Acute Exposure

to Cold Air and Metabolic Responses to Exercise

A. Therminarias Laboratoire de Physiologie, Faculté de Médecine de Grenoble, 38700 La Tronche, France

Abstract A. Therminarias, Acute Exposure to Cold Air

and Metabolic Responses to Exercise. mt J Vol 13,Suppl l,ppSl87—S190, 1992.

Sports Med,

Acute exposure of the whole body to cold air

activates thermoregulatory mechanisms which may influence the physiological responses to exercise. Interactions between cold stress and exercise greatly depend on the intensity of cold stimulation. During exposure to moderate

cold (MC) peripheral vasoconstriction shifts part of the blood from the periphery to the core, increasing the central volume and the ventricular filling. When an incremental exercise is performed in MC, the persistence of cutaneous vasoconstriction alters the cardiovascular pattern. Moreover, a delayed onset of the increase in plasma lactate concentration (LA) is found and LA remains lower for submaximum exercise intensities. Simultaneously a greater plasma norepinephrine (NA) response is observed. In addition to

Introduction Development of winter sports, increasing participation in year round outdoor sports such as triathions, running and cycling have increased the incidence in which human exercises in cold environment. If the body's protection is insufficient, cold stress triggers the thermoregulatory mechaInt.J.SportsMed. 13(1992)S187—Sl90 GeorgThieme Verlag StuttgartNew York

cutaneous vasoconstriction shivering thermogenesis occurs during exposure to severe cold (SC) which increases heat production. During incremental exercise, the oxygen consumption (V02) and the expired minute ventilation (VE) are higher for each exercise intensity. However the ventilatory equivalent (VCO2/ V02) does not change significantly. The increased ventilatory response seems to remain a pure reaction to increasing metabolic demand. The ventilatory threshold occurs at the same exercise intensity but at a higher V02 and VE than in warm conditions. According to the intensity of cold stress the V02 level may be similar, in-

creased or decreased at exhaustion. The LA is higher for light exercise intensities, lower for heavy exercise intensities and recovery. Simultaneously a greater NA was found with no change in plasma epinephrine response. Key words

Thermogenesis, shivering, catecholamines, lactate

nisms which may interact with physiological responses to exercise. The consequences of cold stress depend on the

intensity of the cold stimulation. Whole body exposure to moderate cold induces a cutaneous vasoconstriction which reduces heat loss. If the cold stimulation is severe, shivering thermogenesis occurs which increases heat production. Both these mechanisms may interact with physiological adaptations to

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mt. I Sports Med. 13(1992) S187

S188 Tnt. let. J.J.Sports Sports Med. Med. 13(1992)

diovascular pattern (5, 12, 20). Shivering may occur in many skeletal muscles, particularly those involved in muscular exercise. Possible interactions may exist in the muscle as well as in cardiovascular, metabolic and hormonal responses induced

by muscular activity. The intensity of cold stimulation markedly varies according according to to whether whether exposure exposure to to cold coldoccurs occurs in water or in air. In water water the the convection convection coefficient, coefficient,25 25times times higher than in air, induces a considerably more significant heat loss given the same environment temperature, easily producing hypothermia (25). Because Because heat heat loss loss in in water water isis easier easierto to control, interactions between cold exposure and exercise have often been studied during during water water immersion immersion (8, (8, 11, 11, 15, 15, 18, 18,21, 21, 23). However, immersion alters the status of the respiratory and cardiovascular systems during resting conditions at neutral temperature (7, 9). The present paper is focused on the influence of acute exposure to cold air on metabolic responses to exercise. In addition to the the cold cold intensity, intensity, interactions interactionsbetween between cold and exercise also depend on factors such as the type, intensity tensity and duration of the exercise (3, 4). These metabolic metabolic reresponses have been studied during an incremental aerobic exercise carried out until exhaustion.

Methods

Experiments were performed on young, well trained subjects (20—30 years), essentially cyclists and football players whose body fat was less than 20%. Subjects exercised

in the morning, approximately 2 hours after a continental breakfast. breakfast. The exercises were performed in a climatic climatic chamber chamber in which the controlled temperature ranged from 30 to — 20 °C. Since heat loss in cold air is affected by windchill and by cloth-

ing, air velocity was controlled (10 mIs) and subjects were lightly dressed. After 5-minute rest period they began to exercise on a cycloergometer. The protocole consisted of an incremental exercise up to exhaustion (13, 14, 26, 30). Each experiment lasted from 15 to 35 minutes. We considered cold stress as moderate when the temperature of the cold environment was sufficient to elicit a peripheral vasoconstriction but no shivering. This condition was carried out in an ambient tem-

perature of approximately 10 "C °C (30). We considered cold stress as intense in ambient temperatures below 0 "C °C (13, 14, 26). Consequences of cold exposure on exercise were successively considered during moderate and severe cold stress. Results and Discussion

Exposure to moderate cold Exposure to moderate cold essentially alters the cardiovascular adjustments but also induces metabolic and hormonal changes. Cardiovascular adjustments During exposure to cold air the peripheral vasoconstriction shifts part of the blood from the periphery to the

core increasing the central blood volume, the ventricular filling, and the systolic volume (27). After exposure for 5 minutes at 10 °C, "C, the cutaneous cutaneous vasoconstriction vasoconstriction led led to toaa decrease in mean skin temperature, an increase in systolic and

diastolic blood pressures and a bradycardia (30). When incremental muscular exercise was performed at 10 "C °C cutaneous vasoconstriction persisted inducing a cardiovascular

pattern different from that observed in a temperate environ°C, sysment. For each submaximal exercise intensity at 10 "C, tolic pressure was higher, heart rate was lower but the change in diastolic blood pressure was not significant. With a similar pattern it has been found that for light to moderate exercise intensities stroke volume was higher as compared to temperate conditions resulting in little change in cardiac output (28). At exhaustion, maximal heart rate did not change while systolic blood pressure remained higher at 10 "C, °C, reaching very high levels in some subjects. As in warm conditions the rectal temperature was increased at the end of the exercise period (30). The persistence of peripheral vasoconstriction during exercise is probably required to allow for a beneficial rise in central temperature (1). (1). Metabolic changes During rest and for each exercise intensity, the oxygen consumption (V02) and the expired volume ventilation (STE) (S/E) were wereidentical identicalat at 10 10 °C "C and and in in aa warm warm environment. environment. Plasma concentrations of substrates such as glucose and free fatty acid (FFA), or ions (potassium, sodium, phosphorus) were unaffected by exposure to cold (30). However moderate cold stress influenced plasma lactate concentration (LA) (30). At At 10 10 "C °C as compared to a warm environment, LA was identical cal for low exercise intensities, increased at a slower slower rate rate and and reached lower levels for high submaximal exercise intensities. The LA represents the balance balance between between the the amount amount released released into the blood stream and the amount removed. Lactate metabolism is difficult to evaluate solely on the basis of venous

concentrations. However, during exercise, an increase in blood lactate is generally considered as a indication of lactate

production by the working muscles due to an inadequate supply of oxygen (31). Cutaneous vasoconstriction may have favoured the vascularisation of working muscles, decreasing the anaerobic glycogenolysis rate. It may also have have favoured favoured vascularisation in the liver, a major site of lactate removal, increasing the lactate catabolism. Such a hypothesis has already been considered by Claremont et al. who have similarly found a lower LA in a cold environment (6).

Catecholamine changes At the end of the exercise performed at 10 °C, a further increase in plasma norepinephrine concentration was found. A higher plasma catecholamine level was previously observed when exercise was performed in colder conditions (2). This further rise suggests a greater involvement of the sym-

patho-nervous system. Norepinephrine probably plays a marked role in particular cardiovascular adjustments observed under these conditions. However catecholamines may not be the only hormones involved in the changes. No information is available on the possible effects of exposure to cold on the secretion of hormones or hormonal factors influenced by ventricular filling (arginine vasopressin, atrial natriuretic factor, renin angiotensin system). Nevertheless changes in ventricular filling due to peripheral vasoconstriction may alter these hormonal responses.

Exposure to severe cold When the peripheral vasoconstriction is inadequate to maintain homeothermia, shivering thermogenesis oc-

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exercise. Cutaneous vasoconstriction may alter the car-

A. Therminarias

mt. i Sports Med. 13(1992) S189

Acute Exposure to ColdAir and Metabolic Responses to Exercise

2 °C as compared to 24 °C bin values similarly decreased at — 2 °C (30).

Metabolic changes

Energy expenditure Exposure to a temperature of — 2 °C rapidly in-

duced sustained shivering. After a 5-minute rest. V02 was twice as high as in a temperate environment (14). When an incremental muscular exercise was performed under these conditions for exercise intensities of up to 80% of VO2max, the V02 remained higher than in comfortable conditions reflect-

ing a greater energy expenditure (14). This further increase may be due to a reduction in the mechanical efficiency of working muscles but mainly to the shivering produced by muscles not involved in muscular exercise. As generally observed in cold water (21, 25) the difference in V02 between warm and cold environments tended to decrease for high exercise intensities. When cold stimulation is not too severe, the thermogenesis produced by exercise may obviate the need for shivering thermogenesis. For high exercise intensities and at exhaustion, the energy expenditure varied according to the severity of cold stress. At — 2 °C, a further increase in energy ex-

penditure may persist inducing a light increase in VO2max not coupled with an increase in maximal work load. As a result the

mechanical efficiency decreased. The rectal temperature increased as compared to resting values (14). This extra thermogenesis is presumably necessary to reach an adequate rise in core temperature during muscular exercise. Indeed, it has been previously related that an increase in core temperature is necessary to reach the maximal oxygen consumption (1).

During a very severe cold stress (—20 °C) a decrease in VO2max was found coupled with a decrease in the maximal work load (26). Under these conditions the rectal temperature tended to decrease below 37 °C, reflecting an imbalance in thermic equilibrium. These last findings are similar to those obtained during exercise performed in head-out cold water

immersion. Indeed, most investigators have reported a decrease in VO2max during ergometry or swimming swimming in in cold cold water(11, 18,23). water(1l, Ventilatory and circulatory functions During exposure at — 22 °C, at rest and for each exercise intensity, the VE was higher than that measured in warm conditions. The ventilatory equivalent (VE/ V02) was not significantly affected by exposure to cold. Thus, at rest, as well as during exercise, the increase in ventilatory response

seems to remain a pure reaction to increasing metabolic demand (13). The ventilatory threshold (31) occurred at the same exercise intensity at — 22 °C and in a warm environment. However, as a consequence of shivering thermogenesis, this V02 which is about 0.5 exercise intensity corresponds to a '702 1/mm higher higher and and to a VE approximately 16 1/mm higher than I/mm in warm condition (13). Apparently the ventilatory threshold

The presence of shivering thermogenesis

during exercise increased the energy expenditure and necessarily the amount of substrates used by active muscles. It has been demonstrated in human and in animals, that shivering may use glucose (19, 29), free fatty acid (FFA) (24) and lactate (22) as substrates. Glucose appears to be specially important since hypoglycaemia suppressed shivering (16). During exercise at — 2 °C, we studied only plasma substrate concentrations providing little information on the nature of substrates preferentially used (30). At exhaustion glycaemia was not substantially altered by byexposure exposuretoto— —2 2 °C °Cbut butthe the FFA FFA level stantially altered tended to be higher. Simultaneously the respiratory exchange ratio (RER = VCO2/ V02) tended to be lower, suggesting a preferential oxidation of lipids. Nevertheless, these results are difficult to interpret since RER value greatly depends on lactate metabolism. Acute exposure to —2 OC markedly influences fluences this this metabolism metabolism (30). (30). When When an an incremental incremental exercise exercise °C,the the LA LA was was higher higher during moderate was performed performed at at —2 — 2 °C, exercise intensities, increased at a slower rate than in warm conditions and remained lower during high exercise intensities, at exhaustion and during recovery. These results correspond to studies previously reported in which LA may be lower (6) similar (10) or higher (15, 18) depending on cold conditions. Obviously, severe cold stress alters the metabolism or

the kinetics of lactate more than moderate cold stress. At — —2 °C,aapart partof oflactate lactate production production is is possibly possibly taken taken up and 2 °C, consumed by shivering muscles (22). On the other hand the increase in LA found during during resting resting cold cold conditions conditions isis generally generally

attributed to a greater secretion of epinephrine acting on muscle muscle glycogenolysis (17). In severe cold stress, stress, as as expected expected aa further increase in plasma norepinephrine concentration was

found at exhaustion. However plasma norepinephrine concentration was not higher than that found in moderate cold. Moreover no further increase in epinephrine secretion was found excluding apparently an effect of epinephrine on lactate metabolism (26, 30).

Acute exposure of the whole body to cold stress

greatly influences cardiovascular, metabolic and catecholamine responses to an exhaustive exercise. Effects greatly vary with the intensity of cold stress. They may also differ according to the studied population. These results have been obtained in young well trained subjects. Responses may differ ac-

cording to age, sex, diet or physical fitness. Further studies need to be performed in order to determine the respective roles of these different factors and to better understand the mechanisms involved in the interactions interactions between between cold cold and and exercise. exercise.

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Dr. A. Therminarias Laboratoire de Physiologie Faculté de Médecine de Grenoble F-38700 LaTronche France

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Acute exposure to cold air and metabolic responses to exercise.

Acute exposure of the whole body to cold air activates thermoregulatory mechanisms which may influence the physiological responses to exercise. Intera...
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