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Wildland Firefighter Health and Safety
Recommendations of the April 1999 Conference

Energy Expenditure and Energy Intake During Wildfire Suppression in Male and Female Firefighters

Brent C. Ruby, Ph.D.
Human Performance Laboratory
University of Montana


Historically, wildland firefighters have been required to pass an aerobic fitness test (minimum score of 45 ml/kg-min) before employment. They undergo strenuous physical training before the fire season. Forest fire suppression involves arduous work (average = 7.5 kcal/min) for prolonged periods (12 to 24 hours) in difficult environmental conditions (heat, altitude), including exposure to smoke from forest fires (including respirable particulate, carbon monoxide, formaldehyde, acrolein, benzene from pumps, chain saws, and engines). A job task analysis identifies tasks such as packing heavy loads, building fireline with handtools, and emergency responses, often under arduous and dangerous field conditions. Firefighters perform their duties while wearing personal protective gear that balances the needs for protection, performance, and mobility. In busy fire seasons, firefighters may work as many as 21 days without relief, with meals provided by field rations, a remote fire camp, or an organized camp with full field kitchen. Even in the best camps, firefighters sleep on the ground in tents. When combined with pre- and postseason work on prescribed fires, firefighters may log over 100 days of work in a season and more than 1,000 hours in overtime. For these reasons, wildland firefighting simulates the stresses and strains of extended military operations. The job involves long hours of meaningful work that cannot be duplicated in controlled or make-work studies. Motivation to perform is maintained by a sense of mission and the pride of accomplishment. The fit and highly motivated women working on fire crews represent the capabilities of women who are willing to train to meet the demands of the job.

Because of the job hazards and the unpredictable nature of the job, measurements during the fire season are often difficult to obtain. Similarly, the use of laboratory equipment in the field is often impractical due to expense and the need for a controlled environment. Therefore, it is difficult to obtain samples and data collection from the wildland firefighter during situations of arduous wildfire suppression without affecting the “true” work environment. If the health and safety of the wildland firefighter is the number one concern, it is imperative that we have as much information as possible to establish sound policy regarding work-rest cycles and the nutritional demands associated with the occupation. With the use of the doubly labeled water methodology, day-long energy expenditure during wildfire suppression can be established.

The purpose of this investigation was two-fold:


Seventeen subjects (n=9 females, n=8 males) from three type 1 Interagency Hotshot crews were recruited as subjects for the investigation. The investigation was conducted during the 1997 and 1998 fire seasons and involved five different wildfires (CA, FL, WA, ID). Subjects were selected based on job tasks, gender distribution, and after consultation with the crew bosses, dependability. All subjects provided written consent prior to data collection with an Institutional Review Board-approved consent form. Before the fire season, subjects underwent descriptive measures of nude body weight (kg), height (cm), and body composition measures (skinfold Jackson and Pollock 1978; Jackson et al. 1980). Body density was converted to percent body fat using the age/gender-specific equations of Lohman (1992). Subjects were distributed into groups of similar total body weight to calculate isotopic delivery dose for the total energy expenditure measurement period. Subject’s vials of ²H2O (99% APE) + H2 18O (8 and 10% APE) were mixed before the season to deliver a dose approximately equivalent to .14 g/kg body weight ²HO and .250g/kg body weight H2 18O.

Deployment and Isotopic Delivery

The research team was deployed at the same time as the type 1 crew to reach the crew before wildfire suppression activity began. At each fire, a small number of subjects (three to five) were given the oral dose of doubly labeled water as mentioned above. The basic data collection protocol/timeline is outlined in Figure 1. Before isotopic dosing (at approximately 2200-2300), each subject was allowed a normal dinner that was completed before 2000. After dinner, subjects were instructed not to consume additional foods until 0700 the following morning. A background urine sample was obtained from each subject immediately before isotopic delivery. Subjects consumed the isotopic solution in an approximate fluid volume of 160 to 180 mL. They continued drinking as the isotope container was rinsed three times with approximately 15 to 20 mL of distilled water to ensure complete consumption of the preweighed isotope. Subjects were instructed not to consume any additional water or food sources until additional samples were obtained the following morning. Any overnight voiding of fluid was collected and measured to correct the initially measured total body water. The following morning, the entire first void sample was collected and measured and a nude body weight was obtained. Within 60 minutes, a second void was also collected.

Daily urine samples were collected each morning (at 0400-0600) for the 5 to 7 day experimental period. Duplicate samples for each subject at each time point were placed into two separate tubes (Corning 4.5-mL cryogenic vials) and stored in a cooler. Following 5 to 7 days of wildfire suppression work, subjects were provided with a second dose of 2H2O (approximately 2.0 grams, 99% APE) in the evening to determine total body water at the end of the experiment. Following the collection of a background sample and the 2H2O dose, subjects were instructed to avoid food and beverage until the collection of first and second void urine samples the following morning. After the first void was collected, a nude body weight was obtained. Samples were analyzed for isotopic enrichments using an isotope ration mass spectrometer method as described by Schoeller et al. (1988).

Image showing the relationship of TEE protocol for isotopic delivery and sample collection.
Figure 1—Total energy expenditure (TEE) protocol for
isotopic delivery and sample collection.

Total water turnover rates were calculated from the established average total body water (based on a back extrapolation to the timing of the original dose), and the elimination rates of the 2H and 18O. Average daily CO2 production was calculated from the isotopic elimination rates, the total body water and the rate of water turnover. The total CO2 production (moles/day) was converted to Total Energy Expenditure (TEE, kcals/day) for the experimental period using an assumed food quotient (FQ) of 0.85 for each subject (volume CO2 (1.1+3.9/FQ) • 22.4).

Dietary Record Analyses

Dietary intake during the seasonal measures was completed using a detailed food record/inventory of all ingested materials. When subjects were accessible in camp, tray surveys and measures of pre and post food weights were used. However, this was only possible for some dinner and breakfast times. Total intake patterns were assessed for carbohydrate, fats, and protein.


The doubly labeled methodology allows the assessment of total energy expenditure (average kcals/24 hours for the entire study period) and the assessment of total body water. Table 1 shows the changes in nude body weight and body composition over the 5- to 7-day experimental period. It is important to note that the post body weight value has been normalized to adjust for changes in total body water.

Table 1—Changes in body weight, composition, and total tissue loss during the 5- to 7-day period of wildfire suppression using measures of total body water and skinfold.

  Males Before Fire After 5-7 Days†
  Body weight 75.8±7.0 75.4±7.0
  Fat body mass (kg) 6.4±2.7 6.0±3.1
  Fat-free mass (kg) 69.4±7.2 69.3±7.0

  Females Before Fire After 5-7 Days†
  Body weight 69.3±5.0 68.0±5.2*
  Fat body mass (kg) 15.1±4.5 14.3±4.1*
  Fat-free mass (kg) 54.1±4.0 53.7±3.0

  † Post 5-7 day value is normalized to variations in total body water
  * p < 0.05 that the difference in weight before the fire and after 5 to 7 days is due to chance.

Values of total energy expenditure are expressed in kcal/day and in kcal/kg body weight/day to normalize variations in expenditure due to total body weight. Table 2 shows the mean subject data for the male and female subjects. The TEE (kcal/24 hours) was significantly greater for males compared to females. However, when TEE was expressed relative to total body weight (kcal/kg/24 hours), there was no difference between males and females.

Table 2—Average estimates for total energy expenditure (TEE) during the 5- to-7 day period of wildfire suppression.

    TEE (kcal/24 hours) TEE (kcal/kg/24 hours)
  Males 4758±677 64.5±13.6
  Females 3550±675* 55.0±10.8
  Range 2872-6021 42.5-86.0

  * p < 0.05 that the difference between males and females is due to chance.

Table 3—Reported dietary intake patterns during the 5- to 7-day period of wildfire suppression.

    TEE (kcal/24 hours) % CHO % fat % protein
  Males 4068±939 47±6 36±8 16±3
  Females 3222±712* 59±8 28±8 13±2

  * p < 0.05 that the difference between males and females is due to chance.


The calculated values for TEE measured during wildland fire suppression are similar to values previously measured during military operations (Table 4). The unique aspect of this study is the use of the doubly labeled methodology in an unpredictable “field” environment where conditions cannot be controlled. This subject population may serve as an ideal model to determine the effects of arduous occupational exposure on dietary adjustments during actual field operations. In addition, these data serve as a “minimal” or “average” standard for energy expenditure during work on the fireline.


  Hoyt et al. (1991)
Moderate cold/altitude, heavy exertion
  Jones et al. (1993)
Extreme cold, moderate exertion
  Hoyt et al. (1994)
Moderate cold/altitude, heavy exertion

The doubly labeled water methodology also allows daily body water turnover rates (rH2O) to be calculated. This allows water ingestion during a typical 24-hour period including the arduous physical and environmental conditions associated with the job to be precisely estimated. The average rH2O for the subjects was about 8.0 liters/day (2.1 gallons or 8.5 quarts). These data further emphasize the hydration demands associated with arduous physical activity in the heat and indicate that water intake should be a priority among wildland firefighters during fire suppression. There are also concerns regarding how firefighters would be able to drink this much when it requires additional weight they must carry (2.1 gallons of water weighs 17.5 lb). Fresh drinking water should be available to ensure the hydration requirements of the job are met.

In reference to the dietary intake patterns, it is difficult to rely on “self-report” dietary intake records. Subjects often over and under report. After reviewing the total energy expenditure results, it appears that subjects were as accurate as possible given the unusual field conditions. Further research should be developed to understand the variety of dietary habits that persist among various crews (crew type, ethnic issues).

Although the males appeared to maintain a state of energy balance (maintenance of total body weight by matching energy expenditure with appropriate energy intake), the females did not maintain body weight (Table 1). However, the weight loss that did occur was the result of fat loss (a common occurrence associated with a typical aerobic exercise program designed to enable weight loss). What is troublesome are some of the eating attitudes that persist among crew members related to weight loss. Although our studies did not monitor common eating attitudes, it was alarming to hear anecdotal information regarding weight loss and the fire season. Several subjects stated that weight loss was common and that they typically used the first one or two fires of the season to “get in shape” for the main fire season. From a health and safety perspective, this practice should be discouraged. Figure 2 outlines the hazards associated with an inadequate diet (energy intake) and elevated occupational energy expenditure.

Image showing the schematic effects of negative energy balance.
Figure 2—Schematic of the effects of negative energy balance
on total body protein, cognitive function, and risk for injury.

Reasons for Concern and Projected Further Research

Several aspects of seasonal fire suppression and fireline work make them challenging from a dietary and energy balance perspective. The most apparent aspect is the unpredictable nature of the season that may lead to an abrupt persistent increase in total energy expenditure. It is difficult for most persons to suddenly increase their total intake to 4,000 to 6,000 kcal/day.

We are also uncertain whether the balance of carbohydrates, fats, and proteins are appropriate in the typical “caterer-supplied” diet. Although the caterers are held to a strict standard, the current standard has not been proven appropriate for the maintenance of muscle and liver stores of glycogen (carbohydrate). The wildland firefighter should be presented with the ability to ingest 250 to 500 grams of carbohydrate post shift in liquid and solid forms. If these are not readily available, the typical camp meal may not be restore muscle and liver carbohydrate levels before the next day’s work.

It is also not known whether wildland firefighters are able to meet macronutrient needs associated with the elevated energy expenditure (and perhaps eating fatigue—a general loss of appetite due to extreme physical exertion), creating a state of negative energy balance. We do not know if the typical wildland firefighter needs a multivitamin supplement to ensure micronutrient needs (calcium, iron, B-vitamins). Typically, if the total energy expenditure is matched with a diverse energy intake, the micronutrient requirements are met. However, this requires further study with this unique population that works under adverse conditions.

Although it is unclear whether the dietary intake patterns and standards are optimal, it is clear that the energy demands of wildfire suppression are extreme. Further field research should be conducted in a similar environment investigating:


Hoyt RW, Jones TE, Stein TP, HcAninch GW, Lieberman HR. Doubly labeled water measurement of human energy expenditure during strenuous exercise. Journal of Applied Physiology 71(1): 16-22, 1991.

Hoyt RW, Jones TE, Baker-Fulco CJ, et al. Doubly labeled water measurement of human energy expenditure during exercise at high altitude. American Journal of Physiology 266: R66-71, 1994.

Jackson AS and Pollock ML. Generalized equations for predicting body density of men. British Journal of Nutrition 40: 497-504, 1978.

Jackson AS, Pollock ML and Ward A. Generalized equations for predicting body density in women. Medicine and Science in Sports and Exercise 12: 175-182, 1980.

Jones BH, Bovee MW, Harris J, Cowan DN. Intrinsic risk factors for exercise-related injuries among male and female army trainees. American Journal of Physiology 21(5): 705-710, 1993.

Jukkala AH, Sharkey BJ. An improved wildland firefighting handtool. USDA Forest Service Project Report April 1988.

Lohman TG. Advances in Body Composition. Human Kinetics, Champaign, Il 1992.

Shoeller DA. Measurement of energy expenditure in free-living humans by using doubly labeled water. Journal of Nutrition. 118: 1278, 1988.


These studies have been supported by the Defense Women’s Health Research Program DAMD17-96-1-6329, Principal Investigator–Brent C. Ruby, Ph.D., Director, Human Performance Laboratory, Department of Health and Human Performance, McGill Hall, Missoula, MT 59813.

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