Research

• Carbohydrate Supplements and Immune Function
• Work, Fatigue, and Immune Function
• Carbohydrates and Work Rate
• Nutritional Attitudes of Wildland Firefighters

Carbohydrate Supplements and Immune Function

Short-term exercise studies have shown limited beneficial effect of carbohydrate supplementation on salivary immunogobulin-A (sIgA) levels after exercise. This study evaluated the effects of carbohydrate supplementation during a 12-h work shift on sIgA values immediately after the shift and 12 h later. During an extended shift of firefighting, 29 wildland firefighters were evaluated. In a single-blind, random-crossover design conducted on consecutive days, the firefighters received 200 mL/h of either a 20 percent carbohydrate drink (providing 160 kcal of carbohydrate per hour) or a drink without carbohydrate. At the end of the shift, the group that received a carbohydrate drink also received a drink with an additional 400 kcal of carbohydrate. Four-min forced salivary samples were collected before and after the shift and the morning after each treatment. Samples were analyzed using an Elisa assay. A two-way repeated measures ANOVA (analysis of variance) was used to evaluate the salivary data.

Immediately after the shift, both groups had a significant decrease in sIgA compared to values before the shift, but the decrease in sIgA was significantly less for the group that received supplemental carbohydrates than for the group that had not (figure 2). After 12 h of rest, sIgA values for firefighters who received carbohydrates had returned to the levels before the shift, but the sIgA levels of those who had not received carbohydrates remained significantly depressed. Additionally, during the final 6 h of their shifts, the subjects who received supplemental carbohydrates maintained higher blood glucose values than those who did not receive carbohydrates, and performed significantly (23 percent) more work (486 kcal/h of work compared to 395 kcal/h for firefighters who did not receive carbohydrates).

Figure 2—Changes in salivary IgA levels (µg/mL) before the shift,
immediately after the shift, and 12 h after the shift. Standard
error values are shown above each bar.

A: p < 0.05 that the difference in values compared to those before the shift within the carbohydrate trial was due solely to chance.

B: p < 0.05 that the difference in values compared to those before the shift within the placebo trial was due solely to chance.

C: p < 0.05 that the difference in values compared to the carbohydrate trial was due solely to chance.

D: p = 0.0553 that the difference in values compared to the carbohydrate trial was due solely to chance.

Prolonged arduous work has been shown to decrease sIgA and increase the incidence of upper respiratory tract infection. Carbohydrate supplementation during long shifts helps maintain the immune function in wildland firefighters and improves their recovery after extended arduous work, while allowing them to perform more work. The improved immune function may be related to decreased cortisol concentrations when blood glucose levels are maintained. The effect of improved immune function on upper respiratory tract infection rates in wildland firefighters has yet to be studied.

Effects of Carbohydrate Supplementation on sIgA During Long Duration Arduous Work by S. Harger, S. Gaskill, and B. Ruby, Paper presented at the annual meeting of the American College of Sports Medicine, Indianapolis, IN 2004. Supported by the USDA Forest Service, Missoula Technology and Development Center.

Work, Fatigue, and Immune Function

Maintaining health during the 6- to 8-month season of arduous wildland firefighting is important for crews. Little research has been done to monitor crew health, relating the amount of work accomplished and fatigue to immune function. This study compared sIgA fluctuations during a wildland firefighter’s duty cycle to the energy expenditure during work. We also compared the sIgA data to the results of a field test designed to monitor fatigue. Eleven elite wildland firefighters (hotshot crew members) were monitored during the first 9 d and the following mornings during a 14-d duty cycle. Firefighters wore activity monitors to estimate caloric expenditure. Salivary IgA was collected by a forced 4-min collection each morning before breakfast and immediately after the shift. Additionally, after each saliva sample, a bench step protocol was used to calculate a fatigue index from the sum of the resting heart rate (after 5 min of seated rest), exercise heart rate (after 1 min of bench stepping on an 8-in step, 30 steps per minute) and a seated recovery heart rate (30 s after stepping). Firefighting activities included fireline construction, chain saw work, cutting brush in steep mountainous terrain, and 2 d of lighter mopup duties (on days 3 and 4).

Average energy expenditure was 3,094 kcal/shift (4.3 kcal/min) ranging from 1,247 to 4,853 kcal/shift (figure 3). The peak sustained 10-min average work output (for all subjects) was 17.2 kcal/min. Salivary IgA was depressed at the end of each work shift (p < 0.01), but was fully or partially recovered by the next morning, after 10 to 12 h of rest and sleep. On the mornings after days of moderate and light work (less than 3,000 kcal—days 3, 4, and 5), immune function recovered to baseline values.

Figure 3—Changes in salivary IgA and fatigue index after days of variable
energy expenditure. Values that return to 0 percent reflect recovery. Larger
positive values represent fatigue (the fatigue index). Smaller negative
values indicate immune function suppression.

Error bars (that look like the letter ‘I’) show the standard error.

Vertical bars represent relative work during each shift.

The black line represents the percent change in the fatigue index.

The gray line represents the change in salivary IgA.

After days of heavy work (more than 3,000 kcal—days 1, 2, 6, 7, and 8), recovery the morning afterward was significantly reduced, ranging from 7 to 17 percent below the baseline values. By the 10th morning, after 3 d with an average 4,160 kcal of work per day and a moderate day with 2,340 kcal of work, immune function remained significantly (17 percent) depressed. The fatigue index values were inversely related to sIgA (r = -0.82) and followed a similar pattern of recovery (or lack of recovery), relative to work intensity. These data show that wildland firefighters become immune depressed after hard work (more than 3,000 kcal/d), especially after several days of sustained work of more than 3,000 kcal/d.

The fatigue index, using a simple standardized step test, appears as sensitive as sIgA in identifying patterns of immune depression and fatigue. Although these data suggest that physically fit wildland firefighters can sustain arduous duty cycles, days of lighter activity should be interspersed among arduous days to sustain immune function and to prevent fatigue from accumulating.

Relationship of Work to Salivary IgA and Fatigue in Wildland Firefighters by S. Gaskill and B. Ruby, University of Montana. Supported by the USDA Forest Service, Missoula Technology and Development Center.

Carbohydrates and Work Rate

Although supplemental carbohydrate feedings are common during extended endurance exercise, limited data are available to support the efficacy of supplemental feeding in arduous occupational settings, such as wildland firefighting. Energy expenditure for wildland firefighters may exceed 6,000 kcal/d.

This study determined the effects of liquid and solid carbohydrate supplements on work rates during extended arduous wildland fire suppression. Subjects included 10 hotshot and 5 type II wildland firefighters at two wildland fires in Montana during the 2003 season. Subjects consumed liquid supplements every even hour and a solid supplement every odd hour or a similarly flavored placebo drink in a counter-balanced crossover design. The liquid supplement was 200 mL/h of 20 percent carbohydrate (160 kcal/h). The solid supplement was 25 g carbohydrate, 10 g protein, and 2 g fat (160 kcal/h). Firefighters were allowed to drink water as they wished during the day.

Blood samples were collected at 2-h intervals with automated glucometers. The firefighter wore accelerometers (the Actical made by Mini Mitter, Inc.) that recorded their hourly work rate. Data were analyzed using a two-way ANOVA with repeated measures.

Firefighters who received carbohydrate supplements during their work shift had significantly higher blood glucose levels immediately before lunch and 4 and 6 h after lunch (figure 4). The work rate was significantly higher during the entire work shift for firefighters who received the liquid and solid carbohydrate supplements compared to those who received the placebo. This was especially true late in the shift, when the work rates were nearly twice as high for firefighters who received carbohydrate supplements (figure 5). The results show that firefighters accomplished more work during arduous wildfire suppression when they received 160 kcal/h of carbohydrate supplements. This approach to providing supplemental energy should be evaluated for its ability to enhance cognitive function, critical decision making, and overall safety on the fireline.

Figure 4—Changes in blood glucose during the wildland fire
suppression work shift. The probability is less than 0.01 that
the difference between the blood glucose values for
firefighters who received carbohydrate supplements
and those who received placebos was due to chance.

Figure 5—Work accomplished during a wildland firefighting
shift as monitored with accelerometers worn by firefighters.
Standard error bars are shown.

Carbohydrate Feedings Increase Self-Selected Work Rates During Arduous Wildfire Suppression by B. Ruby, S. Gaskill, S. Harger, D. Heil, and B. Sharkey. Paper presented at the annual meeting of the American College of Sports Medicine, Indianapolis, IN 2004. Supported by the USDA Forest Service, Missoula Technology and Development Center, and Gatorade.

* The probability is less than 0.01 that the difference in values for firefighters who received carbohydrate supplements and those who received placebos was due solely to chance.

Nutritional Attitudes of Wildland Firefighters

The purpose of this study was to determine the beliefs and attitudes of wildland firefighters about carbohydrate, protein, hydration, and nutritional supplements. The survey included 71 questions addressing general nutrition, fluid and hydration needs, protein and carbohydrate needs, attitudes toward nutritional supplements, and sources the firefighters relied on for nutrition information. The 123 wildland firefighters who voluntarily filled out the survey had common misconceptions regarding carbohydrate, fat, protein, and the use of nutritional supplements. The subjects were unaware of the role of carbohydrate, fat, and protein as energy sources during arduous work. The contributions of vitamins and minerals were also unclear. The respondents were best informed in the area of hydration and fluid replacement, where they answered correctly 71 percent of the time. The most influential sources of nutrition information were the catering system, food unit leader, crew supervisor, and family members. In the survey, 75 percent of the respondents agreed that a basic class in nutrition should be one of the requirements for wildland firefighter training. This survey showed that fire crew members had numerous misconceptions and lack information concerning the nutritional demands of wildland firefighting. The authors recommended that nutrition education be incorporated into the basic training of elite wildland firefighters.

Nutritional Attitudes and Beliefs of Wildland Firefighters by K. Kodeski, B. Ruby, S. Gaskill, B. Brown, and A. Szalda-Petree, University of Montana. Paper submitted for publication in the Journal of Nutrition Education and Behavior.