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Heat & Hydration

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Published: 19Aug2009
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Experienced athletes generally have no trouble properly hydrating while exercising or training. Most athletes are aware of the role hydration plays in their performance and will take all measures to insure proper hydration before, during and after activity, including acclimating to the environment in which they will be competing.

Heat acclimatization

It is well-known that exercising in a hot environment accelerates muscle fatigue, challenging the body to regulate its core temperature and dissipate heat. As the core temperature rises, a change in muscle metabolism takes place (e.g. increase in lactate and free radical production, accelerated glycogen usage, etc.). An acute episode of hyperthermia starts off relatively mildly with symptoms like headache and nausea. If activity continues and hydration and temperature regulation are not addressed, symptoms can progress to cramps, heat exhaustion (with dizziness, profuse sweating and weak/rapid pulse), and ultimately heat stroke which is very serious and can cause a loss of consciousness or even in extreme instances death. In fact, among US high school athletes, heat illness is the third leading cause of death. It doesn't help that many distance running races, triathlons and football games occur in warmer times of the year. When exercising intensely for prolonged periods and/or in hot, humid conditions, keeping a watchful eye on hydration is crucial for optimal performance and health. Steps in preventing dehydration or heat stroke include:
- Adequate fluid intake before and throughout practice or competition
- Monitoring of heart rate (any significant change in HR to constant activity shows a poor response to the demands of that activity)
- Monitoring of weight, replacing weight losses with fluid. Remember, a pint weighs a pound

Fortunately, continued exposure to training in warm environments leads to adaptations that make it easier to perform in the heat. These adaptations include an increased capacity to sweat, reduced salt losses in sweat and a lower body temperature and heart rate response during activity. The best part is that these adaptations can take place in a little as 7-14 days.

The primary mechanism for the body's heat loss at high levels of work in a normal environment, and at all levels of work in a hot environment, is the evaporation of sweat which releases heat from the body. Sweat rates increase linearly with increased exercise intensity, and in hot weather sweat losses can reach close to 3.0 liters/hr.

Fluids and performance

If you are not properly hydrated, you do not have the potential to perform your best, even if exercise is only a few minutes long. The greater the duration of the task, the greater decrease one will experience in performance with inadequate hydration. Many athletes and exercisers mistakenly think that their thirst will guide them to ideal hydration. Thirst typically occurs when the one's water deficit is approximately two percent of body mass. Performance begins to be impaired with a fluid loss of as little as 1%, suggesting our natural thirst mechanisms cannot keep up with exercise-induced fluid losses. In other words, by the time we register we are thirsty, performance may be in decline.

How can performance be affected by such small fluid deficits? It may be due in part to the decrease in plasma volume (any decrease in fluids will automatically decrease blood/plasma levels), impairing the delivery of oxygen and nutrients in and out of muscles. Along with the loss of fluids, especially during prolonged exercise and profuse sweating, we also lose electrolytes that are involved in muscle and nerve function. Also with prolonged exercise, there is a significant decrease in muscle and liver glycogen, the body's main energy sources. Replacing fluid, electrolytes and glucose is the formula for performance success, especially in longer duration exercise (more than 60 minutes) in hot environments.

Children and older exercisers/athletes

In young children, adolescents and in older (40+) athletes, we find natural hydration mechanisms (thirst triggers, taste) and heat dissipation functions (skin blood flow and sweat rates) are underdeveloped or diminished, making proper hydration strategies a bigger concern. In these populations cognitive monitoring is generally required.

The other side of the equation starts with "a little bit of knowledge can be dangerous"

This is never more true than in the world of nutrition, and hydration is no exception. In recent years there has been so much attention focused on "water" intake (especially with the increased marketing efforts from makers of bottled water). To put things in perspective, the typical, lightly active person has no trouble replacing the approximate loss of two to three liters of fluids they use daily, and it's often done without drinking one glass of pure water. In spite of this, the average person is exposed to a considerable amount of information/advertising that tells them they require considerably more.

Too much of anything can be bad for you

Yes, this includes water. Though it may be the soup of life, water can actually be toxic if ingested in excessive amounts. When this occurs it is usually based on bad or incomplete advice. Inexperienced runners, including charity race participants, often function under the outdated notion that you can't get too much water. Many participants are not competitive athletes and their pace/level of effort is not extreme; water losses are not that great. Still, in their mind is the knowledge that endurance athletes need to keep up on fluid intake during events. Hyponatremia occurs when blood sodium concentration falls to an abnormally low level prompting a rapid swelling of the brain that can result in seizures, coma and death. The key risk factor is excessive drinking—especially non-electrolyte fluids such as many types of water.

First reported in 1985, the disorder was thought to be a rare phenomenon but has become more common since the early 1990s. A post-race study done after the 2002 Boston Marathon found that 13 percent of the participants experienced hyponatremia and point six percent had critical hyponatremia, a potentially deadly issue. Basically these people had gained weight from excess water consumption going into the start of the event and continued to gain as they drank during the race, partially because their relatively long race times gave them enough time to ingest more fluids than they lost.

The risk of hyponatremia can be reduced by making certain that fluid intake does not exceed sweat loss and by ingesting sodium-containing beverages or foods to help replace the sodium lost in sweat.

For most athletes, dehydration is still the primary obstacle to optimizing performance, not hyponatremia. But the message regarding this increasing problem is for the zealous, less experienced exercisers or competitors: Make sure you start off with the accurate hydration recommendations.

Follow the proper hydration guidelines for before, during and after training, including weighing yourself after the workout/event, and make adjustments as necessary. Additionally, weigh each morning. A stable weight generally indicates proper fluid balance.

Neal Spruce is chairman of the board for the prestigious National Academy of Sports Medicine (NASM) and founder dotFIT, LLC. Neal is a fitness specialist, author, licensed teacher, researcher, bodybuilding champion, personal fitness consultant and speaker. dotFIT is the leading online fitness and weight loss program.

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