The relationships between eating patterns, nutrient intake, and performance during multiple days of arduous physical labor are not well understood. There is a large body of sports nutrition research addressing the effects of consumption of particular nutrients-mostly carbohydrate-at specific times before, during, and after exercise, which suggests that frequent eating or ingestion of carbohydrate throughout a physically demanding work day could improve work output or delay fatigue (5,13). However, the sports nutrition studies are not necessarily applicable to occupational labor scenarios because the subjects in these studies were in overall energy balance and were performing prolonged, high-intensity, steady-state exercise or repeated sprint-type activities at exercise intensities that require a high rate of carbohydrate oxidation. In contrast, the extended work day of the soldier and occupational laborer is characterized as sustained activities of variable, low to moderate intensities, interrupted by short bursts of high-intensity activity (6,14,23,25).
Less studied is the effect of meal frequency on work productivity. Although Diaz et al. (8) reported that no difference in self-paced work productivity of Gambian laborers provided supplemental energy and carbohydrate diets, the supplemental food was apparently only available before, during midshift, and at the end of shift. Therefore, potential effects of snacking behavior were not examined. More recently, Cuddy et al. (6) reported that wildland firefighters consuming a 20% CHO beverage every hour during a 12-h work shift maintained a higher level of physical activity measured using actimetry than when they consumed only the standard sack lunch and placebo beverage. An almost 25% increase in the average activity count per hour was observed when the firefighters consumed supplementary carbohydrate energy compared with placebo. This effect was also present when liquid and solid snack items were consumed in a scheduled manner during the work shift. Thus, the observations of Cuddy et al. suggest that frequent intake of carbohydrate during sustained arduous labor can improve worker productivity. It remains untested, however, if a field ration pack built to make it easy to eat at frequent intervals and eaten ad libitum would increase food intake and improve self-paced physical activity.
There is a growing body of literature supporting the inclusion of caffeine products in field ration packs due to caffeine's ability to reduce sense of effort (4,16) and sustain cognitive performance particularly during vigilance-type tasks (9,15-19) and self-paced physical performance (16,18,19) with sleep restriction. Acute caffeine ingestion has also been shown to improve physical performance during short-duration, high-intensity physical activities (1,2,4,22). Whether these effects singly or combined with altered macronutrient intake translate into increased self-paced physical activity during arduous occupational labor is uncertain.
The purpose of this experiment was to evaluate the effectiveness of a ration pack consisting of eat-on-move items to promote snacking, as well as caffeine items, as a nutritional strategy to improve performance during 2 d of arduous occupational labor. It was hypothesized that regular intake of energy/carbohydrate as well as caffeine would increase self-paced physical activity as measured by actimetry, improve cognitive performance, and improve mood.
Wildland firefighters were selected as the test population as they perform arduous work in hostile environmental conditions; their daily energy expenditure is within the range of dismounted combat soldiers (23), most members of a fire suppression crew perform similar physical tasks, and diligence is necessary for mission success. A within-subject crossover design was used, with volunteers from the same fire crew assigned to consume either two meals, ready-to-eat (MRE) or one first strike ration (FSR) per day during two consecutive days of fire suppression with a 1-d washout separating each 2-d experimental block. Baseline measures were obtained, and practice sessions were conducted 1-2 d before beginning the experimental trials. Baseline measures included the following: height, weight, and questionnaires to assess demographic profiles, health-related behaviors, and typical caffeine intake. Figure 1 provides a graphic summary of the experimental design. The study protocol, consent form,and subject-related materials were approved by the US Army Research Institute of Environmental Medicine scientific and human subjects review committees (protocol ID# H05-07) and by the University of Montana internal review board.
The study was conducted at two separate fires using two Hotshot fire crews. A total of 28 firefighters participated (22 males, 6 females; 28 ± 7 yr, 173 ± 6 cm, 73.2 ± 7.2 kg). The participants were briefed orally and in writing regarding the study, and all signed informed consent forms before participation.
Two days' supply of food was provided the morning of each experimental test block. The test subjects were told to eat no food other than-with one exception-what was provided in their test ration pack. To promote sufficient recruitment and maintain subject compliance, the firefighters were allowed one 12-fl/oz serving of coffee each morning before the work shift. Food consumption (when, what, and how much) was at the subjects' discretion. They had unrestricted access to potable water. Figure 2 provides a breakdown of the composition of the two diets, including one sample menu of the FSR.
The MRE ration pack is packaged by meal and consists of a main entrée and complimentary side items. Although some items are conducive to eat-on-the-go or eat-out-of-hand (e.g., beef jerky, candy), other items including the main entrée are designed to be eaten with a utensil. The main caffeine source is a single serving (57 mg) of instant coffee. For this study, four menus from the MRE version 25 were selected: pork rib, chicken with salsa, pasta with vegetable tomato sauce, and beef teriyaki. The FSR pack is bundled with all the food for a day and consisted almost entirely of items conducive for eating-on-the-go without theneed for utensils. Shelf-stable pocket sandwiches form the foundation of the FSR pack. Complimentary items included beef jerky, carbohydrate-dense food bars, powdered carbohydrate beverage base in a reusable container, and carbohydrate-enriched fruit puree served in a squeeze pouch. Caffeine (∼660 mg) was primarily provided in gum form (5 × 100 mg pieces), with the remainder as part of food or powdered drink. Four FSR menus were included, differing primarily in type or flavor of sandwich.
Actimetry was used to quantify self-selected physical activity. The Actical device (Mini Mitter, Bend, OR) was mounted on a hard card with the same shape and size of the firefighters work shirt and was worn inside the left chest pocket of the firefighters for the duration of each work shift. This configuration has been used successfully in other previous studies with this population (6). Total activity counts, hourly counts, and time spent in activity ranges of 0-50, 51-1000, and >1000 counts·min−1, respectively, were used to assess differences in activity between diets. The partitions for activity levels were determined from the activity level distribution when consuming the MRE and then testing if activity profile differed when consuming FSR.
Cognitive performance was assessed using a simple reaction time test administered on a PDA (Sony Corp.). Volunteers were scored on how rapidly they responded to the random appearance of a marker on the PDA screen. Test duration was 10 min, with 175.0 ± 1.6 stimuli presented during the test. Training on the reaction time task was conducted the day before beginning data collection, with the volunteers completing the test at least three times before experimental testing.
Mood was assessed using the POMS at the beginning of the work shift and at the end of the work shift each experimental test day (20). For this test, the volunteers rated a series of 65 mood-related adjectives on a five-point scale, in response to the question, "How are you feeling right now?" The adjectives were factored into six mood subscales (tension, depression, anger, vigor, fatigue, and confusion).
Energy and macronutrient (carbohydrate, protein, and fat) intakes were calculated from dietary intakes recreated by inventorying each subject's empty food wrappers and leftover food on a daily basis and from diet logs maintained by each volunteer. Opened food wrappers were weighed on a top loading balance (accurate to 1 g) to verify portion of actual consumed. Nutrient intakes were calculated using a food composition database compiled from existing laboratory determinations made by the ration developers, from calculations of the product formulation using Genesis (version 7.6; ESHA), from manufacturer-provided data, or from the US Department of Agriculture Nutrient Database for Standard Reference, release 17, and the US Department of Agriculture Nutrient Database for Individual Food Intake Surveys. The number of eating episodes was determined from diet logs that provided the following descriptive categories: early morning (6-9 a.m.), midmorning (9 a.m. to noon), midday (noon to 3 p.m.), late afternoon (3-6 p.m.), and evening (6-9 p.m.).
Water turnover was quantified for the 10 volunteers from fire 1 using the deuterated water technique. After baseline samples were collected at approximately 2100-2200 h the day before beginning experimental testing, volunteers then drank 9 g of 2H2O (∼0.2 g·kg TBW−1; 99% APE, diluted 1:10 with tap water; Isotec, Inc, Miamisburg, OH, or Cambridge Isotopes, Cambridge, MA). After consumption of the original dose mixture, the dose vial was rinsed three times with ∼20 mL of tap water to ensure complete isotope administration. The following morning (0600-0700 h), subjects were asked to void twice approximately 45 min apart. The second urine sample was collected and used for estimating total body water (23,24). Urine samples were then collected on study day 2 (first morning void) and at the end of the day 2 work shift for measuring isotope elimination. Samples were stored on ice until they were frozen and shipped for hydrogen isotope abundance analysis using a Finnigan MAT 252 gas-inlet isotope ratio mass spectrometer (7).
Caffeine status and stress markers were measured from nonstimulated saliva samples collected (2-3 mL) into sterile screw top tubes the evening preceding the ration trial, the morning of day 1 of each ration trial, and after each work shift. The samples were frozen and shipped for analysis of salivary caffeine and testosterone. Caffeine was determined using Beckman Synchron CX5 using EMIT reagents for caffeine (Dade-Behring Diagnostics, Deerfield, IL). Testosterone was assayed by enzyme-linked immunosorbent assay (Salimetrics, LLC, State College, PA).
Body mass changes were quantified by measuring nude body mass before and after each work shift. Standing height was measured in stocking feet using a stadiometer.
Data were analyzed using the Statistical Program for Social Sciences (version 12.0; SPSS, Inc., Chicago, IL) and Statistica (version 7; Statsoft, Inc., Tulsa, OK) software packages. The Shapiro-Wilk test was used to examine the normality of each variable. A two-way (ration pack × time) repeated-measures ANOVA was used to determine whether significant differences in activity levels, cognitive performance, or mood were present between the two ration pack trials. Significant main or interaction effects were analyzed using Tukey's HSD procedure. Significance was set at P < 0.05. Data were presented as mean ± SD unless otherwise specified.
The firefighters performed long hours of labor because work shifts averaged 11.3 ± 1.6 h·d−1. Although actual energy expenditures were not measured, the physical activities and weather conditions for the first test group (n= 10) resulted in a water turnover of 6.6 ± 2.1 L·d−1 or 95 ± 24 mL·kg−1·d−1 during the experimental period with no apparent effect of diet (FSR = 109 ± 38 mL·kg−1·d−1, MRE = 102 ± 40 mL·kg−1·d−1; P = 0.5). It is likely that energy deficit was present in both fire crews who participated, because body mass declined independent of the diet group 1.6 ± 0.6 kg during the initial 2 d of experimental testing and 0.9 ± 0.9 kg during the final 2 d (P< 0.01). The 1-d recovery period between diet intervention periods did not fully restore body mass, because the body mass of the volunteers was 0.7 ± 0.7 kg lighter before beginning the final 2 d of the dietary intervention (P < 0.01). There was a main effect of time on salivary testosterone levels (pre = 158 ± 77 pg·mL−1, end-shift day 1 = 88 ± 57 pg·mL−1, end-shift day 2 = 93 ± 66 pg·mL−1; P < 0.01) but no difference between diets.
The firefighters' voluntary energy, carbohydrate, protein, and caffeine intakes were higher when subjects ate FSR than when they ate MRE (Table 1). Ration pack intake was also higher (P < 0.01) because 83% of energy provided in FSR was consumed, whereas only 78% of MRE energy was ingested. Similarly, 83% of FSR carbohydrate was consumed, whereas only 75% of MRE carbohydrate was eaten (P < 0.01). There was also a small but statistically significant increase in the number of self-reported eating events when consuming the FSR (FSR = 8.2 ± 1.3 episodes per 2-d diet period, MRE = 7.6 ± 1.1 episodes per 2-d diet period; P = 0.01) because 52% of the volunteers reported at least one additional episode when consuming FSR, whereas only 15% reported additional episodes when consuming MRE. Figure 3 presents daily energy and carbohydrate intake patterns when individuals consumed either MRE or FSR ration packs.
The effect of diet on mood state is presented in Table 2. There seemed to be little difference between the ration packs on mood state, with the exceptions that subjects felt they were less depressed when consuming FSR (P = 0.03) and a trend for subjects to feel less confused (P = 0.08).
Figure 4 presents the activity levels within a work shift. There was a main effect of time on activity level, largely due to greater counts per minute early and late in the work shift when the firefighters walked to and from the fire area over rugged terrain. There was also a main effect of diet, with FSR associated with greater average counts per minute compared with MRE (FSR = 724 ± 177 counts·min−1, MRE = 627 ± 154 counts·min−1; P < 0.01). As a consequence, FSR produced greater total activity counts per work shift than MRE (FSR = 507,833 ± 129,130 counts per shift, MRE = 443,095 ± 142,208 counts per shift; P = 0.046). The greater counts could be attributed almost wholly to subjects eating FSR, spending a greater percentage of the work shift performing activities producing >1000 counts·min−1 (Fig. 5; P = 0.01) and less percent of work shift performing activities producing 0-50 counts·min−1 (P= 0.01). This amounted to approximately 24 min more of activity >1000 counts·min−1 per work shift (P = 0.02) and 30 min less per work shift at 0-50 counts·min−1 (P = 0.07).
Reaction time performance was stable across the two work shifts, and performance was unaffected by diet (Fig. 6). Baseline reaction time averaged 333 ± 50 ms. Multiple PDA malfunctions on the final day of testing resulted in loss of complete data for seven of the volunteers. As a consequence, the effect of diet on cognitive performance could only be assessed for the remaining 21 subjects with complete data.
To determine the efficacy of the FSR feeding platform for enhancing performance, this study used a crossover design in which wildland firefighters consumed both the FSR and MRE during 2 d of arduous work. The firefighter population was chosen for this evaluation because they have daily energy expenditures similar to those reported in military populations (14.6-18.8 MJ·d−1), they perform self-paced physical activity in a situation where there is a sense of urgency, and their tasks are relatively homogeneous in nature. The intense fire situation required study volunteers to perform long hours of labor (11.3 ± 1.6 h·d−1). Associated with the work, there was a greater sense of fatigue. Furthermore, the energy demands resulted in energy deficit as revealed by consistent reductions in body mass during each 2-d diet intervention period.
The results of this experiment support our hypothesis that a ration pack promoting snacking behavior that contains ample carbohydrate and caffeine products would be associated with greater voluntary physical activity during periods of energy deficit. We observed that when firefighters ate the FSR, they consumed a greater portion of the ration pack provided, resulting in greater work shift and total energy and carbohydrate intake. They also chewed the caffeine gum, asevidenced by their elevated end-shift salivary caffeine levels. Associated with this eating behavior, there was anaccompanying increase in the time spent performing moderate-intensity physical activity and less time performing sedentary activities. This behavior change resulted in greater average activity counts per minute and total work shift counts over the two work shifts that were monitored. We interpret this to indicate that the FSR is advantageous for sustaining physical performance relative to eating MRE during repeated days of labor in arduous conditions.
We were unable to detect differences in cognitive performance between diets over the 2-d feeding periods. This contrasts with the consistent observation that caffeine is an effective dietary supplement for sustaining cognition during multiple days of arduous work (17-19). However, there are many differences between the experimental design used in this and those of earlier studies. First, the volunteers slept for up to 8 h between work shifts, whereas in the military operational studies showing ergogenic effects of caffeine, sleep deprivation was part of the experimental design. Second, caffeine was consumed ad libitum, whereas it was administered at set times before experimental tests in earlier studies. Third, the physical work was performed in a self-paced manner and the greater total activity when consuming the FSR compared with the MRE may have put the volunteers in a similar state of tiredness/attentiveness at end of work shift and reduced ability to detect dietary differences in reaction time. Finally, although the simple reaction time test used to study cognitive performance is sensitive to sleep deprivation as well as the stimulating effects of caffeine when sleep deprived, it may have lacked the sensitivity to pick up subtle differences in attentiveness or was administered at times of day where differences were too small to detect. Possibly, tests requiring higher levels of decision making or memory might have been more sensitive to detect differences (if they existed) between diets.
Select mood parameters as measured by Profile of Mood State were modestly affected by diet. It was observed that subjective depression score was lower when volunteers consumed FSR compared with MRE. In addition, there was a tendency for FSR to also reduce confusion scores during the 2-d observation periods. The meaningfulness of these mood parameter changes is unclear because there were no statistical differences in subjective sense of aggression, fatigue, tension, vigor, or total POMS score. However, coupled with the greater self-paced physical activity, wildland firefighters consuming the FSR may have been better able to adhere to the standard fire orders: "Be alert. Keep calm. Think clearly. Act decisively" and "Fight fire aggressively, having provided for safety first"(26).
One potential reason for the improved self-paced physical activity with FSR compared with MRE may be the ease of snacking. Unlike the MRE, the FSR components do not require the use of spoon or other utensils, all items are eat-out-of-hand, and the number of available items enable repeated snacking. Our finding that greater energy intake, greater carbohydrate intake, and increased number of eating episodes were accompanied by increased time spent in moderate-intensity physical activity and less time spent at rest is consistent with the recent observation that programmed snacking (without caffeine) is associated with a greater quantity of self-paced activity (6). It is also in agreement with the observations that soldiers who consumed carbohydrate-electrolyte beverages or practicing good food discipline during repeated days of arduous physical activity were more likely to sustain physical performance than those who did not (21). However, it contrasts with reports that additional energy (and carbohydrate) is ineffective for improving work productivity (8,12,27). A possible explanation for disparate findings may be because latter studies manipulated the quantity of food consumed at discrete meals, and these meals were separated by many hours. Improvements have been observed where either ad libitum eating was encouraged during work and/or snacking was programmed into the work shift. Interestingly, snacking seems to be a desired behavior, as 93% and 89% of volunteers self-reported that they regularly eat midmorning and midafternoon snacks, respectively. Moreover, 61% self-reported that they prefer to eat throughout the work shift as time permits. This eating behavior is consistent with those of competitive endurance athletes where snacking seems to be an adopted behavior, accounting for ∼23% of daily energy intake (3).
The modest improvement in self-reported eating episodes might initially suggest that the FSR had little effect on snacking behavior and the wildland firefighters consuming the FSR generally consumed more of their items per eating episode. However, the data for eating episodes were culled from entries into self-reported diet records providing five time blocks per day for binning entries (e.g., midmorning, noon, etc); as such, the actual number of eating events was not actually captured. Regardless, the provision of FSR was accompanied by greater absolute and relative food intake, and this was associated with greater voluntary physical activity.
The singular and combined effects of caffeine with carbohydrate might also have contributed to the improved time spent performing self-paced physical activity. McLellan et al. (16,18,19) have reported beneficial effects of caffeine on self-paced physical performance with sleep deprivation. Most relevant to this study is the observation that self-paced sandbag piling was performed significantly faster 3 h after ingestion of 400 mg of caffeine despite only 16 h of sustained wakefulness (16). Thus, the beneficial effects of caffeine can be observed during low- to moderate-intensity effort with limited sleep deprivation. Caffeine alone, however, likely only partially explains our outcomes because only 38% of volunteers who had higher activity counts when consuming FSR consumed 400 mg or more of caffeine during 2 d on FSR diet, and 60% of those consuming less than 250 mg of caffeine for 2 d had higher activity counts. Possible interaction with other stimulants, however, cannot be discounted because 13 of 28 volunteers reported use of smokeless tobacco, nine of whom reported that they typically use the product five to ten times per day.
A limitation of this study is that physical activity was measured indirectly via use of actimetry rather than attempting to quantify actual work productivity. Because actigraphy devices rely on accelerometers to quantify movement, the devices do not actually measure energy expenditure. However, the device used has very low intrainstrument coefficient of variation (10) and can reasonably discriminate time spent at different activity levels (11). Our use of a repeated-measures experimental design enabled the subjects to serve as their own activity level control. Moreover, the observation that the differences attributable to diet were not due to two adjacent activity levels (e.g., 0-50 and 51-1000 counts·min−1) but between the percent of work shift time spent in ranges of 0-50 and >1000 counts·min−1 suggests that differences were real versus artifact due to poor instrument sensitivity and/or specificity.
A second limitation of this study is that the two diets were not matched either in energy or in carbohydrate content or in caffeine content. Matching would have enabled discrimination of whether it is snacking per se versus energy and/or carbohydrate intake or caffeine that is responsible for greater voluntary physical activity. Although this criticism is valid, an important practical aspect of ration packs is their size and weight. The FSR was conceptualized in response to the users' desire for a smaller, lighter-weight ration pack that is easier to consume on-the-go. Despite containing greater absolute energy and carbohydrate, the FSR remained lighter in weight and smaller than the MRE. Thus, our initial question was whether manipulation of the contents and the functionality thereof provided performance advantages. The results of this investigation provide evidence that provision of a ration pack that is easy to consume during work and has greater energy, carbohydrate, and caffeine than the existing individual field ration pack, the MRE, offers performance advantages during periods of arduous labor.
Provision of one FSR, compared with two MRE, will better sustain self-selected physical activity and mood during successive days of prolonged physically demanding work. This observation suggests that the delivery of energy and caffeine in a manner that promotes snacking behavior is beneficial for sustaining self-paced work and may have implications for increased safety on the fire line.
The authors acknowledge the technical support of Susan McGraw, Jennifer Rood, Harris Lieberman, Joseph Domitrovich, and Nobuo Yasuda, as well as the efforts of the volunteers who participated in the study. Thanks are also extended to the fire overhead management teams for their cooperation with logistic support and access to the fire camp and crews. Funding was provided by the US Army.
Disclaimer: The views, opinions, and/or findings contained in this publication are those of the authors and should not be construed as an official Department of the Army position, policy, or decision unless so designated by other documentation. The results do not constitute endorsement by the American College of Sports Medicine. This paper is approved for public release; distribution is unlimited.
1. Bridgek CA, Jones MA. The effect of caffeine
ingestion on 8 km run performance
in a field setting. J Sports Sci
2. Bruce CR, Anderson ME, Fraser SF, et al. Enhancement of 2000-m rowing performance
ingestion. Med Sci Sports Exerc
3. Burke LM, Slater G, Broad EM, Haukka J, Modulon S, Hopkins WG. Eating patterns and meal frequency
of elite Australian athletes. Int J Sport Nutr Exerc Metab
4. Cole KG, Costill DL, Starling RD, Goodpaster BH, Trappe SW, Finkielman S. Effect of caffeine
ingestion on perception of effort and subsequent work production. Int J Sport Nutr
5. Coyle EF. Fluid and fuel intake during exercise. J Sports Sci
6. Cuddy JS, Gaskill SE, Sharkey BJ, Harger SG, Ruby BC. Supplemental feedings increase self-selected work output during wildfire suppression. Med Sci Sports Exerc
. 2007;39(6): 1004-12.
7. DeLany JP, Schoeller DA, Hoyt RW, Askew EW, Sharp MA. Field use of D2 18
O to measure energy expenditure of soldiers at different energy intakes. J Appl Physiol
8. Diaz E, Goldberg GR, Taylor M, et al. Effects of dietary supplementation on work performance
in Gambian laborers. Am J Clin Nutr
9. Doan BK, Hickey PA, Lieberman HR, Fischer JR. Caffeinated tube food effect on pilot performance
during a 9-hour, simulated nighttime U-2 mission. Aviat Space Environ Med
10. Esliger DW, Tremblay MS. Technical reliability assessment of three accelerometer models in a mechanical setup. Med Sci Sports Exerc
11. Heil DP. Predicting activity energy expenditure using the Actical activity monitor. Res Q Exerc Sport
12. Immink MD, Blake CC, Viteri FE, Flores R, Torún B. Energy supplementation and productivity of Guatemalan sugar-cane cutters: a longitudinal approach. Arch Latinoam Nutr
13. Jeukendrup AE. Carbohydrate intake during exercise and performance
14. Lieberman HR, Kramer FM, Montain SJ, Niro P, Young AJ. Automated ambulatory assessment of cognitive performance
, environmental conditions, and motor activity during military operations. Proc SPIE
15. Lieberman HR, Tharion WJ, Shukitt-Hale B, Speckman KL, Tulley R. Effects of caffeine
, sleep loss, and stress on cognitive performance
and mood during U.S. Navy SEAL training. Sea-Air-Land. Psychopharmacol (Berl)
16. McLellan TM, Bell DG, Kamimori GH. Caffeine
improves physical performance
during 24 h active wakefulness. Aviat Space Environ Med
17. McLellan TM, Kamimori GH, Bell DG, Smith IF, Johnson D, Belenky G. Caffeine
maintains vigilance and marksmanship in simulated urban operations with sleep deprivation. Aviat Space Environ Med
18. McLellan TM, Kamimori GH, Voss DM, Bell DG, Cole KG, Johnson D. Caffeine
maintains vigilance and improves run times during night operations for Special Forces. Aviat Space Environ Med
19. McLellan TM, Kamimori GH, Voss DM, Tate C, Smith SJR. Caffeine
effects on physical and cognitive performance
during sustained operations. Aviat Space Environ Med
20. McNair DM, Lorr M, Droppleman LE. EDITS Manual for the Profile of Mood States
. San Diego (CA): Educational and Industrial Testing Service; 1971. 27 p.
21. Montain SJ, Shippee RL, Tharion WJ. Carbohydrate-electrolyte solution effects on physical performance
of military tasks. Aviat Space Environ Med
22. Nehlig A, Debry G. Caffeine
and sports activity: a review. Int J Sports Med
23. Ruby BC, Shriver TC, Zderic TW, Sharkey BJ, Burks C, Tysk S. Total energy expenditure during arduous wildfire suppression. Med Sci Sports Exerc
24. Schoeller DA. Measurement of energy expenditure in free-living humans by using doubly labeled water. J Nutr
25. Tharion WJ, Lieberman HR, Montain SJ, et al. Energy requirements of military personnel. Appetite
27. Wolgemuth JC, Latham MC, Hall A, Chesher A, Crompton DWT. Worker productivity and the nutritional status of Kenyan road construction laborers. Am J Clin Nutr
Keywords:©2008The American College of Sports Medicine
CAFFEINE; DIET; MEAL FREQUENCY; SNACKING; ACTIMETRY; PERFORMANCE