Depletion of the body's carbohydrate stores and dehydration are two factors that will limit prolonged exercise.
Sweating is the way in which the body maintains its core temperature at 37 degrees centigrade. This results in the loss of body fluid and electrolytes (minerals such as chloride, calcium, magnesium, sodium and potassium) and if unchecked will lead to dehydration and eventually circulatory collapse and heat stroke. The effect of fluid loss on the body is as follows (Rehrer 1994):
Electrolytes serve three general functions in the body:
The electrolyte composition of sweat is variable but comprises of the following components:
A litre of sweat typically contains 0.02g Calcium, 0.05g Magnesium, 1.15g Sodium, 0.23g Potassium and 1.48g Chloride. This composition will vary from person to person (Hamilton 2005).
Carbohydrate is stored as glucose in the liver and muscles and is the most efficient source of energy as it requires less oxygen to be burnt than either Protein or fat. The normal body stores of carbohydrate in a typical athlete are:
During hard exercise, carbohydrate can be depleted at a rate of 3-4 grams per minute. If this is sustained for 2 hours or more, a very large fraction of the total body carbohydrate stores will be exhausted and if not checked will result in reduced performance.
Recovery of the muscle and liver glycogen stores after exercise will normally require 24-48 hours for complete recovery.
During exercise, there is in an increased uptake of blood glucose by the muscles and to prevent blood glucose levels falling the liver produces glucose from the liver stores and lactate.
Consuming carbohydrate before, during and after exercise will help prevent blood glucose levels falling too low and help maintain the body's glycogen stores. Many athletes cannot consume food before or during exercise and therefore a formulated drink that will provide carbohydrate is required.
Two main factors affect the speed at which fluid from a drink gets into the body:
The higher the carbohydrate levels in a drink the slower the rate of stomach emptying. Isotonic drinks with a carbohydrate level of between 6 and 8% are emptied from the stomach at a rate similar to water. Electrolytes, especially sodium and potassium, in a drink will reduce urine output, enable the fluid to empty quickly from the stomach, promote absorption from the intestine and encourage fluid retention (Unknown 1993).
What's wrong with water?
Calculating personal fluid needs
During an endurance event, you should drink just enough to be sure you lose no more than 2% of pre-race weight. This can be achieved in the following way:
There are three types of sports drink all of which contain various levels of fluid, electrolytes and carbohydrate.
The osmolality of a fluid is a measure of the number of particles in a solution. In a drink, these particles will comprise carbohydrate, electrolytes, sweeteners and preservatives. In blood plasma, the particles will comprise of sodium, proteins and glucose. Blood has an osmolality of 280 to 330mOsm/kg. Drinks with an osmolality of 270 to 330mOsm/kg are said to be in balance with the body's fluid and are called Isotonic. Hypotonic fluids have fewer particles than blood and Hypertonic have more particles than blood.
Consuming fluids with a low osmolality, e.g. water, results in a fall in the blood plasma osmolality and reduces the drive to drink well before sufficient fluid has been consumed to replace losses.
Which is most suitable?
Isotonic - quickly replaces fluids lost by sweating and supplies a boost of carbohydrate. This drink is the choice for most athletes - middle and long distance running or team sports. Glucose is the body's preferred source of energy, therefore, it may be appropriate to consume Isotonic drinks where the carbohydrate source is glucose in a concentration of 6% to 8% - e.g. High Five, SiS Go, Boots Isotonic, Lucozade Sport.
Hypotonic - quickly replaces fluids lost by sweating. Suitable for athletes who need fluid without the boost of carbohydrate e.g. jockeys and gymnasts.
Hypertonic - used to supplement daily carbohydrate intake normally after exercise to top up muscle glycogen stores. In ultra-distance events, high levels of energy are required, and Hypertonic drinks can be taken during exercise to meet the energy demands. If used during exercise Hypertonic drinks need to be used in conjunction with Isotonic drinks to replace fluids.
Want to make your own?
Isotonic - 200ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all the ingredients together and keep chilled
Hypotonic - 100ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all the ingredients together and keep chilled.
Hypertonic - 400ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all the ingredients together and keep chilled.
Sports drinks commonly contain citric acid. All acids have an erosive potential, but the method of drinking will influence whether or not those acids affect the teeth. Sports drinks should be consumed as quickly as possible, preferably with a straw and not be held or swished around the mouth. Retaining drinks in the mouth will only increase the risk of erosion. Refrigerated drinks will have a reduced erosive potential, as the acid dissolution constant is temperature dependent (Milosevic et al. 1997).
Food for thought
In a trial conducted by scientists in the city of Aberdeen, it was determined that a 2% carbohydrate-electrolyte drink provided a more effective combat to exercise fatigue in a hot climate when compared to a 15% carbohydrate-electrolyte mixture (Galloway and Maughan 2000).
Seven Rules of Hydration (Troop 1994)
Intracellular fluid and interstitial fluid have the same osmotic pressures under normal circumstances. The principal cation inside the cell is Potassium and the principal cation outside the cell is Sodium. A fluid imbalance between these two compartments is caused by a change in the Potassium or Sodium concentration. Sodium balance in the body is controlled by aldosterone and antidiuretic hormone (ADH). ADH regulates extracellular fluid electrolyte concentration by adjusting the amount of water reabsorbed into the blood by the kidneys. Aldosterone regulates extracellular fluid volume by adjusting the amount of sodium reabsorbed by the blood from the kidneys.
Certain conditions may result in a decrease in the sodium concentration in interstitial fluid. For instance, during sweating, the skin excretes sodium as well as water. If we replace the lost fluid with plain water, then we may produce a sodium deficit. The decrease in sodium concentration in the interstitial fluid lowers the interstitial fluid osmotic pressure and establishes an effective water concentration between the interstitial fluid and the intracellular fluid. Water moves from the interstitial fluid into the cells, producing two results that can be quite serious:
Alcohol is a high-octane fuel, but it cannot be metabolised to provide energy except in the liver and then only at a very slow rate. Energy provided by alcohol tends to be converted to fat and excessive consumption may cause liver damage. As a diuretic, it will cause dehydration and evidence suggest that vitamin B and C may be depleted. Excessive alcohol will diminish aerobic capacity and impair motor function.
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