 REVENTING DEHYDRATION during exercise is one of the keys of maintaining exercise performance (particularly in hot/humid environments). People engaged in intense exercise or working in the heat need to frequently ingest water or sports drinks (e.g., 6-8 oz every 10 – 15 minutes). Fluid loss during exercise is related to relative humidity, ambient temperature, clothing, and fitness level. Exercise performance can be significantly impaired when 2% or more of body weight is lost through sweat. For example, when a 70-kg athlete loses more than 1.4 kg of body weight during exercise (2%), performance capacity is often significantly decreased. Further, weight loss of more than 4% of body weight during exercise may lead to heat illness, heat exhaustion, heat stroke, and possibly death. For this reason, it is critical that athletes consume a sufficient amount of water and/or sports drinks during exercise in order to maintain hydration status.
The normal sweat rate of athletes ranges from 0.5 to 2.0 L/h depending on temperature, humidity, exercise intensity, and their sweat response to exercise. This means that in order to maintain fluid balance and prevent dehydration, athletes need to ingest 0.5 to 2 L/h of fl uid in order to offset weight loss. This requires frequent ingestion of 6-8 oz of water or a sports drink every 5 to 15-min during exercise. The athlete should consume 3 cups of water for every pound lost during exercise in order to dequately rehydrate themselves. Dehydration can also lead to electrolyte imbalances manifesting themselves in muscular cramping to cardiac arrhythmias. Preventing dehydration during exercise is one of the most effective ways to maintain exercise capacity and good health. It is quite evident that water plays a significant role in maintaining good health and optimizing performance, but water alone is not the answer. In fact, water alone in large quantities can be very dangerous to athletes. Okay, so what really makes an effective sports drink. Good question, right? Well, obviously optimum hydration is extremely important to athletes, so an effective sports drink should be able to replace the fluid lost in sweat. However, fluid replacement is not the only justification for a sports drink. Sweating involves the loss of electrolytes and these must also be replaced. Still yet, supplying carbohydrates (principle fuel for high intensity exercise) to working muscle is vital to continuing high levels of performance. Let’s look a little more closely in the following paragraphs to each component of formulated commercial beverages. Electrolytes Electrolytes are minerals like sodium, chloride and potassium that carry an electric charge, either positive or negative. Electrolytes have many important functions related to water balance, acidbase regulation of the blood along with proper signaling of heart and skeletal muscle. Sodium, chloride, potassium, along with magnesium, zinc, calcium and some vitamins, are excreted with sweat and the composition varies from person to person. Some people’s sweat is very dilute (mainly water) while other “heavy” sweaters have much higher concentrations of electrolytes. Athletes that engage in multi-hour, high intensity workouts, especially in the heat, are at particular risk of depleting electrolytes, sodium, in particular. There are three reasons why replacing electrolytes via a sports drink may be better than drinking plain water: 1.) Although the amounts lost in sweat are generally low, prolonged heavy sweating can lead to significant losses, particularly of sodium. Drinking plain water effectively dilutes the electrolytes that are present in the blood, impairing normal physiological processes. One such example is that of hyponatremia and can be life threatening (see below). 2.) Drinks that contain electrolytes, particularly sodium, are known to enhance voluntary intake of fluid and may also enhance the rate and completness of rehydration after exercise. 3.) In appropriate concentrations, electrolytes may enhance the rate of fluid absorption in the small intestine, especially when combined with glucose. This is important when rapid uptake of fluid is essential in extreme environments or during competition. The American College of Sports Medicine (ACSM) recommends 500-700 mg of sodium per liter of fluid consumed during exercise. The other electrolytes concentrations can be maintained and replaced through a balanced diet.
Hyponatremia As mentioned, sodium is the key electrolyte and must be maintained within normal physiological range for optimal bodily functions and general health. Hyponatremia is a condition of low serum sodium concentration and can cause serious problems. It occurs generally for two reasons: 1) Excessive ingestion of more plain water than required, thereby diluting the sodium concentration; and 2) sweat volume and sodium concentration are very large thereby resulting in sodium depletion. Signs and symptoms of hyponatremia include increased body mass, extreme fatigue, nausea, throbbing headache, lack of coordination and, in fact, severe cases of hyponatremia due to water intoxication (too much plain water) have involved combinations of grand mal seizures, pulmonary edema, increased cranial pressure and respiratory arrest. (Continued in right-hand column)
| (Continued from left-hand column) Hyponatremia can be avoided if fluid consumption does not exceed fluid loss and if adequate sodium is consumed in the diet or in a sports drink during and after exercise. Your fluid and electrolyte needs can be easily determined via calculation of your sweat rate (see the following example). Carbohydrates Carbohydrate is the body’s preferential source for fueling high intensity exercise. The practice of “carb loading” has been in existence for many years and been shown to optimize performance. However, carbohydrate loading is not the prime factor in fluid replacement sports drinks. The addition of carbohydrates in sports drinks can be very beneficial, though. The role of these is to maintain blood glucose, enhance carbohydrate metabolism and delay fatigue by providing exogenous energy. Also important in optimizing performance is maximizing the amount of fluid that can be tolerated without GI distress equaling the rate lost from sweating. Research has also determined that the addition of carbohydrates appears to be a critical factor for rapid absorption in the intestines. Rapid absorption is critical to an athlete during exercise to help maintain hydration levels as close to normal for as long as possible. Another important factor influencing gastric emptying is the volume of fluid in the stomach. Too concentrated of a carbohydrate solution (greater than an 8% solution) actually slows the rate of absorption in the intestines, providing no advantage to the athlete. Therefore, maintaining the largest volume of fluid tolerable (400-600 ml) would be most advantageous with frequent consumption occurring every 15-20 min during exercise ranging from 150-350 ml13. Along with rapid absorption, carbohydrates provide the body with exogenous blood sugar (glucose) to fuel intense exercise while saving the body from dipping into its glycogen (stored glucose) stores. By maintaining glycogen stores for a longer period of time, the athlete is able to prolong their exercise time (train longer) at a higher exercise intensity. This, of course, leads to better performances and greater success. It is generally accepted that exercise lasting less than one hour does not require any special type of rehydration beverage. Plain water is effective in the rehydration process. However, during intense exercise that lasts longer than one hour, fluids containing electrolytes and carbohydrates are recommended. This becomes even more of an issue when extreme environmental factors such as heat and humidity are involved. Let’s now focus on optimal strategies for hydration or rehydration and energy needs of athletes engaged in prolonged activity with the likelihood for significant water loss through sweating. Athletes should not depend on thirst to prompt them to drink because people do not typically get thirsty until they have lost a signifi cant amount of fluid through sweat. Additionally, athletes should weigh themselves prior to and following exercise training to ensure they maintain proper hydration. Calculation of hourly sweat rate 1.) Record body weight before and after 1 hour of exercise or rest in heat. 2.) Calculate the difference (D) between these weights. 3.) If clothing is wet, the increase in clothing weight should be noted by weighing it before and after heat exposure. This increase in weight represents sweat that was produced but remained in the fabric, and should be added to D (step 2 above). If nude body weight is taken, this step can be ignored. 4.) The weight of fluid consumed during step 1 should be added to D. 5.) The weight of urine lost during step 1 should be subtracted from D. 6.) Your hourly sweat rate (in kg/hr) equals D (step 2), after correcting for the items in steps 3-5. 7. Weight lost should be replaced by consuming 1 L of fluid for each kilogram of body weight that was lost. Sample calculation: 1.) Clothed body before exercise = 70 kg. Perform 1 hr of mild exercise in hot environment. Clothed weight after exercise = 69 kg. 2.) D = 70 – 69 = 1 kg
3.) Clothing weight before exercise = 1 kg. Clothing weight after exercise = 1.5 kg. Increase in clothing weight due to sweat = 0.5 kg.
4.) Fluid consumed during exercise = 0.5 L = 0.5 kg.
5.) No urine was lost.
6.) D = 1 kg (step 2) + 0.5 kg (step 3) + 0.5 kg (step 4) = 2 kg.
7.) Weight loss should be replaced by consuming: (1 L/kg) x (2 kg) = 2 L of fluid.
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