Performance: How Sports Drinks Affect Exercise Performance

  Whether you're a casual gym user or a regular endurance athlete, sports drinks are designed to help you exercise. Three things can reduce your performance: dehydration, electrolyte imbalance and carbohydrate depletion. Although all sports drinks contain water to combat dehydration, various brands contain different amounts of electrolytes and carbohydrate. In addition, exercise regimes vary: a gymnast uses energy differently to a marathon runner. It's important to understand how sports drinks work so that you can decide which ones are best for you.   Your body is designed to maintain a core temperature of around 37 degrees Celsius. When muscles burn fuel, they produce heat. When you exercise, your body regulates your core temperature through sweating. Sweating is important to prevent your body from overheating, but the fluid loss involved causes problems. "Heat exhaustion" is actually the result of dehydration, and occurs when you lose more than 5% of your body weight in fluid. However, athletic performance suffers when just 2% of body weight is lost in sweat. That works out at around 1.2 liters lost as sweat for women, 1.5 liters for men. The more water is lost, the worse the effects. Hallucinations, circulatory collapse and heat stroke occur at between 7% and 10% of body weight lost. In order to maintain performance, you need to take on extra fluid before, during and after exercise.   The reason that sweat tastes salty is because it actually contains household salt, sodium chloride, as well as other dissolved chemicals. These are mainly potassium, magnesium and calcium. These chemicals are essential for your body's functioning. Sodium chloride is made up of a sodium ion, which has a positive electrical charge, and a chloride ion, which has a negative charge. These ions allow your cells to conduct the electrical impulses which cause muscles to work and allow nerve cells to transmit information. All body fluids, including sweat, contain dissolved electrolytes. When you exercise you are losing around a gram of sodium and a quarter gram of potassium per liter of sweat. In order for your body to function efficiently, these electrolytes need to be replaced.   Your body is fueled by carbohydrate. This is stored in the liver and the muscles as glycogen (a form of glucose). Around 500g is stored in men and around 400g in women; during strenuous exercise, these stores are used up at a rate of around 3-4 grams per minute. Although the body has enough stores of glycogen to provide energy for several hours of exercise, depleted stores need to be replaced. It's usually not possible or appropriate to eat during exercise, but drinks containing glucose can help to provide the body with an ongoing source of energy.   The effect of a sports drink on your exercise performance will depend on whether it is hypotonic, isotonic or hypertonic. This indicates the amount of carbohydrate and electrolytes dissolved in the drink. Your blood contains electrolytes, proteins and glucose, and the amount dissolved in the blood usually stays within a fixed range. Too little dissolved in your blood, and your kidneys crank up production of urine to stabilize the balance. Too much of these things in your bloodstream, and you feel thirsty. Isotonic drinks have about the same concentration of dissolved materials as your bloodstream; hypotonic drinks have less while hypertonic drinks are more concentrated.   Hypotonic drinks are useful for replacing lost fluid when you don't need to replace carbohydrate. If you've been working at moderate intensity for a short while, or in short bursts, your glycogen stores won't be seriously depleted, but you may begin to see a reduction in performance from dehydration. Hypotonic drinks can correct this. Be aware, though, that drinking lots of hypotonic drink may lower electrolyte levels in your bloodstream. This causes your body to send out signals that you shouldn't drink more, such as feeling bloated and full, even before you have completely replaced the fluid you lost as sweat.   Isotonic drinks quickly replace lost fluid. The dissolved electrolytes and glucose speed up the process of absorption from the stomach and small intestine. The sodium lost in sweat is replaced, and the glucose gives your body the fuel it needs to continue. These drinks are ideal for athletes who are a sustaining activity for an hour or more, e.g. distance runners or football players.   Hypertonic drinks contain high levels of carbohydrate, usually glucose, though occasionally fructose. Studies show that they are absorbed from the stomach at about the same rate as isotonic drinks. They are best used after exercise to replace carbohydrate. Some athletes, such as endurance runners, may consume them during exercise, in conjunction with isotonic drinks.   For most casual athletes, performance will mainly be affected by dehydration: drinking hypotonic or isotonic drinks will counter this. For more serious or sustained exercise, there will be benefits for performance from drinking isotonic drinks during exercise, with a carbohydrate boost from a hypertonic drink afterwards. By combating the effects of dehydration, electrolyte imbalance and carbohydrate loss, sports drinks can have a positive effect on exercise performance.   Author: Stephen Humphreys