15 April 2012

Sports Drinks

A sports drink beverage is designed to help athletes rehydrate when fluids are depleted after training or competition. Electrolyte replacement promotes proper rehydration, which is important in delaying the onset of fatigue during exercise. As the primary fuel utilized by exercising muscle, carbohydrates are important in maintaining exercise and sport performance. 

Types of sport drinks :

Sports drinks can be split into three major types:
  • Isotonic sports drinks contain similar concentrations of salt and sugar as in the human body.
  • Hypertonic sports drinks contain a higher concentration of salt and sugar than the human body.
  • Hypotonic sports drinks contain a lower concentration of salt and sugar than the human body.
Most sports drinks are moderately isotonic, having between 4 and 5 heaped teaspoons of sugar per five ounce (13 and 19 grams per 250ml) serving. They never have a pH comparable to carbonated soft drinks.

The primary purposes of fluids consumed before, during and after exercise are to:

a) Replace sweat loss i.e. add electrolytes
b) Supply a source of carbohydrate that can supplement the limited stores of glycogen  in the muscles and liver.
c) Reduce the problems associated with dehydration i.e. increased heart rate, increases in body heat and loss of fluids.

Reducing risk of water intoxication :

Water is the best beverage for simply rehydrating. However during exercise or just being outside on a hot day electrolytes are also lost. Consumption of excessive amounts of water can cause water intoxication, a potentially fatal imbalance of electrolytes in the body. Water intoxication is extremely rare. It might occur, for example, during intense exercise when heavy sweating removes water and electrolytes from the body, but only large quantities of water are consumed to replace what has been lost. The resulting low concentration of electrolytes adversely affects central nervous system function.
Many sports drinks reduce the risk of water intoxication by replenishing fluids and electrolytes in a ratio similar to that normally found in the human body. However, some sports drinks have low concentrations of electrolytes, so overconsumption of them could still lead to water intoxication. People whose work or exercise puts them at high risk of developing heat illness or water intoxication should seek professional advice about proper rehydration of the body.

Dehydration and Performance :

The decline in physical performance during training and competition accelerates as the glycogen content of the exercising muscles reduces and therefore it becomes increasingly difficult to exercise at high intensities. Low intensity exercise is possible, with the muscles relying mainly on fat as a fuel. Carbohydrate and fatty acids are the main fuels for the skeletal muscle to provide energy during prolonged exercise.
In contrast to fat stores, the glycogen reserve in the body is limited (400 500g). In practice, the muscle glycogen reserves are sufficient for about 45 minutes of high intensity aerobic exercise. When the muscle glycogen stores become depleted, the muscles become more sensitive to cramp.
Ingestion of fluid during prolonged exercise can help maintain cardiovascular function, attenuate the rise in body temperature that occurs, and improve exercise performance. The optimum formulation of such a drink to achieve this will depend on more than just the amount of carbohydrate in the drink. The key to successful hydration and restoration of glycogen levels lies with your body and how quickly the sports drink fluid can be absorbed.
In Sports drinks the two major limiting factors that control the rate of fluid absorption into the blood stream as 1) the combined rates of gastric emptying and 2) intestinal absorption.

Gastric emptying :

The volume of ingested fluid, the energy density of the fluid measured by the osmolality (mOsm/kg H2O) and the balance of the mineral content in the fluid determine the rate of gastric emptying. Ever wondered why you can feel really thirsty and drink water, but feel really bloated? That’s because the body is seeking the replenishment of minerals (electrolytes) as well as fluid. The term osmolality defines the relative concentration of the fluid by identifying the number of dissolved molecules in the fluid.

Intestinal absorption :

The carbohydrate concentration and the osmolality of the beverage are crucially important in influencing the rate at which the fluid is absorbed across the intestinal epithelium.  A drink that is high in energy density, has a high osmolality and is low in sodium chloride (e.g. common carbonated soft drink) will possibly create a decrease in plasma volume as fluid temporarily leaves the blood and passes into the intestine in an attempt to dilute the ingested fluid .
Physiologist Pete Cunningham, believes that the ideal sports drink should have an energy density of 4-6%, with a carbohydrate content from both fructose, glucose and/or maltodextrins, a sodium chloride concentration of approximately 30 mmol/l and most importantly, an osmolality that promotes a sufficient osmotic gradient

Re-hydration and Replenishment :

Once correctly re-hydrated, the key challenge in post exercise is replenishing the body's glycogen reserves as fast as possible. Glycogen is the storage form of glucose and the predominant source of energy for the active muscle. The intake of carbohydrates (such as sugar, glucose, maltodextrin, etc.) directly after endurance exercise increases the rate of muscle glycogen synthesis. However, an overnight rest will normally not be long enough to achieve complete recovery, as full restoration of the glycogen reserves takes longer. The post exercise challenge for sports drinks is to speed up muscle recovery times. Combining hypotonic and Isotonic sports drinks with high-grade proteins such as a series of vegetarian proteins, can significantly increased recovery.

Instead of having Sports drinks, drink at least 64 ounces of plain water each day, so you will feel full, eat less, and get rid of excess salt.

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