Saturday 25 July 2015

Carbohydrate Sports Drinks and Gastric Emptying Rate

The stomach is one of the control points for digestion and absorption. The pyloric sphincter is a ring of muscle at the bottom of the stomach, which acts as a gate to the duodenum and the rest of the small intestine. Contraction of the pyloric sphincter inhibits chyme from exiting the stomach, thus delaying digestion, and particularly, absorption. Gastric emptying rate varies amongst individuals, and women tend to show slower gastric emptying compared to men. Gastric emptying rate follows an exponential time course and is related most closely to the degree of distention in the wall of the stomach muscle and the amount of undigested protein in the stomach. However, other factors do affect the rate of gastric emptying. Consideration of the rate of gastric emptying is important for sports drink consumption as rehydration during and after intense exercise should be as efficient as possible. Factors that slow the passage of water and replacement energy through the stomach are therefore worthy of discussion.
The volume of the drink and its concentration are known to influence the gastric emptying rate. As volume increases up to 700 mL the gastric emptying rate increases. Maintenance of a high gastric volume therefore aids rehydration. This should equate to consumption of around 200 mL of liquid every 15 minutes during intense exercise. As around 1 to 2 litres of water can be lost through sweat during intense exercise, this rate of drinking can almost keep pace with the most extreme loss of water from sweat. Addition of glucose or other some sugars to exercise drinks increases the osmolarity and this can reduce gastric emptying rate. However, fructose appears not to present the same inhibitory effect on gastric emptying, which is why consumption of fruit drinks and soft drinks containing fructose may be particularly bad for the health. Maltodextrins are chains of glucose that vary in molecular weight. Addition of maltodextrin to water does not increase osmolarity to the same effect as an equal weight of glucose.
Even taking into account the different osmolarities of different sugars, the literature generally shows that carbohydrate solutions of around 4 to 8 % carbohydrate, regardless of carbohydrate type, do not cause changes in gastric emptying compared to water. However, above this percentage gastric emptying rate is inhibited and this will affect the ability of the drink to keep pace with water loss. Commercial sports drinks tend to have sugar contents above this 8 % ceiling which is why they should either be diluted with water, or avoided. The duodenal brake that is the pyloric sphincter is important physiologically because it is there to provide a safety valve to prevent a large oversupply of energy to the liver, which may cause detrimental metabolic shifts. Interestingly the bypassing of this break by consumption of fructose is a possible cause of the liver overload syndrome, which may be required to cause obesity. Consuming commercial high sugar sports drink outside of exercise is particularly detrimental.

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