Sports Drinks
Depletion of the body's carbohydrate stores and dehydration are two factors limiting prolonged exercise.
Dehydration
Sweating is how the body maintains its core temperature at 37 degrees centigrade. This results in losing body fluid and electrolytes (minerals such as chloride, calcium, magnesium, Sodium and Potassium). If unchecked, it will lead to dehydration, circulatory collapse, and heat stroke.. The effect of fluid loss on the body is as follows (Rehrer 1994)[1]:
| % body weight lost as sweat | Physiological Effect |
| 2% | Impaired performance |
| 4% | Capacity for muscular work declines |
| 5% | Heat exhaustion |
| 7% | Hallucinations |
| 10% | Circulatory collapse and heat stroke |
Electrolytes
Electrolytes serve three general functions in the body:
- many are essential minerals
- they control the osmosis of water between body compartments
- they help maintain the acid-base balance required for normal cellular activities
The electrolyte composition of sweat is variable but comprises the following components:
- Sodium
- Potassium
- Calcium
- Magnesium
- Chloride
- Bicarbonate
- Phosphate
- Sulphate
A litre of sweat typically contains 0.02g Calcium, a total of 0.05g Magnesium, 1.15g Sodium, a 0.23g potassium, and a 1.48g Chloride. This composition will vary from person to person (Hamilton 2005)[2].
Glucose
Carbohydrate is stored as glucose in the liver and muscles and is the most efficient energy source as it requires less oxygen to be burnt than Protein or fat. The normal body stores of carbohydrates in a typical athlete are:
- 70kg male athlete - Liver glycogen 90g and muscle glycogen 400g
- 60kg female athlete - Liver glycogen 70g and muscle glycogen 300g.
During hard exercise, carbohydrates can be depleted at 3-4 grams per minute. If this is sustained for 2 hours or more, a 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 generally require 24-48 hours for complete recovery.
During exercise, there is 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 carbohydrates before, during and after exercise will help prevent blood glucose levels from falling too low and help maintain the body's glycogen stores. Many athletes cannot consume the food before or during exercise, so a formulated drink that will provide carbohydrates is required.
Hydration
Fluid absorption
Two main factors affect the speed at which fluid from a drink gets into the body:
- the rate at which it is emptied from the stomach
- the rate at which it is absorbed through the walls of the small intestine
The higher the carbohydrate levels in a drink, the slower the rate of stomach emptying. Isotonic drinks with a carbohydrate level of 6 and 8% are drained from the stomach at a rate similar to water. In a drink, electrolytes, especially sodium and potassium, will reduce urine output, enable the fluid to empty quickly from the stomach, promote absorption from the intestine, and encourage fluid retention (Unknown 1993)[3].
What is wrong with water?
- water causes bloating, which will suppress thirst, and therefore drinking
- water contains no carbohydrates or electrolytes
Calculating personal fluid needs
During an endurance event, you should drink just enough to be sure you lose no more than 2% of the pre-race weight. This can be achieved in the following way:
- Record your naked body weight immediately before and after many training sessions, along with details of distance/duration, clothing and weather conditions
- Add the amount of fluid taken during the session to the amount of weight loss - 1 kilogram (kg) is roughly equivalent to 1 litre of fluid (1lb approx. 0.5 litres)
- After a few weeks, you should begin to see some patterns emerging and can calculate your sweat rate per hour
- Once you know what your sweat losses are likely to be in any given set of environmental conditions, you can plan your drinking strategy for any particular event
Sports Drinks
There are three types of sports drinks, all of which contain various fluid, electrolytes and carbohydrate levels.
| Type | Content |
| Isotonic | Fluid, electrolytes and 6 to 8% carbohydrate |
| Hypotonic | Fluids, electrolytes and a low level of carbohydrate |
| Hypertonic | High level of carbohydrate |
The osmolality of a fluid is a measure of the number of particles in a solution. These particles will comprise a drink's carbohydrates, electrolytes, sweeteners, and preservatives. In blood plasma, the particles will contain 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 balance 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 carbohydrate boost. 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. 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 carbohydrates, 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 energy levels 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 and keep chilled
Hypotonic - 100ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all the ingredients and keep chilled.
Hypertonic - 400ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all the ingredients and keep chilled.
Dental Health
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 reduced erosive potential, as the acid dissolution constant is temperature-dependent (Milosevic et al. 1997)[4].
Food for thought
In a trial conducted by scientists in Aberdeen, it was determined that a 2% carbohydrate-electrolyte drink provided more effective combat to exercise fatigue in a hot climate compared to a 15% carbohydrate-electrolyte mixture mixture (Galloway and Maughan 2000)[5].
Seven Rules of Hydration (Troop 1994)[6]
- The rate of water passage from your stomach into your small intestine depends on how much fluid is in your stomach. If there is lots of water there, fluid flow from the stomach to the intestine is like a springtime flood; if there is little water, the movement resembles a lightly dripping tap. Therefore, to increase stomach-intestinal flow (and overall absorption of water), you need to deposit a fair amount of liquid in your stomach just before you begin your exercise. 10-12 ounces of fluid is a good start. This will feel uncomfortable at first, so practice funnelling this amount of beverage into your "tank" several times before an actual competition.
- To sustain a rapid fluid movement into your small intestine during your exertions, take three to four sips of the beverage every 10 minutes, if possible, or five to six swallows every 15 minutes.
- If you exercise for less than 60 minutes, do not worry about including carbohydrates in your drink; plain water is fine. For exercise that is more prolonged, you will want carbohydrates.
- Years of research have suggested that your drink's correct carbohydrate concentration is about 5 to 7%. Most commercial sports drinks fall within this range, and you can make your own 6% drink by mixing five tablespoons of table sugar with each litre of water you use. A bit of sodium boosts absorption; one-third teaspoon of salt per litre of water is about right. Although 5 to 7% of carbohydrate solutions seem to work best for most individuals, there is evidence that some endurance athletes can fare better with higher concentrations. In research carried out at Liverpool John Moores University, for example, cyclists who ingested a 15% maltodextrin solution improved their endurance by 30 per cent compared to individuals who used a 5% glucose drink. The 15% drink also drained from the stomach as quickly as the 5% one, though many other studies have linked such concentrated drinks with a slowdown in water movement.
- A 6% "simple sugar" drink will empty from your stomach at about the same rate as a fancy 6% "glucose polymer" beverage, so do not fall for the idea that the latter can boost water absorption or enhance your performance more than the former, and don't pay more for the glucose-polymer concoction.
- Contrary to what you have heard, cold drinks are not absorbed into your body more quickly than warm ones. However, cold drinks are often more palatable than warm ones during exercise, so if coldness helps you to drink large quantities of fluid while you exert yourself, then keep your drinks cool.
- Swilling drinks during exercise does NOT increase your risk of digestive system problems. In actuality, most gut disorders that arise during exercise are caused by dehydration, not by taking in the fluid. Dehydration induces nausea and discomfort by reducing blood flow to the digestive system, so keep drinking!
Water Intoxication
Intracellular fluid and interstitial fluid have the same osmotic pressures 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). Aldosterone regulates extracellular fluid volume by adjusting the amount of Sodium reabsorbed by the blood from the kidneys. ADH regulates extracellular fluid electrolyte concentration by adjusting the amount of water reabsorbed into the blood by the kidneys.
Certain conditions may result in a decrease in sodium concentration in interstitial fluid. For instance, during sweating, the skin excretes Sodium and water. We may produce a sodium deficit if we replace the lost fluid with plain water. The decrease in sodium concentration in the interstitial fluid lowers the 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 pretty serious:
- The first result is an increase in intracellular water concentration, called overhydration which disrupts nerve cell function. We may see disoriented behaviour, convulsions, coma, and even death in severe overhydration.
- The second result is a loss of interstitial fluid volume that leads to decreased interstitial fluid pressure. As the pressure drops, water moves out of the plasma, resulting in a loss of blood volume that may lead to circulatory shock.
Alcohol
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. The 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.
References
- REHRER, N.J. (1994) The Maintenance of Fluid Balance During Exercise. International Journal of Sports Medicine, vol. 15(3), p. 122-125
- HAMILTON, A. (2005) Sports Drinks or water: What is the best choice for sports performers. Peak Performance, 212, p.1-5
- UNKNOWN (1993) The Effect of Different Forms of Fluid Provision on Exercise Performance. International Journal of Sports Medicine, 14, p. 298
- MILOSEVIC, A. and KELLY, M.J. and McLEAN, A.N. (1997) Sports supplement drinks and dental health in competitive swimmers and cyclists. British Dental Journal 182, p. 303-308
- GALLOWAY, S.D.R. and MAUGHAN, R.J. (2000) The effects of substrate and fluid provision on ermoregulatory and metabolic responses to prolonged exercise in a hot environment. Journal of Sports Sciences, 18 (5), p. 339-351
- TROOP, R. (1994) Drink your way to winning performances: the seven secrets of hydration, Peak Performance (August 1994), p. 11-12
Page Reference
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- MACKENZIE, B. (2000) Sports Drinks [WWW] Available from: https://www.brianmac.co.uk/drinks.htm [Accessed