Introduction:Tactical athletes require consistent physical and mental exertion to succeed in their mission, often under life-threatening conditions. Soldiers as tactical athletes are exposed to a myriad of stressors that may deteriorate performance, especially in harsh military environments. It remains unclear which combination of traits, habits, and characteristics of soldiers affects the likelihood of performance decrement during military operations. Methods:This observational cohort study investigated the differences between a physical-load group and a mental-load group of cadets (n = 49) during a 2-week near-Arctic winter training. One group comprised first-year cadets who had physical demands, while the second-year cadets had more leadership requirements (command and control) and fewer physical demands. The chance occurrence of a viral infection in the first-year cadets provided a naturalistic experiment to study vulnerability to the illness. Demographics, trait resilience, exercise training behavior, and sleep habits were assessed prior to training. Throughout training, wrist-worn accelerometers monitored sleep-wake activity, and cadets completed daily self-report entries on perceived sleepiness, stress, and mood states. Body composition was measured before and after training. Results:Findings showed the following: (1) poorer self-reported mood, stress, and sleepiness during preparation and cleanup phases than during simulated combat among the mental-load group, but for the physical-load group, these measures were poorest during the simulated combat; (2) an overall decrease in fat mass after the training; and (3) survival analysis demonstrated that older age and less frequent endurance training habits significantly predicted the likelihood of illness during training. Discussion:Leaders of tactical athletes should be aware of the taxing mental load not just for the main event itself, but for preparation and cleanup phases. Military experience and aerobic fitness might attenuate the likelihood of succumbing to viral illnesses during missions involving close quarters.

Cold weather operations are logistically difficult to orchestrate and extremely challenging for soldiers. Decades of research and empirical evidence indicate that humans are extremely vulnerable to cold and that individual responses are highly variable. In this context, it may be necessary to develop personalised strategies to sustain soldiers’ performance and ensure overall mission success in the cold. Systematic cold weather training is essential for soldiers to best prepare to operate during, and recover from, cold weather operations. The purpose of this review is to highlight key aspects of cold weather training, including (1) human responses to cold, (2) nutrition, (3) sleep and (4) protective equipment requirements. Bringing science to practice to improve training principles can facilitate soldiers performing safely and effectively in the cold. Cold weather training prepares soldiers for operations in cold, harsh environments. However, decreases in physical, psychological and thermoregulatory performance have been reported following such training, which influences operational ability and increases the overall risk of injuries. When optimising the planning of field training exercises or operational missions, it is important to understand the soldiers’ physical and cognitive performance capacity, as well as their capacity to cope and recover during and after the exercise or mission. Even though the body is fully recovered in terms of body composition or hormonal concentrations, physical or cognitive performance can still be unrecovered. When overlooked, symptoms of overtraining and risk of injury may increase, decreasing operational readiness.

Purpose: This study investigated the effects of heat exposure and previous experience on thermoregulatory and cardiovascular responses to performing a loaded march in the HEAT and on FORCE Combat (TM) circuit performance. Methods: Ten civilians (inexperienced) and 10 infantry reservists (experienced) performed a 60 min loaded march (similar to 35kg), in NORMAL (21 +/- 0.2 degrees C) and HEAT (30 +/- 0.2 degrees C) conditions and the FORCE Combat (TM) military physical performance evaluation. Participant groups were matched for morphology and physiological capacity. Results: Out of the 10 experienced participants that participated in the loaded march in HEAT, 9 completed the full 60 min but only 5 of 10 inexperienced participants were able to do the same. Performing a loaded march in the HEAT caused a state of uncompensable heat stress (continuous increase in core temperature) for both the inexperienced and experienced participants. Heart rate (134 +/- 12vs143 +/- 9bpm,p=0.027), rate of perceived exertion (13 +/- 1vs10 +/- 1,p=0.001) and thermal comfort (1.9 +/- 0.5vs2.4 +/- 0.4,p=0.011) were lower in the experienced compared to the inexperienced group during the loaded march in HEAT. The FORCE Combat (TM) completion times were higher in HEAT compared to NORMAL, but lower in experienced participants in both conditions (p=0.05). Conclusion: Both heat exposure and previous experience had an effect on cardiovascular, thermal and subjective measures during the loaded march and on completion time of the FORCE Combat (TM) circuit.

Purpose: This study aimed to determine if an association exists between basal levels of stress and immune mediators and military physical performance among Canadian Armed Forces members (CAF).Methods: Blood samples from 219 CAF members (86 women and 133 men), were analysed for levels of cortisol, C-reactive protein (CRP), Adiponectin, INF-gamma, TNF-alpha, IL-1 beta, IL-2, IL-6, IL-8 and IL-18. Grip strength, aerobic capacity and performance on six military physical performance tests (sandbag fortification, escape to cover, picking and digging, picket and wire carry, stretcher carry and vehicle extrication) were also assessed. A composite score for total performance was computed, based on rank scores from the six tasks.Results: The results from the linear regression analysis showed that higher CRP values were associated with lower total performance scores (slope -23.0,p0.05), a slower picking and digging time (slope 45.75, p <= 0.05), lower aerobic capacity (slope -71.81,p <= 0.05) and shorter plank time (slope -21.82,p <= 0.05). A positive association between IL-2 values and grip strength was also observed (slope 20.83,p <= 0.05). Adiponectin values were positively associated with plank time (slope 21.42 ,130.05), but negatively associated with grip strength (slope -8.27,p <= 0.05).Conclusion: The results from this study suggest that high levels of immune mediators (like CRP) could be a marker of decreased military physical performance.

Lorsqu’exposé au froid, l’être humain est capable de maintenir sa température corporelle constante en produisant de la chaleur par deux mécanismes le frisson musculaire et l’activation du tissu adipeux brun. Le but de notre recherche est d’étudier les adaptations métaboliques musculaires lors d’une exposition aiguë et chronique au froid. Les participants (hommes sains et jeunes) sont exposés au froid dans une combinaison à refroidissement dont la température peut être ajustée par l’expérimentateur. Lors d’une première session, les participants sont exposés à une température de 4 °C pour 2,5 h L’intensité et le patron de frisson et l’utilisation des substrats énergétiques du corps dans son ensemble (calorimétrie indirecte) sont mesurés pendant toute la période d’exposition. Une biopsie musculaire est prélevée avant et après l’exposition au froid. L’évolution de l’utilisation des substrats énergétiques (glucides et lipides) et du couplage mitochondrial est étudiée sur fibres musculaires perméabilisées (électrode de Clark). Les participants sont ensuite exposés au froid (10 °C) 2 h/jour, 5 jours/semaine pour 4 semaines. Les mêmes tests sont réalisés à la fin de la période d’acclimatation. L’effet de l’exposition chronique au froid sur le typage de fibres ainsi que sur l’expression de gènes musculaires sera étudié. Les résultats préliminaires ne montrent pas de changements de température de peau (indice de perte de chaleur), de dépense énergétique ou d’intensité de frisson suite à quatre semaines d’acclimatation. Les analyses de typages de fibres, d’expression de gènes musculaires et de respiration mitochondriale sont actuellement en cours. Cette étude va permettre de mieux comprendre les adaptations métaboliques suite à une exposition chronique au froid. En raison de l’augmentation de la dépense énergétique lors d’une exposition au froid, cette méthode est vue comme une nouvelle thérapie pour combattre l’obésité. De futures études sont nécessaires afin de prouver que cette méthode serait efficace.

OBJECTIVE: In First Nations communities of northwestern Ontario, where rates of type 2 diabetes mellitus are some of the highest in the world, ascertaining wild food dietary practices is extremely challenging owing to seasonal availability, environmental factors, life circumstances and language/cultural barriers. The purpose of this study was to determine whether analysis of isotopic and fatty acid (FA) profiles could provide more comprehensive information to discriminate between three categories of wild food consumption (that is, plants and animals) in two isolated First Nations communities of northwestern Ontario. In addition, this analysis also highlights whether wild food consumption as practiced in these two communities can increase circulating levels of polyunsaturated FAs (PUFAs), which provide a number of important metabolic benefits that could impact the prevention/treatment of T2DM. RESULTS: 13 C enrichment (in expired CO 2 , plasma and hair), 15 N enrichment (in hair) and FA profiles in plasma phospholipids (phospholipid fatty acid (PL-FA)) were quantified in men and in women consuming various amounts of wild food. 13 C/ 12 C ratios were lower and 15 N/ 14 N ratios were higher in participants consuming wild food at least once a week. In addition, FA results indicated that the relative contributions of 20:4 Ω-6 and 22:6 Ω-3 to total PL-FAs were higher and 18:2 Ω-6 lower in wild food consumers. CONCLUSION: Together, these findings confirm that isotopic and lipid markers discriminate between the different wild food categories in these two First Nations communities. Knowing the close relationship between dietary intake and the potential role of PUFA in the prevention/treatment of obesity and obesity-related diseases, it is critical to accurately measure the composition of diet for individuals in their specific environments.

Aims.- Recent evidence suggests an association between persistent organic pollutants (POPs) and type 2 diabetes. In two First Nations communities where wild food is consumed by a large portion of the population, we compared pollutants in plasma between diabetic and non-diabetic individuals, and investigated the strength of association between pollutants and insulin resistance/secretion in non-diabetic individuals.Methods. - The study population consisted of 72 participants. Oral Glucose Tolerance Tests were used to assess diabetes status. Plasma was used to determine POP concentrations and mercury concentrations were determined from hair samples.Results. - Age-adjusted plasma concentrations of some pollutants were significantly higher in diabetic than in non-diabetic individuals. When taking into account age, adiposity levels, and smoking status, POP levels were not associated with insulin resistance nor with insulin secretion in non-diabetic individuals.Conclusions. - These findings confirm that POP concentrations in plasma may be higher in diabetic than in non-diabetic individuals. No association was however seen between POP concentrations and markers of insulin resistance/secretion in non-diabetic individuals. (C) 2013 Elsevier Masson SAS. All rights reserved.

First Nations populations in Northwestern Ontario have undergone profound dietary and lifestyle transformations in less than 50 years, which have contributed to the alarming rise in obesity and obesity-related diseases, in particular type 2 diabetes mellitus. Even though the genetic background of First Nations peoples differs from that of the Caucasians, genetics alone cannot explain such a high prevalence in obesity and type 2 diabetes. Modifications in lifestyle and diet are major contributors for the high prevalence of chronic diseases. What remains constant in the literature is the persistent view that locally harvested and prepared foods are of tremendous value to First Nations peoples providing important health and cultural benefits that are increasingly being undermined by western-based food habits. However, the complexities of maintaining a traditional diet require a multifaceted approach, which acknowledges the relationship between benefits, risks and viability that cannot be achieved using purely conventional medical and biological approaches. This brief review explores the biological predispositions and potential environmental factors that contribute to the development of the high incidence of obesity and obesity-related diseases in First Nations communities in Northern Canada. It also highlights some of the complexities of establishing exact physiological causes and providing effective solutions.

In cold-exposed adult humans, significant or lethal decreases in body temperature are delayed by reducing heat loss via peripheral vasoconstriction and by increasing rates of heat production via shivering thermogenesis. This brief review focuses on the mechanisms of fuel selection responsible for sustaining long-term shivering thermogenesis. It provides evidence to explain large discrepancies in fuel selection measurements among shivering studies, and it proposes links between choices in fuel selection mechanism and human survival in the cold. Over the last decades, a number of studies have quantified the contributions of carbohydrate (CHO) and lipid to total heat generation. However, the exact contributions of these fuels still remain unclear because of large differences in fuel selection measurements even at the same metabolic rate. Recent advances on the mechanisms of fuel selection during shivering provide some plausible explanations for these discrepancies between shivering studies. This new evidence indicates that muscles can sustain shivering over several hours using a variety of fuel mixtures achieved by modifying diet (changing the size of CHO reserves) or by changing muscle fiber recruitment (increasing or decreasing the recruitment of type II fibers). From a practical perspective, how does the choice of fuel selection mechanism affect human survival in the cold? Based on a glycogen-depletion model, estimates of shivering endurance show that, whereas the oxidation of widely different fuel mixtures does not improve survival time, the selective recruitment of fuel-specific muscle fibers provides a substantial advantage for cold survival. By combining fundamental research on fuel metabolism and applied strategies to improve shivering endurance, future research in this area promises to yield important new information on what limits human survival in the cold.

Heat exchange has been thoroughly studied in cold-exposed humans, but the metabolic substrates used for thermogenesis have received less attention. This review deals with oxidative fuel selection in shivering humans. Lipids provide most of the heat during low-intensity shivering, whereas carbohydrates become dominant under more extreme cold conditions. The contribution from plasma glucose always remains minor, but muscle glycogen plays an important role during intense shivering. Whether the size of muscle glycogen stores influences endurance in the cold remains to be demonstrated. The fuel selection patterns of shivering and exercise are different, but the mechanisms underlying this difference have not been investigated. The simultaneous measurement of metabolic substrate oxidation and muscle fibre recruitment has allowed to characterize two different mechanisms of fuel selection in shivering humans: the recruitment of different pathways within the same fibres and of different fuel-specific fibres within the same muscles. This suggests that muscle fibre composition of each individual may affect survival. Future research promises to provide a combination of theoretical advances on fundamental principles of fuel selection and applied strategies to manipulate fibre composition (through training) or fuel metabolism (through diet) to prolong human survival in cold environments.

SUMMARYThe metabolic consequences of cold exposure and exercise are not well characterized in birds. Ruff sandpipers Philomachus pugnax are migrant shorebirds traveling between Africa and Siberia for up to 30 000 km annually. Our goal was to quantify the fuel selection pattern of these remarkable athletes during shivering and terrestrial locomotion. We used indirect calorimetry and nitrogen excretion analysis to measure their rates of lipid, carbohydrate and protein oxidation at different temperatures (22, 15,10 or 5°C) and different treadmill speeds (15, 20, 25, 30, 35 or 40 m min–1). Results show that lipid oxidation supplies nearly all the energy necessary to support shivering and running, and that the pattern of oxidative fuel selection is independent of shivering or running intensity. During shivering, total ATP production is unequally shared between lipids(82%), carbohydrates (12%) and proteins (6%). During running, lipids remain the dominant substrate (66%), with carbohydrates (29%) and proteins (5%)playing more minor roles. The prevailing use of lipids during intense shivering and high-speed running is not consistent with the fuel selection pattern observed in exercising and cold-exposed mammals. The exact mechanisms allowing birds to use lipids at extremely high rates are still largely unexplored, and quantifying the relative importance of different fuels during long-distance flight remains a major challenge for future research.

The effects of changes in shivering intensity on the relative contributions of plasma glucose, muscle glycogen, lipids and proteins to total heat production are unclear in humans. The goals of this study were: (1) to determine whether plasma glucose starts playing a more prominent role as shivering intensifies, (2) to quantify overall changes in fuel use in relation to the severity of cold exposure, and (3) to establish whether the fuel selection pattern of shivering is different from the classic fuel selection pattern of exercise. Using a combination of indirect calorimetry and stable isotope methodology, fuel metabolism was monitored in non‐acclimatized adult men exposed for 90 mins to 10°C (low‐intensity shivering (L)) or 5°C (moderate‐intensity shivering (M)). Results show that plasma glucose oxidation is strongly stimulated by moderate shivering (+122% from L to M), but the relative contribution of this pathway to total heat generation always remains minor (< 15% of total heat production). Instead, muscle glycogen is responsible for most of the increase in heat production between L and M. By itself, the increase in CHO oxidation is responsible for the 100 W increase in metabolic rate observed between L and M, because rates of lipid and protein oxidation remain constant. This high reliance on CHO is not compatible with the well known fuel selection pattern of exercise, when considering the relatively low metabolic rates elicited by shivering (∼30% for M). We conclude that shivering and exercise of similar energy requirements appear to be supported by different fuel mixtures. Investigating the physiological mechanisms underlying why a muscle producing only heat (shivering), or significant movement (exercise), shows a different pattern of fuel selection at the same power output strikes us as a fascinating area for future research.

To be able to match ATP supply with demand, animals must ensure adequate delivery of metabolic fuels and oxygen to tissue mitochondria. Therefore, the mixture of fuels provided and their individual flux must be tightly orchestrated to cope with changing physiological needs. In exercising mammals, metabolic rate—expressed relatively to the aerobic maximum: %VO2 max—determines what mixture of oxidative fuels is being used. This simple model of fuel selection accurately predicts the relative contributions of lipids and carbohydrates to total metabolism, and it applies widely across body sizes, aerobic capacities, and even to exercise in hypoxic environments. However, it is also becoming obvious that significant exceptions to this pattern exist in other vertebrates that rely more heavily on lipids (e.g., migrating birds) or proteins (e.g., migrating salmonids), or for stresses other than exercise (e.g., cold exposure in mammals). Instantaneous fuel use is determined by multiple interacting mechanisms involving fuel availability, storage location, muscle recruitment, fiber recruitment within each muscle, and metabolic pathway selection within each fiber. These various mechanisms are being characterized in more detail to try designing a general model of fuel selection applicable to a wider range of animals and physiological stresses.

Carbohydrates (CHO) can play an important thermogenic role during shivering, but the effect of their availability on the use of other oxidative fuels is unclear. Using indirect calorimetry and tracer methods ([U-13C]glucose ingestion), we have determined the specific contributions of plasma glucose, muscle glycogen, proteins, and lipids to total heat production (Hprod) in men exposed to cold for 2-h (liquid-conditioned suit perfused with 10 degrees C water). Measurements were made after low-CHO diet and exercise (Lo) and high-CHO diet without exercise (Hi). The size of CHO reserves had no effect on Hprod but a major impact on fuel selection before and during shivering. In the cold, a complete shift from lipid oxidation for Lo (53, 28, and 19% Hprod for lipids, CHO, and proteins, respectively) to CHO-based metabolism for Hi (23, 65, and 12% Hprod for lipids, CHO, and proteins, respectively) was observed. Plasma glucose oxidation remains a minor fuel under all conditions (<13% Hprod), falling to 7% Hprod for Lo. Therefore, adjusting plasma glucose oxidation to compensate for changes in muscle glycogen oxidation is not a strategy used for maintaining heat production. Instead, proteins and lipids share responsibility for this compensation. We conclude that humans can show remarkable flexibility in oxidative fuel selection to ensure that heat production is not compromised during sustained cold exposure.

INTRODUCTION:We examined the effect of prior heating, by exercise and warm-water immersion, on core cooling rates in individuals rendered mildly hypothermic by immersion in cold water.METHODS:There were seven male subjects who were randomly assigned to one of three groups: 1) seated rest for 15 min (control); 2) cycling ergometry for 15 min at 70% Vo2 peak (active warming); or 3) immersion in a circulated bath at 40 degrees C to an esophageal temperature (Tes) similar to that at the end of exercise (passive warming). Subjects were then immersed in 7 degrees C water to a Tes of 34.5 degrees C.RESULTS:Initial Tes cooling rates (initial approximately 6 min cooling) differed significantly among the treatment conditions (0.074 +/- 0.045, 0.129 +/- 0.076, and 0.348 +/- 0.117 degrees C x min(-1) for control, active, and passive warming conditions, respectively); however, secondary cooling rates (rates following initial approximately 6 min cooling to the end of immersion) were not different between treatments (average of 0.102 +/- 0.085 degrees C x min(-1)). Overall Tes cooling rates during the full immersion period differed significantly and were 0.067 +/- 0.047, 0.085 +/- 0.045, and 0.209 +/- 0.131 degrees C x min(-1) for control, active, and passive warming, respectively.DISCUSSION:These results suggest that prior warming by both active and, to a greater extent, passive warming, may predispose a person to greater heat loss and to experience a larger decline in core temperature when subsequently exposed to cold water. Thus, functional time and possibly survival time could be reduced when cold water immersion is preceded by whole-body passive warming, and to a lesser degree by active warming.