decrease water absorption.
Hypertonic drinks have a higher osmolality than body fluid. Drinking this will increase body fluid osmolality and increase water absorption.
Deciding on the type of drink depends on whether you are re-hydrating or replenishing energy stores. Prior to an event re-hydration would be the primary aim. Hypotonic drinks would be first choice as it gets absorbed very quickly. Remember, liquids move from an area of low concentration to an area of a high concentration.
During exercise it is recommended to take isotonic drinks as it provides both carbohydrates for energy and fluid for re-hydration. Isotonic drinks will be absorbed slower as osmotic pressure is the same on either side.
Hypertonic drinks would be considered as an energy source during ultra events such as ultra distance running where the energy demand is very high and the duration of the event is long ( more than two hours). Here the osmotic pressure of the drink is much higher than the actual body fluid and absorption takes much longer.
Just as important as fluid intake is after exercise so are electrolyte and carbohydrate levels. If water or fluid low in sodium is taken in after exercise, plasma volume rises and the sodium concentrations drop. This causes an increase in urine production and therefore counteracting re-hydration.
One of the most important factors when training athletes is to monitor their hydration levels. Knowing exactly how much fluid their body uses in different temperatures at different training intensities and times. This will allow the athlete to maximise his or her performance during the actual event and increase their recovery rate after the event.
If you have decided that your new years resolutions is to get fit, please make sure that you keep yourself hydrated at all times and ensure that you have enough carbohydrates and electrolytes prior and after your exercise.Gerhard