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With all the information available, there are still errors made in hydration at the Olympic and professional level down to the little league level of sports. Heat illness is related to the body's ability to adapt and adjust to an increased core temperature occurring with exertion. If the rate of heat production or gain exceeds the rate of heat dissipation or loss, then hyperthermia or high internal body temperature, results.
The two most important methods of heat dissipation are radiation and evaporation. "Radiation is the transfer of heat to the environment via electromagnetic waves; it accounts for most heat dissipation. As long as there is a temperature gradient between the body and the air, 65% of the body's heat is lost by radiation. Evaporation is the transfer of heat by transformation of perspiration and saliva into a vapor; it accounts for 30% of the body's heat loss." "When air temperature exceeds 95°F (35°C), radiation of heat from the body ceases and evaporation becomes the only means of heat loss. Evaporation is maximally efficient in a dry environment. If humidity reaches 100%, evaporation of sweat is no longer possible and the body loses its ability to dissipate heat." "Initially, the body attempts to control the core temperature. A rise in body temperature activates heat receptors in both the hypothalamus and the periphery, which results in increased shunting of blood to the periphery." When this mechanism is overwhelmed, problems arise. Heat Exhaustion and Heat Stroke
Thirst alone is not a good indicator of the need for fluids. Research demonstrates that exercise in hot adverse conditions can cause dehydration in as little as 15 minutes. Drinking when in a dehydrated state can cause gastrointestinal distress. Athletes who say they cannot drink during a workout because it gives them a stomach ache may be allowing themselves to become dehydrated before they take their first drink. Dehydration is cumulative. If lost fluids and minerals go unreplaced fatigue and heat illness can result. It can manifest as heat cramps, heat exhaustion and heat stroke.
Heat cramps begin can begin as twitching of the muscles and progress to localized contractions of the muscles of the legs, arms, or abdomen. Treatment involves rest and drinking mildly salted fluid.
Heat exhaustion is a form of shock and is the third leading cause of death among young athletes. It is the inability to continue exercise in a hot environment and primarily results from sweat loss and/or inadequate fluid intake. When dehydration (especially greater than 3% of bodyweight) is superimposed on exercise heat stress, the cardiovascular system is simply unable to pump sufficient blood to meet all of the body's needs.
Signs and symptoms can include: profuse sweating, "heat sensations" on the head/shoulders/chest, weakness, "rubbery" legs, chills, anxiety, irritability, nausea, vomiting, muscle cramps, fainting, rapid and weak pulse, pale or flushed skin, disturbance of vision and incoherence. Treatment consists of rest in a cool environment, elevation of legs, pouring cool water on the skin and replacement of fluids orally or intravenously. Recovery is usually complete within 12 to 48 hours, without further complications. Athletes suffering from heat exhaustion should not be allowed to practice or compete for the remainder of that day. Heat exhaustion may be prevented by proper heat acclimatization; consuming water, electrolytes and carbohydrates to replace losses; and monitoring ambient conditions (temperature and humidity) to allow practice sessions to be tailored to the environment.
Heat stroke is a medical emergency requiring rapid diagnosis and treatment. It is an elevated core temperature above 104 degrees F (by rectal thermometer) and altered mental status. Other signs include hot and dry skin, rapid heart rate, rapid breathing, disorientation, confusion, combativeness, decreased responsiveness, coma or seizure. More than half of presenting patients are sweating, especially in cases of exertional heatstroke. Heatstroke can occur when the air temperature is as low as 41 degrees F and often occurs during the early morning. Between 1959 and 1985 there were 77 heatstroke fatalities at high school and college level.
Heatstroke commonly involves more than one factor: obesity, insufficient heat acclimatization, dehydration, lack of sleep, fever, drug or alcohol abuse. Other factors are air temperature, time of day, type of activity, exercise intensity, exercise duration, and clothing. It usually occurs among young, motivated males who push their bodies beyond the point at which they would normally stop exercise if discipline, competition, or peer pressure were not involved. Few cases of exercise-induced heatstroke have been reported among females.
The key to treatment of heatstroke is rapid cooling. Stopping the activity, removing excess clothing, getting into the shade, checking the core temperature and other vital signs and calling 911 must be done immediately. Ice water immersion or pouring ice water on the person has been described as the most efficient and rapid method of core temperature cooling. The majority of patients recover completely within one to six months.
Hyponatremia is a decrease of sodium in the blood and is caused by over-hydration. Slower runners, 4 1/2 hour plus marathoners, are affected more frequently; but this is not absolute. Mild cases may be asymptomatic, but moderate or serious cases may cause elevated BP, vomiting, altered mental status, bloating, respiratory distress, seizure or coma. No fluid should be given until urination has begun and salty foods are encouraged.
For more information on hyponatremia see Hyponatremia in Athletes
Thirst alone is not a good indicator of the need for fluids. Research demonstrates that exercise in hot adverse conditions can cause dehydration in as little as 15 minutes. It takes at least 7-10 days to acclimatize to the heat. Loose, light colored clothing should be worn. No rubberized suits! Weigh yourself before and after training and replace each pound lost with one pint of fluid. A 1-3% weight loss will decrease performance.
Check the Heat Index Tables to see if it is safe to train or compete.
Here is a urine color chart to help you determine your hydration status. Your urine color should be between #1 and #3 throughout the day.
It is recommended that long distance runners consume 1 liter of fluid for every liter lost during a race. For any race longer than one hour, the following procedure is suggested to determine individual fluid needs:
Use the following formula to calculate your hydration requirements:
Pre-workout body weight (kg) from Step 3: _____ kg
(To convert from pounds to kilograms, divide pounds by 2.2)
Post-workout body weight (kg) from Step 7: _____ kg
........................................................... = _____ kg of body weight lost during the workout.
.................................. _____ kg x 1000 = ______ grams of body weight lost during the workout
Grams of body weight lost + fluid (ml) consumed from Step 5 = _____ ml
The above ml of fluid is the ml of fluid per hour needed to properly hydrate.
To convert oz to ml: oz x 30 = ml.
To convert ml to oz: ml/30 = oz.
Carbohydrates consumed immediately after and two hours after exercise can enhance muscle glycogen restoration.(12) Most oral rehydration solution contain less than 6% carbohydrates (about 55 calories per 8oz.) because research has shown that the greatest rates of fluid absorption are usually stimulated by solution containing from 2-6% carbohydrates. Increasing carbohydrate content beyond 6% often reduces the rate of fluid absorption.(20) Research shows the 6% carbohydrate concentration is absorbed at least 30% faster than water. Higher glucose levels are maintained with ingestion of this type of beverage.(9)
Gatorade (6% carbohydrate/electrolyte solution) was absorbed significantly faster than water during both exercise and recovery periods. Exercise did not influence the absorption process.(11) Exercise performance improved significantly with carbohydrate feedings. This study recommends ingesting 30-60g/hour of carbohydrate during exercise to improve performance.(13)
Solutions with multiple transportable substrates that can stimulate several different solute transport mechanisms produce greater solute and water absorption than solutions with only a single transport medium. The 6% CHO solution containing a combination of free glucose and fructose are suggested for maximizing waster and CHO absorption.(18)
To calculate the carbohydrate percentage of any beverage:
Divide the number of grams of carbohydrate per serving (in milliliters) and multiply by 100, e.g. 14 grams per 8oz. (1oz.x30=milliliters)
14/240 mlx100= 5.83 or 6%
Sodium 110 mg/8oz
When the subjects ingested the beverage every 20 minutes during two hours of exercise, the fructose beverage was not as readily available for energy as the glucose-containing beverages. The glucose polymer did not provide any metabolic advantage over the glucose-free beverage.(14) Consumption of fructose was associated with greater incidence of gastrointestinal distress due to its slow absorption rate, a greater loss of plasma volume, greater stress hormone response and substantially poor exercise performance.(17)
Carbonated soft drinks contain carbohydrates in a concentration of 10-11%. Compared to the typical sport drinks, such beverages are absorbed slowly. In addition, the carbonation turns into carbon dioxide gas when warmed in the stomach and can cause gastrointestinal distress. The caffeine found in many soft drinks actually increased fluid lost by stimulating urine production.(19)
A new concept of super oral rehydration has been proposed in which nutrients other than carbohydrates are also included in the beverage like amino acids. The theory needs to be confirmed.(20)
The volume of fluid ingested is a powerful regulator of gastric emptying. There was greater gastric emptying with rehydration of 150% of fluid lost. However, this was attained via relatively large gastric fluid volume, with concomitantly large urine volumes. Thus, forced intake of a relatively dilute solution is not an effective method of rapid rehydration and may be detrimental to subsequent performance.(16)
Recruits at higher risk for developing EHI (exertional heat illness) had a BMI of >22/kg/mm and 1.5 mile run time of >12 minutes. These recruits had an eightfold higher risk for developing EHI during basic training when compared to those with BMI <22 and a 1.5 mile run time of <10 minutes. Only 1/5 of male recruits met these criteria for high risk, but they accounted for nearly half of the EHI cases occurring during the 12 weeks basic training course.(10)
Studies have shown that the main maturational differences to exercise in the heat (mostly sweating rate and composition) occur at late puberty and adulthood, rather than from pre- to mid-puberty.(15)
Individuals with spinal cord injury possess limited autonomic control of heat dissipation (sweat gland secretion, redistribution of cardiac output, vasodilation in cutaneous vessels) below the level of injury. The magnitude of their thermoregulatory impairment is related to the level and completeness of the spinal cord lesion. Other factors are impaired sympathetic cardiac stimulation which results in lower stroke volume and cardiac output and decreased venous return to the heart than that of able-bodied individuals; reduced thermoregulatory response for a given core temperature; unique movements of upper-body wheelchair propulsion, when compared to lower body exercise of equal intensity, produce enhanced strain, evidenced by greater plasma volume loss and catecholamine concentrations; medications (oxybutynin and phenoxybenzamine for bladder control) may negatively affect sweating and thermoregulation.(8)
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