For the human body, water is the most important nutrient.
Water makes up about 60% of the average person’s weight but can range from 45% to 75% (Campbell & Spano, 2011). Euhydration is a state in which the body water content is adequate to meet a person’s physiologic needs. Hypohydration, a condition in which body water content is insufficient, and hyperhydration, when the body water content is in excess, both present potential dangers (Campbell & Spano, 2011; McArdle, Katch, & Katch, 2014; Tipton, ed., 2006).
Dynamic exercise, especially endurance exercise, causes increases in volume in most fluid compartments compared to sedentary counterparts (Tipton, ed., 2006). Shifts in fluid volume during exercise promote homeostasis in the vascular compartment to maintain cardiovascular function (Tipton, ed., 2006). During training, fluid accumulation in active skeletal muscle averages at about 18, 30, and 45 ml/kg of active muscle for light, moderate, and heavy exertion, respectively (Tipton, ed., 2006). To compensate for this large fluid shift, the vascular compartment, via complex mechanisms, redirects blood flow from inactive tissues to preserve plasma volume during intense exercise, which is ultimately critical to thermoregulation (Tipton, ed., 2006). This includes fluids from passive muscle, the skin, and the gastrointestinal tract (Tipton, ed., 2006).
The list of possible factors that influence plasma volume during exercise is quite extensive, including hormones, exercise mode, posture, exercise intensity, the muscle mass of the activity, gender, phase of the menstrual cycle, individual fitness, and hydration status (Tipton, ed., 2006).
Maintaining adequate cardiovascular function takes priority over thermoregulation (McArdle, Katch, & Katch, 2015) but maintaining cardiovascular function during exercise helps prevent heat stress (McArdle, Katch, & Katch, 2015; Tipton, ed., 2006).
Water is the most important ergogenic aid when exercise and heat stress are combined (McArdle, Katch, & Katch, 2014).
The strategies that I will review that will lower the risk for exercise heat stress are acclimatization, monitoring hydration status, and fluid intake/replacement.
The first strategy I will discuss to prevent heat stress is heat acclimatization.Most heat-related injuries occur at the beginning of hot weather seasons so prevention is important. Click To Tweet
Heat acclimatization causes biological adjustments that reduce heat stress’s adverse effects (McArdle, Katch, & Katch, 2014; Tipton, ed., 2006). The acclimatization process requires repeated exposures to heat sufficient to elevate core and skin temperature to induce perfuse sweating (Tipton, ed., 2006). Even though heat acclimatization is specific to the heat environment (i.e., deserts), there will be cross-over effects to other hot climates (Tipton, ed., 2006).
The biological adjustments associated with heat acclimatization are alterations in thermoregulatory control, fluid balance, and cardiovascular response (Tipton, ed., 2006).
It has been suggested that for full acclimatization to take place exposure must occur over about a two-week period with most of the benefit occurring in the first 7 to 10 days (McArdle, Katch, & Katch, 2015).
It is recommended that exposure occur daily for about two hours with endurance exercise (McArdle, Katch, & Katch, 2015; Tipton, ed., 2006). The athlete may break this into short periods, and light exercise of short duration is recommended at the start of acclimatization (McArdle, Katch, & Katch, 2015). It should be noted that the benefits of heat acclimatization can dissipate within 2 to 3 weeks of a more temperate environment (McArdle, Katch, & Katch, 2015; Tipton, ed., 2006) but that a few days of cool weather will not interfere with the benefits (Tipton, ed., 2006).
Another consideration during acclimatization are the types of equipment worn with the activity.
For example, at the beginning of American Football preseason workouts, the training should occur without the pads allowing the athlete to become accustomed to the heat before adding insulation layers. Adding the gear used may extend the time for acclimation, and adjustments in workouts may be needed.
The major benefits of heat acclimatization
The major benefits of heat acclimatization are improved cutaneous blood flow, effective redistribution of cardiac output, lower threshold for sweating, increased sweating, lower sodium content in sweat, lower skin and core temperature and heart rate with standard exercise, and carbohydrate sparing effect (McArdle, Katch, & Katch, 2015; Tipton, ed., 2006). However, even with good acclimatization, exercising in hot and humid environments in which sweat does not evaporate off the skin will require optimum hydration to maintain thermoregulation (Porcari, Brant, & Comana, 2015).
Assessment of hydration status, fluid adjustments, and intake
There are ways for athletes, both elite and recreational, to monitor ongoing hydration status. If an athlete begins a practice, exercise session, or competitive event in a hypohydration state, their risk for heat-related stress/illness is significantly increased (Campbell & Spano, 2011; McArdle, Katch, & Katch, 2014; McArdle, Katch, & Katch, 2015; Porcari, Brant, & Comana, 2015; Terrados & Maughan, 1995). Given this, monitoring one’s hydration status can mitigate problems with heat stress during exercise. Again, prevention is key.
Depending on the resources available, there are several ways for an athlete to monitor hydration status. Without access to a refractometer to evaluate urine-specific gravity, athletes have other ways to assess hydration levels at his or her disposal. First, urinating frequently is a good sign of hydration level with a frequency of about two hours adequate (Reuter, ed., 2012).
Urine color can be an excellent gage of hydration level, but this has a few caveats.
After the athletes, the first void in the morning, the color of urine being pale yellow to almost clear indicate good to excellent hydration respectively (see chart reference below “REC Urine Chart,” n.d.). However, athletes ingesting certain supplements or foods (b-complex vitamins, beta-carotene, betacyanins, and some food colorings and medications) may have urine color changes (Campbell & Spano, 2011).
Measuring pre and post-exercise body weight to evaluate the hydration strategies’ efficacy to see if deficits exist is helpful. Any loss in weight reflects fluid loss. It has been suggested that it would take 150% of fluid replacement of weight loss to fully rehydrate (Campbell & Spano, 2011). Finally, weighing daily can help. An athlete weighing in the morning after voiding should attempt to maintain weight within 1% (if the goal is weight maintenance).
Adjustment and hydration occur in several ways.
First, nothing replaces a solid total nutrition plan. Oral hydration accounts for only a part of daily fluid intake, and the athlete should understand that food supplies a significant amount of water.
People who are underfed can be may have problems hydrating.
Along with oral fluids, ingesting fruits and vegetables provide valuable sources of fluids with vitamins and minerals (Campbell & Spano, 2011).
Fluid regulation pre and during exercise
At least 4-hours before exercise, the athlete should consume approximately 5-7 ml of fluid per kilogram of body weight (Campbell & Spano, 2011). If the person’s urine is not light 2 hours before another fluid ingestion of 3-5 ml/kg of body weight is warranted (Campbell & Spano, 2011). Simultaneously, consuming sodium-rich foods can stimulate thirst and help retain fluids (Campbell & Spano, 2011).
During activities lasting 60 minutes or longer to prevent significant dehydration, one recommendation is the athlete ingest 3-8 oz of a 6-8% carbohydrate-electrolyte beverage every 10 to 20 minutes (Campbell & Spano, 2011). Consuming a beverage (or snacks) with adequate sodium (460-1,150 mg/l) and possibly some protein will help retain fluids (Campbell & Spano, 2011).
Post-exercise rehydration is important to replenish and lost fluid and electrolytes. Consuming 150% of lost weight in fluids in a six-hour period can achieve normal hydration. A recommendation is to consume 20 to 24 ounces of fluid for every pound lost during exercise (Campbell & Spano, 2011). The athlete may wish to consider a drink with electrolytes to replace lost sodium and chloride (Campbell & Spano, 2011). Depending on the interval to the next bout of exercise, consuming sodium-rich foods and beverages may facilitate rehydration as sodium is one of the key nutrients for fluid retention and maintaining thirst (Campbell & Spano, 2011).
Campbell, B. I., & Spano, M. A. (2011). NSCA’s guide to sport and exercise nutrition (Science of Strength and Conditioning). Champaign, IL: Human Kinetics.
McArdle, W. D., Katch, F. I., & Katch, V. L. (2014). Sports and exercise nutrition (4th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise physiology: Nutrition, energy, and human performance (8th ed.). Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Porcari, J., Brant, C., & Comana, F. (2015). Exercise Physiology. Philadelphia, PA: F.A. Davis Company.
REC urine chart [Digital image]. (n.d.). Retrieved from http://1.bp.blogspot.com/-vLJ9-k7Uhx8/TgsztUW94SI/AAAAAAAAAEM/iNf0JaIALKo/s1600/REC+Urine+Chart+2011.png
Reuter, B. (Ed.). (2012). Developing endurance (NSCA Sports Performance Series). Champaign, IL: Human Kinetics.
Terrados, N., & Maughan, R. J. (1995). Exercise in the heat: Strategies to minimize the adverse effects on performance. Journal of Sports Sciences, 13(Sup1). doi:10.1080/02640419508732278
Tipton, C. M. (Ed.). (2006). ACSM’s advanced exercise physiology. Philadelphia, PA: Lippincott Williams & Wilkins.
Backwoods wanderer with a passion for backpacking, hiking, kayaking, and exploring the wilds of the Catskills and Adirondacks in New York. A Catskill 3500 Club Member and Adirondack Forty-Sixer. Climbed Mount Rainier. Professionally an Exercise Physiologist.