Field Notes

Assessing Hydration

During work in the heat, individuals never voluntarily drink as much water as they lose, replacing only two-thirds of the water loss. This phenomenon is called voluntary dehydration. Because of the delay between fluid loss and thirst, some individuals maintain a chronic state of dehydration equivalent to a water deficit of 2 L, or almost 3 percent of body weight. This voluntary dehydration increases with rising ambient temperature, work rate, and sweat rates. Workers in hot environments should be considered dehydrated unless they have recently been forced to drink more water. Remember:

Thirst always underestimates fluid needs.

Dr. Lawrence Armstrong of the University of Connecticut Human Performance Laboratory validated a urine color chart in a series of studies that confirmed the relationship of urine color to laboratory measures of hydration, including the specific gravity and osmolality of urine. Wildland firefighters can use the chart to evaluate hydration status before, during, and after work and exercise training. Urine darker than a pale yellow or straw color indicates dehydration.

Urine color is best observed against a white background, such as a toilet bowl. Observing urine color may be more difficult in fire camp or in the field. A disposable white paper cup may be used in fire camp (dispose of cups in the trash, not in portable toilets). While urine color may change temporarily in response to illness, some medications, vitamin supplements, and food pigments, Dr. Armstrong's studies found these effects to be minimal.

Armstrong, Lawrence E. 2000. Performing in extreme environ-ments. Champaign, Il: Human Kinetics (E-mail: humank@hkusa.com).

Food and Immunity

Upper respiratory problems account for 30 to 50 percent of the visits to fire camp medical tents. Ailments include coughs, colds, and sore throats. The fall 2000 edition of this report included a discussion of the human immune system and how factors in the wildland fire environment can suppress immune function. Exhaustion, stress, inadequate nutrition, dehydration, rapid weight loss, sleep loss, and exposure to smoke can suppress immune function. Conversely, good nutrition has been shown to maintain or enhance immune function, even in physically demanding environments.

Firefighters can burn up to 6,000 kcal daily in arduous work duties. The calories need to be replaced to avoid rapid weight loss and fatigue. How those calories are replaced can influence the function of the immune system. A recent study by Pedersen and others (2000) indicates that during a 7-week period of exertion, a high-fat diet (62 percent of calories from fat) was detrimental to the immune response when compared to a high carbohydrate diet (65 percent of calories from carbohydrate). Natural killer (NK) cell activity, one measure of immune response, increased 11 percent on the carbohydrate-rich diet, while it decreased 6 percent on the high-fat diet.

Fire camp meals can contain as much as 50 percent of calories from fat. Firefighters should limit fat intake and increase the intake of complex carbohydrates (whole grain breads, pasta, potatoes, corn, and rice). This is especially true during an extended fire season. A diet containing 55 to 65 percent of calories from carbohydrates, 15 percent from protein, and 25- to 30-percent fat provides the energy needed to fuel muscles during long days and weeks on the fireline.

Other studies indicate the value of maintaining energy during activity, using carbohydrate-rich energy bars and a carbohydrate/electrolyte (C/E) beverage. Immune function declines during and after periods of exhausting work. Maintaining energy with energy bars between meals and C/E beverages minimizes the decline in immune function. In addition to the effect on the immune system, energy is available to fuel muscular contractions and maintain mental function during work. The body's use of carbohydrates increases during work in the heat. The brain and nervous system depend on carbohydrate energy. When blood-sugar levels drop, the worker becomes confused, irritable, and more prone to poor judgment and accidents.