Research

• Supplemental Feeding and Cognitive Function
• Core Temperature and Hydration
• Water, Electrolytes, and Hydration

This section reviews project-related field studies conducted on wildland firefighters during the 2004 fire season. The studies were conducted by researchers from the University of Montana Human Performance Laboratory in cooperation with MTDC and with support from the National Wildfire Coordinating Group.

Supplemental Feeding and Cognitive Function

Field studies have shown that wildland firefighters can maintain their blood glucose levels by ingesting supplemental carbohydrate, either as a liquid or solid. Supplemental carbohydrate allows firefighters to increase their total daily work without affecting their perception of how hard they have worked. This study examined the effect of supplemental carbohydrate on cognitive performance and decisionmaking.

Hotshot crews performed reaction and mathematical processing (cognitive) tests on a hand-held microprocessor. The cognitive test was completed and a blood glucose sample was taken before breakfast, immediately before the work shift, and immediately after the work shift. Subjects recorded hourly estimates of work intensity and wore an activity monitor that measured energy expenditure throughout the shift. Data were collected on each subject during two 12-hour days of wildland fire suppression, once with subjects drinking supplemental carbohydrate every hour, and once with subjects drinking a flavored placebo.

When subjects ingested 200 milliliters of a 20-percent carbohydrate liquid every hour (160 kilocalories), they maintained blood glucose and sustained a significantly higher level of work output (figure 1).

Figure 1— Firefighters worked harder when
they received supplemental carbohydrate.
The hourly average activity counts are
measured over a 12- hour shift.

Source: Effect of Supplemental Feeding on Cognitive Function in Wildland Firefighters During Arduous Fire Suppression. S. Gaskill, B. Ruby, A. Goodson, A. McClaughry, and J. Cuddy. Paper delivered at the annual meeting of the American College of Sports Medicine, 2005. (Cognitive function test equipment provided by the U.S. Army Research Institute for Environmental Medicine).

Core Temperature and Hydration

Wildland firefighters work long shifts in extreme environmental conditions. Temperature regulation and hydration status are among the factors that affect cognitive and physical performance. This investigation determined the effects of the work performed during wildland fire suppression on temperature regulation and drinking behavior during an arduous day on the fireline.

Subjects included male (n = 16) and female (n = 4) firefighters from various hotshot and district crews. Core, skin, and ambient temperature and self-selected work rate (recorded by activity monitors) were measured using a wireless physiological monitoring system.

Core temperature was measured using an ingested temperature sensor. Drinking characteristics were recorded with a previously validated digital flow meter (Medicine and Science in Sports and Exercise 33(5): S257, 2001), which allowed researchers to measure drink volume (milliliters per hour), drink frequency (drinks per hour), and total volume consumed (liters per workshift).

Urine specific gravity was measured at the second morning urination, late morning, late afternoon, and 1 hour after the shift. Data were analyzed throughout the day by comparing average morning and afternoon workshift values using repeated measures analysis of variance.

Ambient temperature increased from 24.8 °C in the morning to 34.0 °C in the afternoon (figure 2). Core temperatures increased significantly throughout the day (from 37.2 °C in the morning to 37.8 °C in the afternoon), as did skin temperatures (from 32.7 ° C in the morning to 34.67 °C in the afternoon).

Figure 2—Firefighters’ core temperatures and ambient
temperatures throughout the workshift.

 

Statistical significance means that the possibility that these results were solely due to chance is less than 5 percent, or p < 0.05. Drinking volume was significantly higher during work hours 8 to 15 (275 milliliters per hour in the morning and 583 milliliters per hour in the afternoon). However, drinking frequency (drinks per hour) was similar from hours 1 to 7 and 8 to 15 (5.3 drinks per hour in the morning and 7.4 drinks per hour in the afternoon).

Nude body weight decreased significantly after the shift (0.7 kilograms or 1.54 pounds). Urine specific gravity increased significantly throughout the workshift (from 1.019 in the morning to 1.023 in the afternoon, values higher than 1.020 indicate dehydration).

Self-selected work rate (mean activity counts per hour) declined between early and later segments of the workshift, but the difference was not statistically significant (502 activity counts per hour in the morning, compared to 450 counts per hour in the afternoon).

These data demonstrate that extended arduous work in the heat is associated with a rise in ambient, core, and skin temperatures during the day and with self-selected drinking volume. The similarity in hourly activity counts during the early and later segments of the workshift suggests that drinking behavior may be related more to temperature changes than work rate. The reduction in body weight and the increase in urine specific gravity suggest that although drinking volume increased throughout the day, it was not sufficient to maintain adequate hydration.

Source: Factors Affecting Core Temperature and Hydration During Extended Arduous Work. J. Ham, S. Harger, S. Gaskill, and B. Ruby. University of Montana. Paper delivered at the annual meeting of the American College of Sports Medicine, 2005. (Supported by a grant from the Mini Mitter Co., Bend, OR. Hydration monitoring system provided by the U.S. Army Research Institute for Environmental Medicine.)

Water, Electrolytes, and Hydration

Our laboratory recently demonstrated a mean rate of water turnover of 95 milliliters per kilogram per day in elite wildland firefighters during 5 days of extended work shifts (Medicine and Science in Sports and Exercise 35(10): 1760–1765, 2003). For example, a 165-pound (75-kilogram) firefighter would have a mean rate of water turnover of 95 milliliters times 75 kilograms or 7.13 liters per day.

This study evaluated the effects of water or water and supplemental electrolytes on core body temperature, self-selected work, hydration, and drinking behavior during arduous wildland fire suppression. Subjects included male (n = 16) and female (n = 4) wildland firefighters from various hotshot and district crews during the 2004 Fischer Fire in Washington.

Subjects were randomly placed into a group (n = 10), that consumed only water during the entire workshift or a group (n = 10) that consumed water with an electrolyte additive (45 milligrams of magnesium, 125 milligrams of sodium, 390 milligrams of chloride, and 130 milligrams of potassium per liter). Each subject was outfitted with a wireless data recording system for continuous measurement of core, skin, and ambient temperatures and self-selected activity (Mini Mitter activity monitor).

Nude body weight was recorded before and after the workshift. Urine specific gravity was measured at the second morning urination, late morning, late afternoon, and 1 hour after the shift. Drinking behavior was measured with a previously validated drinking reservoir system. Data analysis used a mixed design analysis of variance with repeated measures.

Nude body weight decreased significantly during the workshift (from 79.8 kilograms to 79.1 kilograms, a loss of 0.7 kilograms or 1.54 pounds) for the entire group. While the group that consumed electrolyte additives lost more weight (0.96 kilograms compared to 0.44 kilograms), the difference was not statistically significant. There were no differences between ambient, core, or skin temperatures throughout the day for the two groups. Average hourly self-selected work rate was similar for both groups (439 activity counts per hour without electrolyte additives compared to 487 counts per hour with additives).

Urine specific gravity increased significantly during the workshift in the group that received electrolyte additives (from 1.019 in the morning to 1.025 in the afternoon). However, urine specific gravity was unchanged in the group that did not receive additives (1.019 in the morning compared to 1.021 in the afternoon).

Total drinking volume was significantly higher for the group that did not receive electrolyte additives (7.5 liters compared to 4.3 liters, figure 3). Based on the disparity in drinking volume and the similarity in weight loss, these data indicate that consuming electrolyte additives in water may act to preserve total body water during arduous work in extreme environments, when access to water may otherwise be limited.

Figure 3—Hourly drinking volume for the groups
of firefighters that just received water, compared
to the group that received electrolyte additives in water.

Source: Effects of Water and Water + Electrolytes on Changes in Body Temperature, Hydration Status, and Drinking Behaviors During Arduous Work. B. Ruby, S. Montain, S. Harger, J. Ham, and S. Gaskill. University of Montana and U.S. Army Research Institute for Environmental Medicine. Paper presented at the annual meeting of the American College of Sports Medicine, 2005. (Supported by a grant from the Mini Mitter Co., Bend, OR. Hydration monitoring system provided by the U.S. Army Research Institute for Environmental Medicine.)