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Heat Stress Resolution 
Hyperthermic Performance-Inhibiting Cause And Effect
*Bill Misner Ph.D.


The Problem: The Body Attempts To Regulate Core Temperatures But Fails

The normal human body temperature displays a circadian rhythm, ranging from 97 F. or lower in predawn hours to 99.3 F. or higher by afternoon. Body temperatures that exceed the norm of 98.6 F. are often observed in healthy people. Normal body fluid levels help maintain normal body temperature, cooling by evaporation from the skin and lungs, throwing off heat when blood is circulated near the body's surface, and what most people do not realize is normal hydrated fluids also help maintain warmth. Abnormal elevation of temperature (pyrexia) is categorized as hyperthermia or fever. Hyperthermia is the result of a failure of thermal control mechanisms. Acclimatized hyperthermic-conditioned athletes deplete 50% of the fluids and electrolytes as those athletes who have exposed themselves to the humidity and heat in prolonged aerobic training. During the wintertime, distal capillary beds are least stressed and are adaptively changed from vasodilatation resulting in evaporative heat-dispersion to vasoconstrictive heat-saving activities. The average athlete requires 10-21 days exposure to exercise sessions in either cold or heat in order to maximize heat preserving or heat dispersing for keeping narrow controls over core body temperature fluctuations.

Electrolyte Loss

The sedentary human body requires only 500 mg sodium either already stored within [endogenous] or as consumed [exogenous] from a drink or diet dose. Sodium transit and homeostatic balance is time- and dose- dependent. Taking an either too much sodium to accommodate hyperthermic perspiration loss may force the stomach to "extract" fluids out of the body system already fluid-poor in order to dilute the excessive sodium to maintain body fluids at a preferential osmolar pressure 280-303 mOsm. Conversely, if not enough sodium is consumed to replace losses sodium will be is extracted from the serum circulates in system which is already electrolyte-poor due to losses in evaporative perspiration. Either way stomach and/or muscle may cramp in response when an insufficient fluid or electrolyte condition exists.

A human body's total body water (TBW) content averages 60% of body weight in young men. Since fat tissue has a lower water content; thus the fraction of TBW to body weight is slightly lower on average in women (55%) and is substantially lower in obese people and the elderly. About 2/3 of TBW is intracellular and 1/3 extracellular. About 3/4 of the extracellular fluid (ECF) exists in the interstitial space and connective tissues surrounding cells, whereas about 1/4 is intravascular. Most of our fluid losses therefore come from extracellular spaces and vascular fluids. Loss of fluids forces the heart to work harder and muscle fibers contractile fail to produce energy and torque efficiently.

Salt incrustation typically substantiates high sweat loss, more so in unacclimatized subjects. This rate of loss is coupled with distal vascular capillary cooling capacity, which fails to reduce core temperatures when exposed to unexposed heat conditions. However, exercise-induced evaporative-cooling adaptive changes take place after being exposed to 10-21 hyperthermic prolonged training sessions. As the body develops capillary bed vasodilatation to heat, it will commence to cool itself more efficiently reducing the rate of electrolyte and fluid losses resulting in controlled evaporative cooling. Simply put, it takes an appropriate amount of time in the heat for capillary beds to adopt an increased vasodilatation response for cooling an overheated body. Slowing the pace during too-high heat events and pouring water over head, face, and upper torso helps with cooling and reduces losses if acclimatization training is incomplete. Take notice the differences in electrolyte losses recorded comparing those who were unfit, fit against those who were both fit-and-acclimatized:

Electrolytes Lost Each Hour [In Milligrams 1, 2]


Unfit unacclimatized

Fit unacclimatized

Fit acclimatized

















[1] Verde T., Shepherd R.J., Corey P., Moore R. Sweat Composition in Exercise and Heat, J Appl Physiol, 1982;53;6:1541.

[2] Costill D.L., Sweating Its Composition and Effects on Body Fluids, Annals of the New York Academy of Sciences, 1977; 301:162.

Fluids, Fuels, Electrolytes Losses Vs Repletion Rate

Fluids, Fuels, Electrolytes are lost faster than they can be replaced...Fluid losses vary widely between individuals, individual fitness exposed to heat, and in heat multiplied by humidity, which would negate the figures proposed for the normal population above. Glycogen losses are less with decreased pace in moderately cool to warm temperatures, though they too are a property of individual training and diet, but tend to stay fixed in comparison to sodium or fluid dynamics. Sodium loss rate is always greater than fluid loss in the heat, but like fluid loss, sodium depletion may vary widely between individuals, weather conditions, and to the individual's acclimatization. Recent shows drinking water and sodium solution is preferred for replenishing each. During the three-hour rehydration period, subjects who drank water alone restored 68% of the fluid they lost, while subjects who drank the sodium solution replaced 82% of their lost fluids. As always, balance is key. Taking too much of either or not enough will further reduce performance outcome proportionate to imbalance or insufficiency.

Substrate lost

Max rate lost

Max percent replaced


1 liter per hour

60-80% or 24-28 of 34 fl. Oz.


600-900 calories per hour

27-40% or 240-280 of 600-900

Sodium electrolyte*

2000 mg per hour

15-30% or 300-600 mg of 2000

*Sodium is listed because it is the electrolyte lost in sweat at the highest rate. Other electrolytes lost include chloride, potassium, magnesium, and manganese.

Minimal Electrolyte Repletion Versus Maximal Electrolyte Repletion Dose

The need for a survival requires 500-mg dietary sodium in sedentary subjects every 24 hours or at an average rate of 21 milligrams sodium per hour. During exercise at 75% VO2 Max HR aerobic pace, it has been reported that 10-12 times the free radicals are generated above sedentary state.

Therefore, it is hypothetically suggested applying a multiple of 10-12 for accommodating electrolyte repletion rate in order to determine ELECTROLYTE MINIMAL REPLETION DOSE:
------>10 or 12 X 21 mg = 210-252 milligrams per hour

Colgan's highly regarded theory of "Biochemical Individuality" compares individual differences in excretion versus absorption rate based on absorption mechanics of Vitamin C as 5:1. This model numerically recalculates the minimal dose to maximal optimal dose between 200-1200 mg sodium per hour during hyperthermic conditions. Since a 3:1 ratio covers 90% of the population independent of BMI, fitness level, heat acclimatized exposure, the sodium electrolyte requirements range present 200-600 mg sodium transit per hour are suggested rational dose for the general population. This hypothetical equation may differ by individual, gender, size, acclimatized-fitness state, rate of pace, humidity, medications taken, diet, genetics, and individual body fat percent.

Fluid Losses Vs Repletion Rate

EXTREME FLUID LOSS from evaporation occur via expired air and the skin, constitutes between 0.4 to 0.5 mL/hour/kg body wt or a minimum of 650 [650.00 mL is 19.50 fluid ounces per hour] to a maximum of 850 mL [850.00 milliliters is 25.50 fluid ounces per hour] in an average 70-kg adult. Sweat losses are generally negligible but can be significant with fever or in warmer climates. GI water losses are also negligible in health but can be significant in severe diarrhea, nausea, or protracted vomiting. With fever, an additional 50 to 75 mL/day may be lost for each degree of temperature elevation above normal.

A 154-lb. person has 2 compartments filled with 85 lbs. (total) fluids that must be kept in constant osmotic balance. Inside our cells potassium ions are 15 times higher than outside mixed within 25 liters of water stored [or 53 lbs.] in homeostatic balance with the more water is stored outside cell walls. Outside the cells, sodium ions are 10 times higher than inside, within an additional 15 liters or 32 more lbs. of water are stored. When the heat goes up an athlete may lose up to 2.2 pounds of water per hour.

During exercise sessions lasting 2 or more hours, a 2% body water weight loss is expected. When muscle glycogen is metabolized during the first 70-120 minutes water is given off then perspired to an expected 2% body weight loss. More than 3% water weight loss suggests dehydration while no water weight loss suggests overhydration and mild to moderate dilutional hyponatremia [diluted sodium serum], both of which are performance-inhibiting, hyponatremia may be more life-threatening than dehydration. Dehydration also produces symptoms proportionate to severity:

Symptoms that have been observed when a percentage body water weight is lost [1]:

0% -- normal heat regulation and performance

1% -- thirst is stimulated, heat regulation during exercise is altered, performance decline

2% -- further decrease in heat regulation, increased thirst, hinders performance

3% -- more of the same (worsening performance)

4% -- exercise performance cut by 20 - 30%

5% -- headache, irritability, "spaced-out" feeling, fatigue

6% -- weakness, severe loss of thermoregulation

7% -- collapse is likely unless exercise is stopped

10% --comatose

11% --death likely

Resolutions suggested: what really works?

The following practices have been reported to resolve most problems encountered especially those in hyperthermic events [above 60 degrees F. & 60% Humidity] early in the season:

A. HYDRATION REPLETION RATE as a preventative, requires consuming 10-12 glasses or 80-100 fluid ounces liquid daily.

B. REHYDRATION RATE during an event requires between 16-24 fluid ounces liquid each hour. Warning: Athletes who consume 34 fluid ounces per hour or more may predispose themselves to diluting serum sodium stores resulting in hyponatremia, a clinical medical emergency.

C. ELECTROLYTE REPLETION RATE is generally adequate if 300-600 milligrams sodium are consumed each hour in a divided dose format in the presence of other electrolytes such as potassium, magnesium, chloride, and manganese. Remarkable dose variation exists between athletes. One female ultramarathoner successfully won a 100-mile running race on 100 mg Sodium per hour for 17 consecutive hours. Her measured blood serum sodium levels varied only -2% and were within normal reference range both before and after the event. Others have reported requirements of over 6 times the former low dose, or as high as 600 mg Sodium each hour in addition to other electrolytes.

D. FUEL REPLETION RATE suggested is 4.0-4.6 calories per minute, or approximately 240-280 calories each hour in 3-4 divided doses. This dose recommendation represents how much fuel the stomach and liver are able to return as an energy substance in working muscles. The human body prefers small portions and shows its appreciation by absorbing a greater amount of a small dose than a larger volume. High complex carbohydrate energy gels or powdered drinks are reported to absorb at body fluid osmolality in a higher solutions [15%] with less gastric side effects than products with containing simple sugar solutions [6-8%], such as fructose, sucrose, dextrose, or maltose.

E. REDUCED RACE PACE will help reduce core body temperature. The faster pace generates more core heating than a slower pace. Adding a walk break during a run or splashing the whole face and upper torso will reduce body core temperature.

F. BODY WEIGHT is expected to decrease -2% in hyperthermic events lasting 3 hours or more. Always weigh before and after events to determine if the hydration protocol was adequate, excessive, or inadequate. The 2% lost reflects water produced during muscle glycogen metabolization.


Each year starting in May, our "IN" box grows increasingly full of questions asking how to overcome hyperthermic performance-limiting fatigue, nausea, muscle cramps, and gastric stress. The following is a HEAT STRESS RESOLUTION FORM for athletes to list information required for determining cause of performance inhibiting heat stress. Unexpected early-season hyperthermic disorders are often resolved when this post-race information is complete. We base this on reliable research and literally 1000's of trial subjects who have resolved hyperthermic performance-deterioration as opposed to great performance in spite of the heat.

If you are experiencing hyperthermic performance-limiting problems as related to FUEL, FLUID, or ELECTROLYTE requirements prior to or after a hot endurance event, fill out the following form and send it to either Steve Born or myself:

"Steve Born" AT <e-caps.com> "Steve Born" <steve@e-caps.com>

"askdrbill" AT <e-caps.com> "Dr. Bill Misner Ph.D." <askdrbill@e-caps.com>


We will make every effort to assist you in overcoming heat related performance limitations.

DISCLOSURE STATEMENT: E-CAPS, INC., manufactures an electrolyte supplement. As the author of this paper, I declare a competing interest, but have excluded the name of the product.























*Bill Misner Ph.D.

E-Mail: <askdrbill@e-caps.com>






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