Thermal energy is transferred within and between bodies via several avenues, but for most unprotected human cold exposures, particularly during immersion, convective heat loss dominates. Lower tissue temperatures stimulate thermoreceptors, and the resultant afferent flow elicits autonomic homoeostatic responses (thermogenesis and vasoconstriction) that regulate body temperature within a narrow range. The most powerful effector responses occur when both superficial and deep thermoreceptors are cooled simultaneously, but thermoeffector activation can also occur as a result of peripheral cooling alone. The responses to cold, and the hazards associated with cold exposure, are moderated by factors which influence heat production and heat loss, including the severity and duration of cold stimuli, accompanying exercise, the magnitude of the metabolic response, and individual characteristics such as body composition, age, and gender. Cold stress can quickly overwhelm human thermoregulation with consequences ranging from impaired performance to death. This review provides a comprehensive overview of the human physiological responses to acute cold exposure.

Human physiological responses to cold exposure.

Cold exposure in humans causes specific acute and chronic physiological responses. This paper will review both the acute and long-term physiological responses and external factors that impact these physiological responses. Acute physiological responses to cold exposure include cutaneous vasoconstriction and shivering thermogenesis which, respectively, decrease heat loss and increase metabolic heat production. Vasoconstriction is elicited through reflex and local cooling. In combination, vasoconstriction and shivering operate to maintain thermal balance when the body is losing heat. Factors (anthropometry, sex, race, fitness, thermoregulatory fatigue) that influence the acute physiological responses to cold exposure are also reviewed. The physiological responses to chronic cold exposure, also known as cold acclimation/acclimatization, are also presented. Three primary patterns of cold acclimatization have been observed, a) habituation, b) metabolic adjustment, and c) insulative adjustment. Habituation is characterized by physiological adjustments in which the response is attenuated compared to an unacclimatized state. Metabolic acclimatization is characterized by an increased thermogenesis, whereas insulative acclimatization is characterized by enhancing the mechanisms that conserve body heat. The pattern of acclimatization is dependent on changes in skin and core temperature and the exposure duration.

/pdf/human-physiological-responses-to-cold-exposure-acute-5ejr0u82oc.pdf

Human physiological responses to cold exposure: Acute responses and acclimatization to prolonged exposure

We tested the hypothesis that key endocrine responses to semistarvation would be attenuated by changing only the food intake in a multistressor environment that also included sustained workload, inadequate sleep, and thermal strain. Serum hormones were compared within and between two groups of healthy young male volunteers participating in the 8-wk US Army Ranger course, with four repeated cycles of restricted energy intakes and refeeding: group 1 (n = 49) and group 2 (n = 48); energy deficits averaged 1,200 and 1,000 kcal/day, respectively. After 8 wk, most of group 1 achieved a minimum body fat, serum 3,5,3'-triiodothyronine (T(3)) was below normal (78 +/- 20 ng/dl), testosterone (T) approached castrate levels (4.5 +/- 3.9 nmol/l), insulin-like growth factor I (IGF-I) declined by one-half (75 +/- 25 microg/l), and cholesterol rose from 158 +/- 31 to 217 +/- 39 mg/dl. Bioavailable T(3) and T were further reduced by increases in their specific binding proteins in response to declining insulin. Refeeding, even with continuation of the other stressors, produced prompt recovery of T(3), T, and IGF-I. Higher energy intakes in group 2 attenuated the subclinical hypothyroidism and hypercholesterolemia, whereas consistent luteinizing hormone suppression indicated centrally mediated threshold effects on gonadal hormone suppression. We conclude that low T, T(3), and IGF-I remained reliable markers of acute energy deficits in the presence of other stressors; elevated cholesterol and cortisol provided information about chronic status, corresponding to diminishing body fat stores.

Endocrine markers of semistarvation in healthy lean men in a multistressor environment.

Cardiovascular disease (CVD) follows a seasonal pattern in many populations Broadly defined winter peaks and clusters of all subtypes of CVD after 'cold snaps' are consistently described, with corollary peaks linked to heat waves Individuals living in milder climates might be more vulnerable to seasonality Although seasonal variation in CVD is largely driven by predictable changes in weather conditions, a complex interaction between ambient environmental conditions and the individual is evident Behavioural and physiological responses to seasonal change modulate susceptibility to cardiovascular seasonality The heterogeneity in environmental conditions and population dynamics across the globe means that a definitive study of this complex phenomenon is unlikely However, given the size of the problem and a range of possible targets to reduce seasonal provocation of CVD in vulnerable individuals, scope exists for both greater recognition of the problem and application of multifaceted interventions to attenuate its effects In this Review, we identify the physiological and environmental factors that contribute to seasonality in nearly all forms of CVD, highlight findings from large-scale population studies of this phenomenon across the globe, and describe the potential strategies that might attenuate peaks in cardiovascular events during cold and hot periods of the year

Seasonal variations in cardiovascular disease.

Challenging environmental conditions, including heat and humidity, cold, and altitude, pose particular risks to the health of Olympic and other high-level athletes. As a further commitment to athlete safety, the International Olympic Committee (IOC) Medical Commission convened a panel of experts to review the scientific evidence base, reach consensus, and underscore practical safety guidelines and new research priorities regarding the unique environmental challenges Olympic and other international-level athletes face. For non-aquatic events, external thermal load is dependent on ambient temperature, humidity, wind speed and solar radiation, while clothing and protective gear can measurably increase thermal strain and prompt premature fatigue. In swimmers, body heat loss is the direct result of convection at a rate that is proportional to the effective water velocity around the swimmer and the temperature difference between the skin and the water. Other cold exposure and conditions, such as during Alpine skiing, biathlon and other sliding sports, facilitate body heat transfer to the environment, potentially leading to hypothermia and/or frostbite; although metabolic heat production during these activities usually increases well above the rate of body heat loss, and protective clothing and limited exposure time in certain events reduces these clinical risks as well. Most athletic events are held at altitudes that pose little to no health risks; and training exposures are typically brief and well-tolerated. While these and other environment-related threats to performance and safety can be lessened or averted by implementing a variety of individual and event preventative measures, more research and evidence-based guidelines and recommendations are needed. In the mean time, the IOC Medical Commission and International Sport Federations have implemented new guidelines and taken additional steps to mitigate risk even further.

https://bjsm.bmj.com/content/bjsports/46/11/770.full.pdf

International Olympic Committee consensus statement on thermoregulatory and altitude challenges for high-level athletes

The purpose of this study was to determine how chronic exertional fatigue and sleep deprivation coupled with negative energy balance affect thermoregulation during cold exposure. Eight men wearing ...

Exertional fatigue, sleep loss, and negative energy balance increase susceptibility to hypothermia

Objective To evaluate an ambulatory physiological monitoring system during a mountaineering expedition. We hypothesized that the Environmental Symptoms Questionnaire, combined with frequent measurement of oxygen saturation and core temperature, would accurately identify cases of environmental illness. Methods Twelve military mountaineers took a daily Environmental Symptoms Questionnaire, monitored fingertip oxygen saturations, and recorded core temperatures while climbing a 4,949-m peak. Illnesses identified by the system were compared with those identified by spontaneous reports. Results The system correctly identified one case of high-altitude pulmonary edema and two illnesses that were not reported to the physician (one case of acute mountain sickness and one of self-limited symptomatic desaturation). However, it did not identify two illnesses that were severe enough to preclude further climbing (one case of sinus headache and one of generalized fatigue). Conclusions Our monitoring system may complement, but cannot replace, on-site medical personnel during mountaineering expeditions.

Ambulatory physiological status monitoring during a mountaineering expedition.

Thermoregulatory responses to cold water at different

exercise during cold exposure: effects of prior

: In February 1997, ten Ranger students were studied during four days of swamp phase training at Camp Rudder. Objective: to confirm that core temperatures remain above 35.5 deg C (95.9 deg F) when the Ranger Cold Weather Immersion Limits Table guidelines were followed. Methods: an accurate, unobtrusive, lightweight (3 oz) Body Core Temperature Monitor (BCTM) and an FDA-approved temperature pill were used to record and store data for later analysis. A logbook of date, time, grid coordinates, debarkation water temperature, and immersion depths was kept. Weather data, including river depth and water temperature, was obtained from automated weather stations in the Ranger training area. Results: Due to the unusually warm weather conditions (air temp. = 56 to 76 deg F) and limited immersion time, the data needed to validate the Immersion Limits Table could not be collected. Nevertheless, the volunteers exhibited a remarkably wide range of core temperatures. Average core temperature ranged from 35.9 deg C (96.6 deg F) to 38.2 deg C (100.8 deg F), with individual values ranging from 35.4 deg C (95.7 deg F) to 39.1 deg C (102.4 deg F). Recommendations: (a) return to Camp Rudder next winter to continue effort to validate Cold Weather Immersion Limits Table, (b) initiate similar studies of Mountain Phase of Ranger Training, (c) define design for an improved BCTM capable of transmitting core temperature data to a data collecting system that would warn when a soldier's temperature was outside safe limits.

Warfighter Physiological Status Monitoring (WPSM): Body Core Temperatures During 96 h of Swamp Phase Ranger Training.

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