Heat, Hydration, Electrolytes and Exercise: A Delicate Balance

Anyone who has ever taken a run around the park or played outside on a hot day knows we sweat. Not everyone who has had that experience has thought about why. If you have been that person with a black shirt on you might have noticed later that a funny white residue showed up on your clothes. What is that all about? The delicate balance between heat, hydration, sodium and exercise (or just sweating for that matter) is always present. What hasn’t always been known is how does that affect us? I have always known that sodium is excreted when we sweat, and I knew the physiologic role sodium played, but until recently I didn’t think about the possible effect that sodium loss could have on the body. I guess I assumed that everyone got enough (usually too much) in the processed world that we live in, but I never took the time to think about the person like me who drinks 6 liters of water a day and doesn’t salt their food. The concept of not enough sodium (in the non-hospital setting) honestly never crossed my mind. We are sold the idea that salting your food is bad for you, but the question you have to ask yourself is, where are you starting from and what are you doing? I drink 6 liters of water and do 1 hour of cardio (not counting my resistance workout) a day. I have actually started drinking Gatorade (well, G2 anyway) for the electrolytes after cardio.

I knew that hydration was important, and I understood the principles of water being a conduit, a mechanism of transport for the body. Hormones, mediating factors, oxygen delivery, pretty much everything that allows us to “be” requires water as a transport mechanism for delivery. Until recently I never thought of water as being compulsory as an expulsion mechanism. Water (in the form of sweat) is required as a means of dissipating heat through evaporation. We are also able to lose heat through conduction, convection and radiation, but evaporation through sweat is the primary mechanism of heat loss, especially in hot weather. “Sweat” is a combination of water and electrolytes, so in contemplating sweat loss one has to factor in losses in both. Sweat rates can be variable (as little as 0.5 and as much as 3 or more liters per hour) and is influenced by variables such as exercise intensity and duration, age, gender, training, heat acclimatization, air temperature, humidity, cloud cover, wind velocity, clothing, and individual sweat rates.(Grandjean, Reimers & Buyckx 2003) Water is also lost from respiratory, renal, metabolic and gastrointestinal function, but sweat is the main mechanism of water loss in exercise-heat stress.(Sawka et al. 2007) With water composing 60-70% of our body mass, water loss is something to worry about. We wouldn’t have so much of it as a part of us if it wasn’t important.
Although dehydration has been found to have little effect on muscle strength and ballistic power, adequate hydration is crucial in our ability to cool down.(Montain 2008) Heat production is 15-20 times greater during exercise than at rest and can raise core body temperature 1 degree Celsius every 5 minutes without thermoregulatory measures.(Coris, Ramirez & Van Durme 2004) Without the ability to lose heat through evaporation, a myriad of conditions can arise. Dehydration can be linked to all kinds of health problems such as kidney stones, constipation, urinary tract infections and more, but the main health concerns in athletic populations are heat related illnesses. (Grandjean, Reimers & Buyckx 2003) The main heat related illnesses are exertional heat cramps, heat syncope, heat exhaustion and heat stroke. There are many risk factors for heat related illnesses such as obesity, lack of acclimatization, certain medications, alcohol consumption, and some illnesses to name a few, but dehydration is among the top offenders.
Exertional or “exercise associated” heat cramps are tri-factoral. They are caused by a combination of muscle fatigue, dehydrati()on, and sodium loss.(Armstrong et al. 2007) Because conduction of electrical impulses of nerves and muscle tissue are effected by sodium, chloride and potassium concentrations, imbalances in any of these electrolytes can cause dysfunction. This condition can be caused by water loss, or over hydration; both can lead to imbalance. Treatment for exertional muscle cramps includes rest, stretching of cramped areas, and NaCl ingestion in fluids and foods. It is recommended to take 1-2 salt tablets or 1/8-1/4 table spoon of salt and 300-500 ml of fluids as a treatment with IV normal saline fluids as an option for severe cases. The preferential treatment is always the least invasive one that delivers the desired effect.(Armstrong et al. 2007) Although dehydration is often a cause it must be noted that excessive water consumption can dilute electrolytes and precipitate cramping so treatment with more fluid should proceed only after ruling out overhydration otherwise risking dilutional hyponatremia.(Wexler 2002)
Heat syncope is also a heat related syndrome and involves a loss of consciousness resulting from inadequate cardiac output and postural hypotension, but recovery is fast once placed in a supine position. Fluid replacement is usually recommended. (Coris, Ramirez & Van Durme 2004)

Heat exhaustion has 2 forms, water depleted and sodium depleted, although both forms can overlap. A defining characteristic of heat exhaustion is a core body temperature greater than 38˚C but less than 40 ˚C. Signs and symptoms include weakness, vertigo, headache, heat cramps, nausea, vomiting, hypotension and elevated heart rate (tachycardia).(Wexler 2002) Treatment is dependent on the type but usually includes rest in a cool environment, oral fluid replacement and sodium replacement if required.
Heat stroke is the most severe heat related illness. It is defined by a rectal temperature > 40˚C with central nervous system changes and multiple organ system failure.(Armstrong et al. 2007) Extreme elevation in temperature leads to cell disruption causing multiple organ system failure and eventual death. The Triad of symptoms includes hyperpyrexia, anhidrosis, and mental status changes.(Wexler 2002) Organ damage and mortality rate are both directly proportional to length of time of elevated core body temperature so immediate cooling measures are required.
Another hydration issue that has been talked about a lot as of late is hyponatremia, defined as a serum sodium level < 130 mmol/L.(Hsieh 2004, Cleary, Casa 2005) Symptoms include nausea, vomiting, light headedness, lethargy, edema and cramps with severe symptoms including mental status changes, encephalopathy, seizures, pulmonary edema and death. This condition is usually associated with overhydration with hypotonic fluids in the presence of sodium loss from sweat. Problems have been faced with the treatment with hyponatremia because symptoms are very similar to those of dehydration related illnesses, but the pathology and treatment are very different. Treatment of an overhydration syndrome with dehydration tactics can exacerbate symptoms, and has been associated with rhabdomyolysis, pontine myelinolysis and death. Athletes with symptoms of heat related illness should have their core temperature and blood glucose levels checked before treatment to rule out heat related illness verses hyponatremia.(Hsieh 2004)
As far as what I would recommend for hydration in the heat, my views have changed a bit. In the past I wouldn’t have really even considered sodium replacement in hydration recommendations. I usually drink when I am thirsty, and I drink a lot, but one has to consider losses of sodium and water. Although Goulet disagrees with some of ACSM’s guidelines on their 2007 position statement, I would have to stick with ACSM.(Sawka et al. 2007, Goulet 2008) Using body weight as a measure of sweat loss and assuming that 1ml of sweat loss represents 1g of loss in body weight is a great measure of fluid replacement needs. Body weight has been shown to accurately reflect individual fluid replacement needs. Rehydration variables should include duration of the event, intensity, environmental conditions, acclimatization state, and individual sweat rate so giving one generalized number doesn’t really work. I have seen 0.6-1.2 liters per hour of activity as a guideline, but that doesn’t take into account the above stated variables.(Cleary, Casa 2005) It is inappropriate to use a single fluid replacement for everyone, as each individual has unique fluid loss rates. Post event recommendations of a carbohydrate-electrolyte containing fluid has been a consistent finding in the research I have done, with sodium concentrations of 20-30 meq, 2-5 meq of potassium and 5-8% carbohydrates. Prehydrating 4 hours before a long event with 5-7 ml/kg and including 20-50 meq/l of sodium has been recommended to avoid risks of hypovolemia and hyponatremia.(Sawka et al. 2007) All in all, it’s about putting back in what is coming out. Water, electrolytes, carbohydrates, fuel; we need them all to function so think about it.

Armstrong, L.E., Casa, D.J., Millard-Stafford, M., Moran, D.S., Pyne, S.W. & Roberts, W.O. 2007, “Exertional Heat Illness during Training and Competition”, Medicine & Science in Sports & Exercise, vol. 39, no. 3, pp. 556-572.

Cleary, M. & Casa, D. 2005, “Exertional Hyponatremia: Considerations for Athletic Trainers”, Athletic Therapy Today, vol. 10, no. 4, pp. 61-66.
Coris, E.E., Ramirez, A.M. & Van Durme, D.J. 2004, “Heat illness in athletes: the dangerous combination of heat, humidity and exercise”, Sports Medicine, vol. 34, no. 1, pp. 9-16.

Goulet, E. 2008, “Pre-Exercise Hyperhydration: Comments on the 2007 ACSM Position Stand on Exercise and Fluid Replacement”, Journal of Exercise Physiology Online, vol. 11, no. 2, pp. 64-74.

Grandjean, A.C., Reimers, K.J. & Buyckx, M.E. 2003, “Hydration: issues for the 21st century”, Nutrition reviews, vol. 61, no. 8, pp. 261-271.
Hsieh, M. 2004, “Recommendations for treatment of hyponatraemia at endurance events”, Sports Medicine, vol. 34, no. 4, pp. 231-238.

Montain, S.J. 2008, “Hydration recommendations for sport 2008″, Current Sports Medicine Reports, vol. 7, no. 4, pp. 187-192.
Sawka, M.N., Burke, L.M., Eichner, E.R., Maughan, R.J., Montain, S.J. & Stachenfeld, N.S. 2007, “Exercise and Fluid Relacement”, Medicine & Science in Sports & Exercise, vol. 39, no. 2, pp. 337-390.
Wexler, R.K. 2002, “Evaluation and treatment of heat-related illnesses”, American Family Physician, vol. 65, no. 11, pp. 2307-2314.

Tags: Electrolytes, Exercise, Heat, Hydration

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