Published by Fit Pro Magazine - June 2003

Confused by all the different sports drinks currently available? Not sure what is meant by the terms ‘isotonic’, ‘hypotonic’ and ‘hypertonic’? Want to know how to check whether you are dehydrated? The brief guide from Team Nutrition -Active8 answers your questions.

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(1).

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 (2).

Ingestion of fluid during prolonged exercise can help maintain cardiovascular function, attenuate the rise in body temperature that occurs, and improve exercise performance (3). 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.

The science is well known.  In ‘Sports drinks: Basic Science and Practical Aspects’, Ron Maughan and Robert Murray (4) identify 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 (5).

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 (6).

Hypotonic, Isotonic and Hypertonic

Hypotonic- There is good evidence to suggest that sports drinks that are termed hypotonic in osmolality (when compared to the blood) and have a sufficient energy density are optimal for hydration purposes.  A figure of about 210 mOsm/kg H2O is believed ideal for rehydration purposes (7).

Isotonic drinks normally have an osmolality measure equal to the osmolality of the blood (i.e. about 280 mOsm/kg H2O) and therefore have a slower hydration and glycogen synthesis ability due to the time required for fluid to be secreted into the small intestine to dilute the intestinal contents and create an osmotic gradient that is conducive to fluid absorption (8).

Drinks with a high osmolality (>400 mOsm/kg H2O) are often referred to as hypertonic with slow rates of fluid ingestion in the small intestine (9).

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 (e.g. Hyprol ™, from Quest International), can significantly increased the up-take of glucose from the bloodstream (10, 11, 12).

References

1. Coyle, E.F. and Montain, 1993. In perspect. Exerc. Sci. Sports Med. Vol 6.
2. LJC Van Loon, WHM Saris, M Kruijshoop and AJM Wagenmakers, “Maximizing post exercise muscle glycogen synthesis”
3. Coyle and Montain, 1992. Med. Sci. Sports Exerc., 24, 671,
4. Ron Maughan and Robert Murray, “Sports drinks: Basic Science and Practical Aspects” (2000)
5. (Shi, Summers, Schedl, Flanagan, Chang, and Gisolfi, 1995).
6. Peter Cunningham, (2000) Chichester University
7. Maughan and Shirreffs, 1997. J.Sports science., 15, 297-303,
8. Peter Cunningham, (2000) Chichester University
9. Peter Cunningham, (2000) Chichester University
10. LJC Van Loon, WHM Saris, M Kruijshoop and AJM Wagenmakers, “Maximizing post exercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures”, Am. J. Clin. Nutr., 72 (1) 2000,
11.LJC Van Loon, WHM Saris, H Verhagen and AJM Wagenmakers, “Plasma insulin responses after ingestion of different amino acid or protein mixtures with carbohydrate”’ Am. J. Clin. Nutr., 72 (1) 2000.
12. A Siemensma and Dr M Hakkaart, Sports Drink Concept for Improved Muscle Energy, Quest International
13. Peter Cunningham, (2000) Chichester University
14. Armstrong,1985med. Sci. Sports Exercise. 17, 456-261.