An isotonic or sports drink has the benefit of possessing electrolytes, which aid fluid absorption, and carbohydrates that provide fuel for the body to maintain performance. Water constitutes 60% of male body weight and 55% of female body weight and is eliminated through perspiration, defecation, and urination - consequently the levels in the body need to be replenished constantly. An average adult needs to consume approximately 2.5 litres of water per day, a figure that increases during exercise.

Historically, water was always the preferred choice for remaining hydrated throughout any endurance exercise, but it causes bloating and stimulates urine output - thereby making it inefficiently retained. An isotonic or sports drink has the benefit of possessing electrolytes, which aid fluid absorption, and carbohydrates that provide the necessary fuel for the body to maintain peak performance and enhance post exercise recovery. In ancient Greek, ‘iso’ referred to equal and ‘tonos’ meant pressure - joining the two together gave the word ‘isotonic’.

The requirement for isotonic drinks.

Sweating is the way in which the body maintains its core temperature of 37C but this will result in the loss of body fluid and electrolytes like chloride, calcium, phosphate, magnesium, sodium and potassium. If the levels of electrolyte are not checked and replenished then, potentially, this can lead to dehydration followed eventually by circulatory collapse and heat stroke if the percentage body weight lost as sweat exceeds 10% or more.

Electrolytes are essential minerals that help control the osmosis of water between the different body compartments, as well as maintaining the acid-base balance required for normal cellular activities. Carbohydrate is stored as glucose in the liver and muscle, it is the most efficient source of energy as it requires less oxygen to be burnt that either protein or fat.

During a period of exercise, the muscles increase the uptake of blood glucose, which the liver produces from its stores, to prevent the levels from falling. The carbohydrate stores of the body are depleted gradually and if they are not restocked will result in a reduced performance.

Hence, consuming carbohydrate before and after exercise will prevent blood glucose levels from dropping too low and maintain the body’s glycogen stores. As many athletes cannot consume food before or during exercise, many opt for a formulated isotonic drink to provide the necessary carbohydrate required.

The absorption of fluid.

Factors that influence the speed at which fluid from a drink can get into the body include the speed at which it is emptied from the stomach and also the rate at which the drink is absorbed through the walls of the small intestine. The higher the level of carbohydrate in a drink, the slower the rate at which the stomach empties. Isotonic drinks with a carbohydrate level of 8% will be disposed from the stomach at a rate similar to that of water.

Electrolytes like sodium that are also present in the drink will reduce the urine output, enabling the fluid to be quickly emptied from the stomach - promoting the absorption from the intestine and encouraging fluid retention. Ingesting drinks with no electrolyte content over short periods will result in a rise in plasma volume and a fall in the sodium concentration and osmolality (number of particles in solution) of the blood, making it more dilute.

The dilution can lead to an increase in urine production, promoting a loss of bodily fluids which counteract the effects of re-hydration. The fall in blood osmolality and sodium concentration can reduce the drive to drink - which works against complete re-hydration.

Isotonic drinks provide the same amount of sodium as blood plasma. Drinking plain water can be a poor choice when a high fluid intake is necessary, as it contains none of the carbohydrate or electrolytes that promote a more rapid recovery after a session of strenuous exercise.

During extreme endurance activities such as marathons, it is important that competitors drink beverages containing sodium electrolytes to avoid the potentially life threatening condition known as hyponatremia (a low blood sodium level).

Conclusion.

Isotonic drinks quickly replace fluids lost by sweating, and supply a boost of carbohydrate, so are usually the preferred choice for athletes, distance runners and those participating in team sports events.

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