Comparison of thermoregulatory responses between men and women immersed in cold water

Dietary tyrosine benefits cognitive and psychomotor performance during body cooling

Supplemental tyrosine is effective at limiting cold-induced decreases in working memory, presumably by augmenting brain catecholamine levels, since tyrosine is a precursor for catecholamine synthesis. The effectiveness of tyrosine for preventing cold-induced decreases in physical performance has not been examined. This study evaluated the effect of tyrosine supplementation on cognitive, psychomotor, and physical performance following a cold water immersion protocol that lowered body core temperature. Fifteen subjects completed a control trial (CON) in warm (35 degrees C) water and two cold water trials, each spaced a week apart. Subjects ingested an energy bar during each trial; on one cold trial (TYR) the bar contained tyrosine (300 mg/kg body weight), and on the other cold trial (PLB) and on CON the bar contained no tyrosine. Following each water immersion, subjects completed a battery of performance tasks in a cold air (10 degrees C) chamber. Core temperature was lower (p=0.0001) on PLB and TYR (both 35.5+/-0.6 degrees C) than CON (37.1+/-0.3 degrees C). On PLB, performance on a Match-to-Sample task decreased 18% (p=0.02) and marksmanship performance decreased 14% (p=0.002), compared to CON, but there was no difference between TYR and CON. Step test performance decreased by 11% (p=0.0001) on both cold trials, compared to CON. These data support previous findings that dietary tyrosine supplementation is effective for mitigating cold-induced cognitive performance such as working memory, even with reduced core temperature, and extends those findings to include the psychomotor task of marksmanship.

American College of Sports Medicine position stand: prevention of cold injuries during exercise

It is the position of the American College of Sports Medicine that exercise can be performed safely in most cold-weather environments without incurring cold-weather injuries. The key to prevention is use of a comprehensive risk management strategy that: a) identifies/assesses the cold hazard; b) identifies/assesses contributing factors for cold-weather injuries; c) develops controls to mitigate cold stress/strain; d) implements controls into formal plans; and e) utilizes administrative oversight to ensure controls are enforced or modified. The American College of Sports Medicine recommends that: 1) coaches/athletes/medical personnel know the signs/symptoms and risk factors for hypothermia, frostbite, and non-freezing cold injuries, identify individuals susceptible to cold injuries, and have the latest up-to-date information about current and future weather conditions before conducting training sessions or competitions; 2) cold-weather clothing be chosen based on each individual's requirements and that standardized clothing ensembles not be mandated for entire groups; 3) the wind-chill temperature index be used to estimate the relative risk of frostbite and that heightened surveillance of exercisers be used at wind-chill temperatures below -27 degrees C (-18 degrees F); and 4) individuals with asthma and cardiovascular disease can exercise in cold environments, but should be monitored closely.

Physiological response to water immersion: a method for sport recovery?

Recovery from exercise can be an important factor in performance during repeated bouts of exercise. In a tournament situation, where athletes may compete numerous times over a few days, enhancing recovery may provide a competitive advantage. One method that is gaining popularity as a means to enhance post-game or post-training recovery is immersion in water. Much of the literature on the ability of water immersion as a means to improve athletic recovery appears to be based on anecdotal information, with limited research on actual performance change. Water immersion may cause physiological changes within the body that could improve recovery from exercise. These physiological changes include intracellular-intravascular fluid shifts, reduction of muscle oedema and increased cardiac output (without increasing energy expenditure), which increases blood flow and possible nutrient and waste transportation through the body. Also, there may be a psychological benefit to athletes with a reduced cessation of fatigue during immersion. Water temperature alters the physiological response to immersion and cool to thermoneutral temperatures may provide the best range for recovery. Further performance-orientated research is required to determine whether water immersion is beneficial to athletes.

Impaired defense of core temperature in aged humans during mild cold stress

Aged humans often exhibit an impaired defense of core temperature during cold stress. However, research documenting this response has typically used small subject samples and strong cold stimuli. The purpose of this study was to determine the responses of young and older subjects, matched for anthropometric characteristics, during mild cold stress. Thirty-six young (YS; 23 ± 1 years, range 18–30) and 46 older (OS; 71 ± 1 years, range 65–89) subjects underwent a slow transient cold air exposure from a thermoneutral baseline, during which esophageal (Tes) and mean skin temperatures (Tsk), O2 consumption, and skin blood flow (SkBF; laser-Doppler flowmetry) were measured. Cold exposure was terminated at the onset of visible sustained shivering. Net metabolic heat production (Mnet), heat debt, predicted change in midregion temperature (ΔTmid), and tissue insulation (It) were calculated. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial pressure and expressed as percent change from baseline (ΔCVC%base). There were no baseline group differences for Tes, but OS Mnet was lower (OS: 38.0 ± 1.1; YS: 41.9 ± 1.1 W · m−2, P < 0.05). Tes was well maintained in YS but fell progressively in OS ( P < 0.01 for all timepoints after 35 min). The skin vasoconstrictor response to mild cold stress was attenuated in OS (42 ± 3 vs. 53 ± 4 ΔCVC%base, P < 0.01). There were no group differences for Tsk or It, while Mnet remained lower in OS ( P < 0.05). The ΔTmid did not account for the drop in Tes in OS. Healthy aged humans failed to maintain Tes; however, the mechanisms underlying this response are not clear.

Shivering in the cold: from mechanisms of fuel selection to survival

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.

Responses to mild cold stress are predicted by different individual characteristics in young and older subjects

Older individuals' ability to maintain core temperature during cold stress is impaired; however, the relative importance of individual characteristics that influence this response are unknown. The purpose of this study was to determine the relative influence of individual characteristics on core temperature and tissue insulation (I(t)) during mild cold stress. Forty-two young (23 +/- 1 yr, range 18-30 yr) and 46 older (71 +/- 1 yr, range 65-89 yr) subjects, varying widely in muscularity, adiposity, and body size, underwent a transient cooling protocol during which esophageal temperature (T(es)) was measured continuously and I(t) was calculated using standard equations. Multiple-regression analyses were performed to determine predictors of T(es) and I(t), and standardized regression coefficients were analyzed to determine the relative influence of each predictor. Candidate predictors included age, sex, weight, body surface area, body surface area-to-mass ratio, sum of skinfolds, percent fat, appendicular skeletal muscle mass, and thyroid hormone concentrations (triiodothyronine, thyronine). The sum of skinfolds explained 67% (P < 0.01) of the T(es) variance in young subjects vs. 2% (P = 0.30) in older subjects. Conversely, appendicular skeletal muscle mass explained a greater portion of the variance in older subjects for both T(es) (older: 28%, P < 0.01; young: 8%, not significant) and I(t) (older: 46%, P < 0.01; young: 17%, P < 0.01). The T(es) residual variance was considerably larger in older subjects (59-72% vs. 14-42% in young subjects), possibly due to varying rates of physiological aging. These results suggest that the relative influence of individual characteristics changes with aging.

Fuel selection in shivering humans

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.

Thermoregulatory model for prediction of long-term cold exposure

A multi-segmental mathematical model has been developed for predicting shivering and thermoregulatory responses during long-term cold exposure. The present model incorporates new knowledge on shivering thermogenesis, including the control and maximal limits of its intensity, inhibition due to a low core temperature, and prediction of endurance time. The model also takes into account individual characteristics of age, height, weight, % body fat, and maximum aerobic capacity. The model was validated against three different cold conditions i.e. water immersion up to 38 h and air exposure. The predictions were found to be in good agreement with the observations.

Partitioning oxidative fuels during cold exposure in humans: muscle glycogen becomes dominant as shivering intensifies

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 degrees C (low-intensity shivering (L)) or 5 degrees 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 (approximately 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.

Can you be large and not obese? The distinction between body weight, body fat, and abdominal fat in occupational standards

Weight control is an important early intervention in diabetes, but the nature of the association between weight and disordered metabolism has been confused because fat mass and its distribution are only partly associated with increasing body size. Weight, fat, and regional fat placement, specifically in the abdominal site, may each have distinctly different associations with diabetes risk. Abdominal circumference may be the common marker of poor fitness habits and of increased risk for metabolic diseases such as diabetes. This is an important question for public health policy as well as for occupational standards such as those of the military, which are intended to promote fitness for military missions and include strength and aerobic capacity, as well as military appearance considerations. U.S. soldiers are heavier than ever before, reflecting both increased muscle and fat components. They also have better health care than ever before and are required to exercise regularly, and even the oldest soldiers are required to remain below body fat limits that are more stringent than the current median values of the U.S. population over age 40. The body fat standards assessed by circumference-based equations are 20-26% and 30-36%, for various age groups of men and women, respectively, and the upper limits align with threshold values of waist circumference recommended in national health goals. The basis and effects of the Army standards are presented in this paper. U.S. Army body fat standards may offer practical and reasonable health guidelines suitable for all active Americans that might help stem the increasing prevalence of obesity that is predicted to increase the prevalence of Type 2 diabetes.

Magnesium Sulfate Increases the Rate of Hypothermia Via Surface Cooling and Improves Comfort

BACKGROUND AND PURPOSE

Therapeutic hypothermia shows promise as a treatment for acute stroke. Surface cooling techniques are being developed but, although noninvasive, they typically achieve slower cooling rates than endovascular methods. We assessed the hypothesis that the addition of intravenous MgSO4 to an antishivering pharmacological regimen increases the cooling rate when using a surface cooling technique.

METHODS

Twenty-two healthy volunteers were studied. Hypothermia was induced using a surface technique with a target tympanic temperature (Ttym) of 34.5 degrees C (target range 34 to 35 degrees C). Subjects received 1 of the following pharmacological regimens: (1) meperidine monotherapy (n=5); (2) meperidine plus buspirone, 30 to 60 mg PO administered at the time of initiation of cooling (n=4); (3) meperidine and ondansetron, 8 to 16 mg IV administered as an 8 mg bolus at the time of initiation of cooling with an optional second dose after 4 hours as needed for nausea (n=5); or (4) meperidine, ondansetron, and MgSO4, 4 to 6 g IV bolus followed by 1 to 3 g per hour infusion (n=8). Thermal comfort was evaluated with a 100-mm-long visual analog scale.

RESULTS

More subjects who received MgSO4 were vasodilated during hypothermia induction (7 of 8 [88%] versus 4 of 14 [29%]; P=0.024). MgSO4 (coefficient -17.265; P=0.039), weight (1.838, 0.001), and the initial 2-hour meperidine dose (0.726, 0.003) were found to significantly impact the time to achieve Ttym of 35 degrees C. Subjects who received MgSO(4) had significantly higher mean comfort scores than those who did not (48+/-15 versus 38+/-12; P<0.001).

CONCLUSIONS

Administration of intravenous MgSO(4) increases the cooling rate and comfort when using a surface cooling technique.

Regulated hypothermia reduces brain oxidative stress after hypoxic-ischemia

Regulated hypothermia produces a decrease in core temperature by lowering the brain's temperature set-point while maintaining thermoregulation at that lower set point. In contrast, forced hypothermia lowers core temperature by overwhelming the body's capacity to thermoregulate, but does not change the set-point. Regulated hypothermia has been shown to be cerebral protective in hibernating mammals. The effect of regulated hypothermia on the brain during reperfusion from hypoxic-ischemia has not been well studied. We induced regulated hypothermia with a neurotensin analogue (NT77) to determine whether it could reduce oxidative stress in the brain during reperfusion from asphyxial cardiac arrest (ACA) in rats. Mild hypothermia (32-34 degrees C) was induced by brief (4 h) external cooling (BC), NT77 or prolonged external cooling (24 h) (PC) 30 min after resuscitation from 8 min of ACA in rats. Malondialdehyde (MDA) levels in the brain were measured during reperfusion to quantitate oxidative stress.

RESULTS

MDA levels in the hippocampus were elevated at 16 h of normothermic reperfusion versus 48 h with BC reperfusion. There was no increase in hippocampal MDA levels in the NT77 and PC groups at 24-72 h of reperfusion. Regulated hypothermia induced by NT77 reduced oxidative stress in the hippocampus during reperfusion from hypoxic-ischemia in comparison to forced brief external cooling of the same duration. In addition, the duration of external cooling after resuscitation also alters oxidative stress in the brain during reperfusion.

Lifestyle and demographic factors in relation to vasomotor symptoms: baseline results from the Study of Women's Health Across the Nation

Results of recent trials highlight the risks of hormone therapy, increasing the importance of identifying preventive lifestyle factors related to menopausal symptoms. The authors examined the relation of such factors to vasomotor symptoms in the multiethnic sample of 3,302 women, aged 42-52 years at baseline (1995-1997), in the Study of Women's Health Across the Nation (SWAN). All lifestyle factors and symptoms were self-reported. Serum hormone and gonadotropin concentrations were measured once in days 2-7 of the menstrual cycle. After adjustment for covariates using multiple logistic regression, significantly more African-American and Hispanic and fewer Chinese and Japanese than Caucasian women reported vasomotor symptoms. Fewer women with postgraduate education reported vasomotor symptoms. Passive exposure to smoke, but not active smoking, higher body mass index, premenstrual symptoms, perceived stress, and age were also significantly associated with vasomotor symptoms, although a dose-response relation with hours of smoke exposure was not observed. No dietary nutrients were significantly associated with vasomotor symptoms. These cross-sectional findings require further longitudinal exploration to identify lifestyle changes for women that may help prevent vasomotor symptoms.

Fuel selection during intense shivering in humans: EMG pattern reflects carbohydrate oxidation

The thermogenic response of humans depends critically on the coordination of muscle fibre recruitment and oxidative fuel metabolism. The primary goal of this study was to determine whether the electromyographic (EMG) pattern of muscle recruitment could provide metabolic information on oxidative fuel selection during high-intensity shivering. EMG activity (of 8 large muscles) and fuel metabolism were monitored simultaneously in non-acclimatized adult men during high-intensity shivering. Even though acute cold exposure elicited similar changes in metabolic rate among subjects, lipid and carbohydrate use was very different. Depending on the subject, the cold-induced increase in carbohydrate (CHO) oxidation ranged between 2- and 8-fold, with CHO accounting for 33-78% of total heat production, and lipids for 14-60%. This high variability in fuel selection was primarily explained by differences in 'burst shivering' rate, indicating that the recruitment of type II fibres plays a key role in orchestrating fuel selection. This study is the first to show that the pattern of muscle recruitment can provide quantitative information on energy metabolism. Future work should focus on the study of shivering bursts that may provide essential clues on what limits human survival in the cold.

Effects of carbohydrate availability on sustained shivering I. Oxidation of plasma glucose, muscle glycogen, and proteins

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.

Heat balance precedes stabilization of body temperatures during cold water immersion

Certain previous studies suggest, as hypothesized herein, that heat balance (i.e., when heat loss is matched by heat production) is attained before stabilization of body temperatures during cold exposure. This phenomenon is explained through a theoretical analysis of heat distribution in the body applied to an experiment involving cold water immersion. Six healthy and fit men (mean +/- SD of age = 37.5 +/- 6.5 yr, height = 1.79 +/- 0.07 m, mass = 81.8 +/- 9.5 kg, body fat = 17.3 +/- 4.2%, maximal O2 uptake = 46.9 +/- 5.5 l/min) were immersed in water ranging from 16.4 to 24.1 degrees C for up to 10 h. Core temperature (Tco) underwent an insignificant transient rise during the first hour of immersion, then declined steadily for several hours, although no subject's Tco reached 35 degrees C. Despite the continued decrease in Tco, shivering had reached a steady state of approximately 2 x resting metabolism. Heat debt peaked at 932 +/- 334 kJ after 2 h of immersion, indicating the attainment of heat balance, but unexpectedly proceeded to decline at approximately 48 kJ/h, indicating a recovery of mean body temperature. These observations were rationalized by introducing a third compartment of the body, comprising fat, connective tissue, muscle, and bone, between the core (viscera and vessels) and skin. Temperature change in this "mid region" can account for the incongruity between the body's heat debt and the changes in only the core and skin temperatures. The mid region temperature decreased by 3.7 +/- 1.1 degrees C at maximal heat debt and increased slowly thereafter. The reversal in heat debt might help explain why shivering drive failed to respond to a continued decrease in Tco, as shivering drive might be modulated by changes in body heat content.

Effect of cold exposure on fuel utilization in humans: plasma glucose, muscle glycogen, and lipids

The relative roles of circulatory glucose, muscle glycogen, and lipids in shivering thermogenesis are unclear. Using a combination of indirect calorimetry and stable isotope methodology ([U-13C]glucose ingestion), we have quantified the oxidation rates of these substrates in men acutely exposed to cold for 2 h (liquid conditioned suit perfused with 10 degrees C water). Cold exposure stimulated heat production by 2.6-fold and increased the oxidation of plasma glucose from 39.4 +/- 2.4 to 93.9 +/- 5.5 mg/min (+138%), of muscle glycogen from 126.6 +/- 7.8 to 264.2 +/- 36.9 mg glucosyl units/min (+109%), and of lipids from 46.9 +/- 3.2 to 176.5 +/- 17.3 mg/min (+376%). Despite the observed increase in plasma glucose oxidation, this fuel only supplied 10% of the energy for heat generation. The major source of carbohydrate was muscle glycogen (75% of all glucose oxidized), and lipids produced as much heat as all other fuels combined. During prolonged, low-intensity shivering, we conclude that total heat production is unequally shared among lipids (50%), muscle glycogen (30%), plasma glucose (10%), and proteins (10%). Therefore, future research should focus on lipids and muscle glycogen that provide most of the energy for heat production.

Gender differences in thermoregulation

Women differ from men in thermal responses to exogenous heat load and heat loss as well as to endogenous heat load during exercise, because they usually have a larger ratio of body surface to body mass, a greater subcutaneous fat content, and lower exercise capacity. When these differences are eliminated in experimental studies, it appears that women's sweating response to heat load is still smaller than that of men, but they are able to maintain their core body temperature on a similar level to that of men as a result of greater evaporative efficiency of sweating. In addition, the changing rate of sex hormone release during the menstrual cycle modifies thermoregulation in women, so there are differences in resting body temperature and thermal responses to positive or negative heat loads depending on the phase of the cycle. In this review, the changes in thermoregulation in young women taking oral contraceptives and those associated with the menopause and hormonal replacement therapy are also described.