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American Journal of Clinical Nutrition, Vol. 83, No. 5, 1068-1075, May 2006
© 2006 American Society for Nutrition

ORIGINAL RESEARCH COMMUNICATION

Negative energy balance in male and female rangers: effects of 7 d of sustained exercise and food deprivation1,2,3,4

Reed W Hoyt, Per Kristian Opstad, Ann-Helen Haugen, James P DeLany, Allen Cymerman and Karl E Friedl

1 From the US Army Research Institute of Environmental Medicine, Natick, MA (RWH, AC, and KEF); the Norwegian Defence Research Establishment, Kjeller, Norway (PKO and A-HH); and the Pennington Biomedical Research Center, Baton Rouge, LA (JPD)

2 The opinions and assertions contained in this manuscript are the personal views of the authors and do not necessarily represent the official views or policy of the US Department of the Army.

3 Supported by the Norwegian Defence Research Establishment, Kjeller, Norway, and the Military Operational Medicine Research Program, US Army Medical Research and Materiel Command, Fort Detrick, MD.

4 Reprints not available. Address correspondence to RW Hoyt, MCMR-BMD, US Army Research Institute of Environmental Medicine, Natick, MA 01760-5007. E-mail: reed.hoyt{at}us.army.mil.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background:A challenging 7-d ranger field exercise (FEX) by cadets in the Norwegian Military Academy provided a venue in which to study the effects of negative energy balance.

Objective:We quantified total energy expenditure (TEE), food intake, and changes in body composition in male and female cadets.

Design:TEE (measured by doubly labeled water), food intake, activity patterns (measured by accelerometry), and body composition (measured by dual-energy X-ray absorptiometry) were measured in 16 cadets (10 men and 6 women aged 21–27 y).

Results:The physically active ({approx}23 h/d) and semistarved (0.2–2.2 MJ/d) cadets lost weight (x ± SD: men, –7.7 ± 1.1 kg; women, –5.9 ± 1.1 kg; P < 0.05). Absolute TEE differed by sex (men, 26.6 ± 2.0 MJ/d; women, 21.9 ± 2.0 MJ/d; P < 0.05) but body weight–specific TEE did not (men, 343 ± 26 kJ · kg–1 · d–1; women, 354 ± 18 kJ · kg–1 · d–1; NS). Fat-free mass (FFM) loss differed significantly by sex (men, –4.0 ± 1.2 kg; women, –2.5 ± 1.1 kg; P < 0.05), but percentage FFM loss did not (men, –6.3 ± 1.9%; women, –5.6 ± 2.4%). In contrast, absolute FM loss did not differ significantly by sex (men, –3.45 ± 0.72 kg; women, –3.42 ± 0.22 kg), but fat oxidation (men, 5.2 ± 1.0 mg · min–1 · kg FFM–1; women, 7.3 ± 0.5 mg · min–1 · kg FFM–1) and the relative contribution of FM to TEE (men, 74 ± 14%; women, 89 ± 6%) were significantly greater in women than in men (P < 0.05).

Conclusion:Female cadets maintained a significantly more fat-predominant fuel metabolism than did male cadets in response to sustained exercise and semistarvation.

Key Words: Starvation • sustained exercise • body composition • sex • water intake • military rangers • Norway


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
As part of their training, male and female Norwegian Military Academy cadets participate in a challenging 5–7-d field exercise (FEX) course characterized by sleep and food deprivation and sustained physical activity (1). The inclusion of women in this annual FEX ranger training course provides a unique opportunity to compare responses to extreme physical demands between the sexes. The US Army, in contrast, excludes women from participating in combat arms training courses (2).

Scientific studies of male cadets participating in the FEX course have provided scientific insights into the effects of sleep deprivation (3, 4), alterations in physical and psychological performance under stress (5, 6), and endocrine adaptations (1, 7, 8). However, only a limited amount of information is available on energy expenditure and substrate use by cadets in the FEX, and little is known about the physiologic responses to the FEX in female cadets.

Total energy expenditure (TEE) during FEX training was estimated to be 33–40 MJ/d by the heart rate method (3, 9). A similar TEE was evident in a FEX study in which an average food energy intake of {approx}33 MJ/d was needed to maintain body weight (6). These TEEs, at 4–5-fold the resting metabolic rate (RMR), are similar to the high rates of energy expenditure of US Marines training in mountainous, cold-weather conditions (3–4-fold the RMR) (10) and of Tour de France bicycle racers ({approx}5 x the RMR) (11).

The FEX offered an opportunity to study sex differences in substrate use under extreme conditions of physical activity and food deprivation. Laboratory studies of short-term exercise suggest that females maintain a more fat-predominant and less carbohydrate-dependent fuel metabolism than do males (12, 13). Across a range of moderate exercise intensities, both the fractional contribution of fat oxidation to TEE and the rate of fat oxidation per kg fat-free mass (FFM) were reported to be greater in women than in men (12). The apparently greater rate of fat oxidation during submaximal exercise in women appears to be promoted by estrogen (14). However, not all studies report sex differences in substrate oxidation during exercise (15–17), which suggests that further study is warranted.

The present study assessed the responses of healthy male and female soldiers in response to the extreme demands imposed by 7 d of sustained exercise and food deprivation. The specific goals were to accurately quantify TEE and changes in body composition and to compare the responses of men and women.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The test subjects gave their informed consent to be studied during their FEX training course after being informed of the purpose, risks, and benefits of the study. The subjects understood that they were free to withdraw from the study at any time. This study was approved by the Institutional Review Boards of the US Army Research Institute of Environmental Medicine and the US Army Medical Research and Materiel Command.

The test volunteers were 16 healthy and physically fit classmates (10 men and 6 women) from the Norwegian Military Academy, Kjeller, Norway, who participated in the FEX course. The FEX, which is held each summer, seeks to teach cadets combat leadership, an appreciation of personal and team-member responses to stressful conditions, and an ability to endure physical and mental stress. The FEX typically involves severe food restriction, sleep deprivation, and periods of sustained physical activity. Water is freely available. Activities include long-distance foot marches, simulated combat patrols, traversing obstacle courses, and marksmanship training.

The cadets participated in 1 of 2 similar 7-d FEX iterations; the data from these 2 FEX iterations were combined. In the first FEX (study 1), 6 men and 4 women were studied; in the second FEX (study 2), 4 men and 2 women were studied. In addition, urine samples were obtained from a separate set of 4 male cadets (2 per FEX iteration) who were not given doubly labeled water (DLW). As described below, changes in background isotopic 2H2O and H218O enrichments in these samples were used to correct DLW TEE calculations (18). These 4 cadets participated in the same training activities and lost amounts of body weight (7.93 ± 1.16 kg) similar to those in male cadets in the main study group.

The experimental test schedule is shown in Figure 1Go. The FEXs were conducted in forested military training areas northwest of Oslo, Norway, at an altitude of {approx}500 m, in June, when the weather was warm by day (20–30°C) and cool at night (5–15°C). Meteorologic conditions in the training area were obtained from local weather stations operated by the Norwegian government weather service. Field training activities started at {approx}0400 on day 1 and ended at {approx}1000 on day 8 (total FEX duration: 7.25 d).


Figure 1
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  		FIGURE 1. Study schedule. The energy balance of 10 male and 6 female military cadets participating in a 7-d field exercise (FEX) involving sustained exercise and food restriction was assessed. Data were collected during 2 FEX iterations (study 1: n = 6 men and 4 women; study 2: n = 4 men and 2 women). Total energy expenditure (TEE) during the FEX was measured by the doubly labeled water (DLW) method. Total body water was determined before and after the FEX by deuterium oxide dilution. Food energy intake was monitored during the FEX. Activity-inactivity patterns were assessed in a subset of subjects with the use of wrist-worn activity monitors (study 1 only; n = 3 men and 2 women). Body composition was assessed before and after the FEX by dual-energy X-ray absorptiometry (DXA).

 
Total energy expenditure and total water turnover determinations
Daily TEE was assessed by the DLW method. On the morning of day 0, each volunteer, who had refrained from eating or drinking for ≥12 h, reported to the testing area with a baseline sample of their first-void urine. After providing baseline saliva samples, each subject drank 0.25 g H218O/kg total body water (TBW) (Isotec Inc, Miamisburg, OH), 0.18 g 2H2O/kg TBW (Cambridge Isotope Laboratories, Andover, MA), and 100 mL tap water, which was used to rinse the dose container. TBW was assumed to be 73% of estimated FFM for the purposes of calculating the DLW dose. To confirm isotopic equilibration, saliva samples were collected 3 and 4 h after tracer ingestion. Actual TBW was calculated from isotopic enrichments in the 4-h saliva sample (19). The subjects were free to eat and drink only after the final saliva sample was collected. First-morning void urine samples were collected at the start of the energy expenditure period on day 1 and on day 7 and day 8. A second and final determination of TBW was made on the morning of day 8 with the use of a dose of 0.18 g 2H2O/kg TBW.

Daily TEE was calculated by using the 2-point method, and changes in baseline isotopic abundances and changes in TBW were corrected for. The rate of carbon dioxide production (rCO2) was calculated by using the equations of Schoeller et al (20) with modification (21, 22):

Formula 1(1)
where N is the average TBW. Average TBW was calculated as initial TBW, determined from the 18O enrichment in the 4-h saliva sample, minus one-half the change in TBW. The change in TBW during the FEX was calculated as the difference between initial and final TBW by using the respective 4-h postdose salivary 2H2O enrichments. rH2Of is the rate of fractionated evaporative water loss, estimated as 1.05N(1.007kO – 1.041kD). Energy expenditure was calculated from rCO2 by using a metabolic fuel quotient of 0.75 that was derived from food intake, changes in body energy stores, and conventional calorimetric relations (23).

Water turnover (RH2O), or total water flux, was calculated as the product of the 2H2O dilution space and the deuterium elimination rate (kD). The isotopic enrichment of body water declines because of the elimination of labeled water via excretion and evaporation and the influx of unlabeled water from dietary, metabolic, and atmospheric sources. The tritiated water elimination method has been validated in animals (24, 25), and the 2H2O elimination method of measuring preformed dietary water intake has been validated in humans (26). Typically, {approx}10% of the apparent total water influx can be attributed to respiratory and cutaneous water influx and metabolic water production, with the balance associated with preformed dietary water intake.

Isotopic analyses of 18O and deuterium were performed as previously described (27). Briefly, 18O abundances were measured by equilibration of fluid with carbon dioxide. Measurements were done in duplicate, with an SD of 3 x 10–5 atom% (0.15{per thousand}). Deuterium abundances were measured by the zinc reduction method. Measurements were performed in duplicate, with an SD of 1.7 x 10–5 atom% (1–2{per thousand}). Isotope enrichments were calculated by taking the arithmetic difference between the per mil ({per thousand}) enrichment of each sample and the respective predose sample.

Food intake
Nutrient values for the food consumed during the FEX were determined by using Moore’s Extended Nutrient (MENu) database (version 2.0, 1996; Pennington Biomedical Research Center, Baton Rouge, LA). The dietary analysis software portion of this personal computer database uses information from the USDA Nutrient Data Base for Standard Reference, release 10 (1992; US Department of Agriculture, Human Nutrition Information Service, Hyattsville, MD); the USDA Nutrient Data Base for Individual Food Intake Surveys, release 7; and the Extended Table of Nutrient Values (mainframe version).

Food intake was severely restricted. In study 1, the weekly total for each 6-person squad was one-half of a smoked eel ({approx}250 g), a container of liver paste (500 g), and half of a chicken ({approx}750 g). Individual weekly dietary intake totals were {approx}1.43 MJ, 6 g carbohydrate, 50 g protein, and 24 g fat. In study 2, each cadet was provided 2 US Army Long Range Patrol rations, one on day 2 and one on day 5. These restricted rations, intended to be used for periods of ≤10 d, provided an individual weekly total of 10.2 ± 1.7 MJ dietary energy, 323 ± 68 g carbohydrate, 96 ± 13 g protein, and 85 ± 12 g fat. Supplemental food, consisting of potato, bread, beer, and sausage, added {approx}3.1 MJ energy, 113 g carbohydrate, 16 g protein, and 26 g fat to the diet. The estimated total individual dietary energy intake during study 2 was 13.3 MJ/wk or 1.9 MJ/d.

Activity monitoring
In study 1, activity patterns were monitored in a subset of subjects (n = 5; 3 men and 2 women) by wrist-worn, accelerometer-based, activity monitors (Actigraph; Precision Control Design Inc, Fort Walton Beach, FL). The amount of inactivity (apparent sleep) was calculated immediately before (from day –2 to day 0) and during the first 4 d of the FEX (day 1 to day 4) by using the Cole-Kripke algorithm (10, 28). Technical problems prevented activity data from being recorded on study days 5 through 7. Activity data were recorded as the number of zero crossings that occurred during a continuous series of 1-min epochs, where zero crossing was defined as the number of times the output voltage of the motion sensor crossed the zero voltage threshold.

Body composition, body energy stores, and RMR
Body composition (FM, FFM, and bone mass) was assessed by dual-energy X-ray absorptiometry (DXA, Hologic QDR-2000, Enhanced Array Whole Body V5.60A; Hologic Inc, Waltham, MA) before the FEX, either on day –4 (study 1) or day 0 (study 2), and immediately after the FEX on day 8. Body weight was measured before and after the FEX with a calibrated electronic scale accurate to ±0.05 kg. In addition, in study 1, total weight (ie, body weight plus items worn or carried) was measured on day 3 (n = 5) and on day 4 (n = 9). In study 1, DXA measurements were made on day –4 rather than on day 0 due to limited access to the test volunteers. We evaluated our assumption that the subjects were in energy balance between day –4 and day 0 (start of the FEX) by calculating the change in total-body energy content by using body weight, height, and DXA- and isotope dilution–derived TBW (29). The fractional hydration of FFM was calculated as TBW/FFM by using measured TBW values. Energy equivalents of 18.42 kJ/g protein and 39.77 kJ/g fat were used (30).

The change ({Delta}) in body energy stores was estimated as follows:

Formula 2(2)
where {Delta}ES is the change in body energy stores, {Delta}FM is the change in fat mass, {Delta}FFM is the change in FFM, and (1 – FFM hydration) is the nonaqueous fraction of FFM.

Estimated fat energy reserves were calculated as FM – structural FM. Structural FM, defined as that portion of body fat unavailable to meet fat fuel needs, was estimated by using 5% and 10% body fat as the minimums in healthy, physically active, underfed men (31) and women. Because the minimum percentage body fat of healthy women is not well-defined, we assumed a 10% minimum—a value corresponding to a body mass index (BMI; in kg/m2) of {approx}16.5 in the female cadets.

Resting metabolic rate (MJ/d) was estimated as [370 + (21.6 x FFM)] x 0.004186 (32), where FFM was measured by DXA, and the factor 0.004186 was used to convert kilocalories to megajoules. Physical activity level (PAL) was calculated as TEE/RMR, whereas absolute activity energy expenditure (AEE) was calculated as TEE – RMR.

Statistics
Two-sample t tests and one-factor analysis of variance were used to assess between-sex differences. Analysis of covariance was performed with either FFM or body weight as covariates and sex as the between-subjects variable. Coefficients of determination (R2) were calculated for the relations of the dependent variables AEE and TEE to the independent variables body weight and FFM (SPSS 12.0 statistical software; SPSS Inc, Chicago, IL). Instead of using simple ratio scaling to calculate mass-specific TEE and total RH2O values, corrected values were computed to account for x intercepts different from zero (33). A P value <0.05 was considered significant. Unless otherwise noted, n = 16 (10 men and 6 women). Values are expressed as means ± SDs.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The physical characteristics of the subjects reflected the expected sex dimorphism (Table 1Go). Significant sex differences were evident, both before and after the FEX, in percentage body fat (before: see Table 1Go; after: men, 12.7 ± 3.3%; women, 22.6 ± 4.8%), body FM index (in kg/m2; before: men, 3.9 ± 0.9; women, 5.9 ± 1.1; after: men, 2.8 ± 0.8; women, 4.6 ± 1.1), and FFM index (in kg/m2; before: men, 20.4 ± 1.0; women, 16.8 ± 1.4; after: men, 19.1 ± 1.1; women, 15.9 ± 1.3; P < 0.05 for all). However, no significant sex differences were evident before or after the FEX in BMI (before: men, 24.6 ± 1.2; women, 23.0 ± 1.5; after: men, 21.9 ± 1.5; women, 20.5 ± 1.4), estimated fat energy reserves (before: men, 8.4 ± 2.6 kg; women, 9.9 ± 2.9 kg; after: men, 4.9 ± 2.3 kg; women, 6.4 ± 2.8 kg), or the percentage loss of initial FFM (men, –6.3 ± 1.9%; women, –5.6 ± 2.4%; NS).


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  		TABLE 1 Baseline characteristics of the subjects1

 
The differences in food energy intake and the timing of the pre-FEX DXA measurements between study 1 and study 2 did not appear to be significant factors in the data analysis. Food energy intake in study 1 (0.2 MJ/d; n = 6 men and 4 women) and study 2 (1.9 MJ/d; n = 4 men and 2 women) met {approx}1% and 9% of the cadet’s energy needs, respectively. This difference in food energy intake was not associated with any significant between-study differences in the absolute or relative change in FM, FFM, or body weight (P = 0.60–0.85). In addition, body weight measurements and total body energy calculations indicated the study 1 subjects were in energy balance between the day –4 DXA measurements and the day that the FEX started (day 0). Body weight increased from day –4 to day 0 (1.57 ± 1.87 kg; n = 10; P < 0.05), and no significant difference in the amount of weight gained by the men (1.67 ± 2.47 kg; n = 6) and women (1.43 ± 0.58 kg; n = 4) was observed. This body weight gain was within the normal day-to-day variation in TBW in army recruits (34) and probably did not reflect a change in body energy stores. Estimated total body energy did not change significantly from day –4 to day 0 (–19 ± 64 MJ; n = 10).

Activity monitor data from a subset (n = 5) of subjects from study 1 showed that the cadets were not sleep deprived immediately before the FEX (apparent sleep: day –2, 8.6 ± 2.0 h/d; day –1, 6.4 ± 0.6 h/d; day 0, 5.4 ± 1.1 h/d). In contrast, during the FEX, the subjects were inactive (apparently asleep) <1 h/d. During the first 4 d of the FEX (day 1 to day 4), apparent sleep averaged 0.8 ± 0.7 (day 1), 0.8 ± 0.5 (day 2), 1.1 ± 0.8 (day 3), and 1.1 ± 0.3 (day 4) h/d.

The group mean DLW TEE was 24.8 ± 3.1 MJ/d (range: 19.1–29.5 MJ/d; n = 16). In study 2 (n = 6), the estimated intake-balance TEE (23.6 ± 3.6 MJ/d; range: 20.1–28.7 MJ/d) did not differ significantly from the corresponding DLW TEE (23.6 ± 3.4 MJ/d; range: 19.1–27.8 MJ/d).

Data derived from the DLW measurements are shown in Table 2Go. Baseline TBW was 46.8 ± 3.8 kg for men and 34.5 ± 2.5 kg for women (P < 0.05). The oxygen-18 elimination rate (kO) on day–1 was –0.1456 ± 0.0167 for men and –0.1552 ± 0.0326 for women; kD on day–1 was –0.1056 ± 0.0152 for men and –0.1090 ± 0.0307 for women. Total RH2O averaged 4.4 ± 1.0 L/d (n = 16). Absolute RH2O was greater in the men than in the women (P < 0.05), but no significant difference was evident when RH2O was expressed relative to FFM or body weight. The relations of RH2O to FFM [RH2O = (–0.705 x FFM) – 0.368; x intercept = –5.2 kg; R2 = 0.46; n = 16] and of RH2O to body weight [RH2O = (–0.777 x body wt) +1.179; x intercept = 15.2 kg; R2 = 0.46; n = 16] were used to calculate mass-specific RH2O. No significant sex differences were evident in FFM-specific RH2O (men, –69.7 ± 9.6 mL · kg corrected FFM–1 · d–1; women, –72.5 ± 19.2 mL · kg corrected FFM · d–1) or in body weight–specific RH2O (men, –77.2 ± 11.2 mL · kg corrected body wt–1 · d–1; women, –78.0 ± 21.6 mL · kg corrected body wt–1 · d–1).


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  		TABLE 2 Daily total energy expenditure (TEE), food energy intake, water turnover, physical activity level (PAL), and activity energy expenditure (AEE) in male and female cadets participating in a 7-d ranger field exercise with sustained exercise and food restriction1

 
The significant correlation of TEE to body weight is shown in Figure 2Go. Similar relations of AEE to body weight (AEE = 0.189 x body weight + 4.086; R2 = 0.57; SEE = 1.54; n = 16), of AEE to FFM [AEE = 8.166 + (0.166 x FFM); R2 = 0.55, SEE = 1.57; n = 16], and of TEE to FFM [TEE = 10.234 + (0.256 x FFM); R2 = 0.73, SEE = 1.65; n = 16] were also evident. In addition, in study 1, TEE was correlated with total weight, ie, with estimated load plus body weight [TEE = (0.199 x total weight) + 4.654; R2 = 0.65, n = 10]; the load averaged 32.9 ± 3.1 kg (men: 34.3 ± 3.0 kg, n = 6; women: 30.7 ± 2.1 kg, n = 4; P < 0.05).


Figure 2
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  		FIGURE 2. Correlation of total energy expenditure (TEE) with body weight in male (n = 10; •) and female (n = 6; {circ}) cadets participating in a 7-d field exercise involving sustained exercise and semistarvation. TEE was measured by the doubly labeled water method.

 
Absolute TEE was greater in the men than in the women (P < 0.05; n = 10 men and 6 women). However, mass-specific TEEs did not differ significantly between sexes (n = 10 men and 6 women). Specifically, the use of FFM and body weight values corrected for the nonzero intercepts in the linear relations of TEE to FFM [TEE = (0.256 x FFM) + 10.234; x intercept = –40 kg; R2 = 0.73; n = 16) and of TEE to body weight [TEE = (0.286 x body weight) + 4.342; x intercept = –15 kg; R2 = 0.72; n = 16] showed that no significant sex differences (n = 10 men and 6 women) were evident in FFM-specific TEE (men, 255 ± 15 kJ · kg corrected FFM–1 · d–1; women, 257 ± 21 kJ · kg corrected FFM–1 · d–1) or in body weight–specific TEE (men, 287 ± 21 kJ · kg corrected body wt–1 · d–1; women, 285 ± 15 kJ · kg corrected body wt–1 · d–1). Similarly, no significant sex differences in TEE were evident after correction for covariance by either body weight or FFM (n = 10 men and 6 women).

During the FEXs, body weight decreased from 71.6 ± 9.1 kg (day 0) to 64.6 ± 8.1 kg (day 8) (n = 16; P < 0.05). On average, the daily loss of body weight was {approx}1 kg (men, –1.06 kg/d; women, –0.85 kg/d; P < 0.05) including a 0.5-kg loss in FM (men, –0.49 kg/d; women, –0.50 kg/d; NS). On average, the cadets lost {approx}10% of their body weight during the FEX, with the absolute body weight loss of the men exceeding that of the women (P < 0.05; n = 10 men and 6 women; Table 3Go); the men lost more FFM than did the women (P < 0.05). Both the men and the women lost {approx}6% of FFM. However, FFM loss expressed as a percentage of body weight loss was significantly greater in the male than in the female cadets (men, 53 ± 11%; women, 41 ± 11%; P < 0.05). Absolute FM loss was not significantly different between the men and the women ({approx}3.4 kg), but the men had a significantly larger percentage loss of initial FM (28.3 ± 5.9%) than did the women (22.1 ± 3.6%). About 50% of the total loss of FM was derived from the trunk (men, –2.1 ± 0.6 kg; women, –1.7 ± 0.3 kg); the men lost 45.3 ± 12.4% and the women lost 36.1 ± 8.7% (P = 0.064) of initial truncal fat mass.


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  		TABLE 3 Changes ({Delta}) in body composition and the associated change in body weight in male and female cadets in response to a 7-d ranger field exercise with sustained exercise and food restriction1

 
Absolute and relative fat oxidation and the percentage contribution of FM to TEE are shown in Table 4Go. Absolute rates of fat oxidation were not significantly different between the women and the men, despite differences in body size; however, fat oxidation per kg FFM in the women exceeded that in the men (P < 0.05). Regression analysis showed that fat oxidation per kg FFM was positively related to FM (Figure 3Go). Fat oxidation per kg FM, or, alternatively, {Delta}FM/initial FM (men, –29 ± 6%; women, –22 ± 4%), were significantly greater in the men than in the women (P < 0.05). However, this difference was not significant when apparent sex differences in fat energy reserves were taken into account. In other words, fat oxidation per kg fat reserves, or {Delta}FM/estimated fat reserves (men, –44 ± 13%; women, –37 ± 10%), did not differ significantly between the male and the female cadets. Finally, the fractional contribution of FM to TEE of the women significantly exceeded that of the men.


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  		TABLE 4 Fat oxidation rates and the relative contribution of fat mass (FM) to total daily energy expenditure (TEE) in male and female cadets participating in a 7-d ranger field exercise with sustained exercise and food restriction1

 

Figure 3
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  		FIGURE 3. Correlation of fat oxidation per kg fat-free mass (FFM) with fat mass in male (n = 10; •) and female (n = 6; {circ}) cadets participating in a 7-d field exercise involving sustained exercise and semistarvation. Body composition was assessed by dual-energy X-ray absorptiometry (DXA).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In response to a week of sustained exercise and food deprivation, the female cadets oxidized more body fat per kg FFM and had a greater fractional contribution of FM to TEE than did the male cadets. Our findings are consistent with most (12–14, 35), but not all (15–17), less-extreme, short-term exercise studies that have found that women use more fat and less carbohydrate and protein than men. In addition, the positive relation of fat oxidation per kg FFM and FM among the cadets suggested that fuel metabolism becomes more fat-predominant as FM increases, although other factors, such as circulating hormones (14), are also likely to be important.

Our finding that female cadets maintained a more fat-predominant fuel metabolism than did males implies reduced glycogen use, a significant capacity for endurance exercise, and less loss of FFM. Reduced glycogen use would tend to limit decrements in maximum sustainable endurance exercise intensity that normally accompanies carbohydrate deprivation (36). Running performance in female athletes apparently approaches that of men as race distances increase, reaching parity in a 90-km ultramarathon race (37).

With prolonged underfeeding, FFM loss is {approx}25% of the weight lost, with fat accounting for the balance, although extreme energy deficits, as in the present study, can increase the contribution of FFM (38). Using less glycogen would tend to decrease protein use for gluconeogenesis and reduce the loss of FFM (38, 39). Women are reported to use less glycogen and excrete less urea nitrogen than men in response to {approx}95 min of moderate-intensity exercise (35). The loss of FFM during the FEX was a smaller percentage of body weight loss in the women than in the men.

Absolute FM loss and the absolute rate of fat oxidation to meet energy needs were not significantly different between the male and the female cadets, but fat oxidation per kg FFM and the percentage contribution of FM to TEE were greater in the female cadets. This contrasted with the typical pattern in which absolute differences between the sexes were not evident when calculated on a relative basis. For example, the male cadets lost more FFM than did the female cadets, but no sex difference was evident in the percentage loss of initial FFM. The female cadets were physically smaller and had lower TEEs than did the male cadets, but PAL, AEE, and relative TEE were similar between the sexes.

The body weights and body fat levels of the cadets were near the ideal for fit young men ({approx}76 kg, or {approx}15%) and women ({approx}60 kg, {approx}25%) and were similar to those of US Army soldiers (40) and cadets in previous FEX studies (6, 41). Although body FM index values were normal, the FFM index values for the fit cadets were at the high end of the normal range (42).

The cadets were inactive {approx}1 h/d, as in previous FEX studies (3, 43), and they had PALs equivalent to 23 h/d of heavy work (44); the PALs exceeded those of climbers of Mount Everest [2.2 x basal metabolic rate (BMR)] (45), soldiers training for jungle warfare (2.5 x BMR) (46), hill walkers ({approx}2.8 x BMR) (47), US Army Rangers in training (48), and others (49). However, the cadets were less physically active than were US Marines training for mountain warfare (PAL: {approx}4) (10), Arctic explorers (PAL: {approx}4.5) (50), and Tour de France cyclists (4.3–5.3 x BMR) (11). The TEEs of our cadets, determined either by intake-balance or DLW methods, were less than the TEE estimates for previous FEXs (3, 6, 9). Our cadets had TEEs that were {approx}50–75% of the TEEs estimated in previous studies (33–46 MJ/d) in which the duration of the FEXs was 3–5 d (3, 6, 9), as opposed to 7 d in the present study. On the other hand, average daily losses of body weight (–0.76 to –0.90 kg/d) (6, 41, 51) and of FM (–0.60 to –0.69 kg/d) (6, 41) in previous FEX studies suggest TEEs of {approx}27–29 MJ/d.

Our male cadets used 45 ± 15% of their body fat reserves, assuming 5% body fat as a minimum (31). This was similar to the 50% fat reserve depletion with FEX training in male cadets (6). Female cadets used 37 ± 10% of their fat reserves, assuming 10% as the minimum percentage body fat in healthy young women. The disproportionately large contribution of trunk fat to the total FM loss, found in both the male and the female cadets, is consistent with earlier findings in male cadets (41). Rognum et al (41) sampled cadets’ fat cells from 3 sites and found large decreases in fat cell size in abdominal and gluteal adipocytes but not in the more peripheral femoral site adipocytes.

In addition to the known relation of RMR to FFM, an analogous but less-defined relation between TEE and FFM or body weight has been reported (18). Among the cadets, both TEE and AEE were correlated with FFM, body weight, and estimated total weight. This was expected, given that body weight is a key determinant of the metabolic cost of locomotion (52), and the primary activity of the cadets was prolonged foot marches. Kram and Taylor (52) showed that the metabolic cost of locomotion is primarily determined by the cost of supporting body weight and the rate at which this force is generated. In one study, Schoeller and Fjeld (53) attributed most of the variance in DLW TEE to individual differences in FFM (men, R2 = 0.87; women, R2 = 0.68). In a study of obese women, DLW TEE was correlated with FFM (R2 = 0.52) and body weight (R2 = 0.59) (54). During a winter trek on Mount Rainier, DLW TEE was also correlated with FFM (R2 = 0.89, P < 0.01) and total weight (R2 = 0.95, P < 0.01) (19). In US Marines engaged in varied cold-weather training activities, DLW TEE was less-well correlated with FFM (R2 = 0.35, P < 0.05) (10). The relations between TEE and body weight and of TEE to FFM were also less evident in sedentary soldiers (27). The relation of TEE to FFM appears to be more evident when subjects share a common locomotion task and is less evident in sedentary groups.

The present study had limitations. A definitive examination of sex differences in fuel oxidation during sustained stress requires a larger sample size. We were restricted by the limited number of female cadets participating in FEX training. Second, because of a schedule conflict, the DXA measurements in study 1 were made 4 d before the FEX rather than immediately before the FEX. The subjects were probably in energy balance during this period, given that calculated total body energy was unchanged. The difference in food energy intake between study 1 (1% of TEE; n = 6 men and 4 women) and study 2 (9% of TEE; n = 4 men and 2 women) probably contributed to the variability of the data. Finally, aerobic capacity was not measured. However, previous studies found the cadets to be moderately fit (maximal oxygen uptake = 50–58 mL/kg · min–1) (9, 55), which suggests that our cadets, who were engaged in common training, were also moderately fit. We conservatively assumed that relative aerobic fitness and exercise intensity during the FEX were not significantly different between the men and women. If the women worked at a higher intensity than the men, the effect would be to decrease fat oxidation and minimize any sex difference in fuel oxidation.

In conclusion, most of the sex differences in energy expenditure, RH2O, and FFM loss in response to the FEX were attributed to differences in body size. However, the female cadets maintained a more fat-predominant fuel metabolism and achieved an absolute rate of fat oxidation similar to that of the physically larger male cadets.


   ACKNOWLEDGMENTS
 
We thank the selfless test volunteers from the Norwegian Military Academy, who made this study possible; Daniel P Redmond for assistance with the actigraphy data analysis; and Arne Høiseth for help with the DXA measurements.

RWH, PKO, JPD, AC, and KEF participated in the study design and manuscript preparation. RWH, PKO, AHH, and AC conducted the study. All authors participated in one or more aspects of data analysis and interpretation. None of the authors had a conflict of interest.


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 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
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Received for publication September 6, 2005. Accepted for publication January 12, 2006.





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