the Impact of Physical Activity and Cold Exposure

8/10/2019 the Impact of Physical Activity and Cold Exposure

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Journal of Wilderness Medicine 1 265-278 (1990)

he impact of physical activity and cold exposureon food intake and energy expenditure n manE.T. POEHLMAN 1 , A.W. GARDNER, 2 and M.l. GORANI

Endocrinology, Metabolism and Nutrition, College of Medicine, Department of Medicine, University of

Vermont, Burlington, VT05405, USA2Exercise and Sport Research Institute, Arizona State University, A Z 85287, USASupport: Dr E T Poehlman is supported by The National Institute of Aging AG07857) and the GeneralClinical Research Center RR1109) at the University of Vermont and American Association of Retired PersonsAndrus Foundation

The interrelationships of physical activity, food intake and extremes of environmental temperatureare important considerations influencing nutritional intake and physical performance in wildernessactivities. This brief review familiarizes the reader with the components responsible for regulatingfood intake and energy expenditure, addresses work that has examined the regulatory role ofexercise on the caloric content of food intake, considers the influence of regular participation inphysical activity on two components of energy expenditure (resting metabolic rate and energyexpenditure during submaximal exercise), and discusses the effects of acute and chronic exposure tocold temperature on food intake and energy expenditure. The adaptive changes in food intake andenergy expenditure in response to environmental challenges are relevant considerations for

wilderness activities.Key words: Food intake, metabolic rate, cold, energy expenditure, body composition

ntroduction

Regulation of body energy reserves occurs through changes in the energy content of foodconsumed or the energy lost by total daily energy expenditure. Energy intake is episodicin nature. On the other hand, total daily energy expenditure is relatively constant and, fortheoretical and analytical purposes, can be divided into several components (Fig. 1 ).Resting metabolic rate represents the largest portion of daily energy expenditure ( 6075 ) d i t f g d d f th i t f l b d f ti


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266 Poehlman Gardner and Goran

24hr ENERGY EXPEN ITURE

kc l

3 000

2 500

2 000

1 500

1 000

500

0

I

I

AIIII

Fig 1 The three major components of daily energy expenditure. RMR, resting metabolic rate;TEF, thermic effect of feeding; and TEA, thermic effect of activity From Poehlman eta/. [2] .

above resting metabolic rate, the thermic effect of feeding due to voluntary exercise, andenergy devoted to involuntary activity such as shivering, fidgeting and postural control.In sedentary individuals, the thermic effect of activity may comprise as little as 100 kcalper day; in highly active individuals it may approach 1000 kcal per day. In addition tothe direct energy cost associated with physical exercise, the thermic effect of activity hasbeen reported to influence the other two components of energy expenditure, resting

metabolic rate and the thermic effect of feeding [1,2].In this brief review, we discuss adaptive changes of food intake and energy expenditure to various environmental challenges. We first consider the regulatory role of exerciseon the energy content of food intake, then discuss the influence of chronic participationin physical activity on resting metabolic rate and on energy expenditure during submaximal exercise. Finally, we consider the influence of acute and chronic exposure to


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267ffect of physical activity and cold exposure on energy balance

that subjects are aware that food intake is being measured does not permit true assessment of changes in spontaneous food intake in response to an environmental challenge.Furthermore, previous studies have either imposed exercise sessions on volunteersubjects and made assumptions about their activity during nonexercise times of the day,or have ignored the food questions and made assumptions about energy intake fromsmall changes in body composition. t is apparent that the most meaningful data regarding food intake behavior can only be obtained under controlled living conditions.

In a classic study, Mayer and colleagues [3] described three different activity zones inwhich different food intake responses were provoked. Rats were exercised at each of thespecified exercise durations for a minimum of 14 days. Exercise durations less than60 min were not followed by increases in food intake; in fact, small decreases in foodintake were observed. At 2-6 h of exercise, food intake was tightly coupled to exerciseenergy expenditure. This suggests that exercise is a strong modulator of food intake andthat various thresholds of exercise modify food intake in a specific manner.

Mayer et a/ [4] examined the relationships between energy intake, body weight and

physical work in a group of workers in West Bengal, India. The range of activity variedfrom sedentary to very hard strenuous labor. The investigators found that energy intakeincreased within a certain normal activity zone . This suggested that food intake andenergy expenditure correlate proportionally in individuals who engage in medium andheavy work. Interestingly, the food intake of sedentary workers was higher than that ofworkers in a normal activity zone. One might speculate that below a certain threshold ofphysical activity, uncoupling of food intake from energy expenditure may occur, so thatsedentary individuals may overshoot and consume more calories than actually dictatedby their energy expenditure needs. Considerations of appetite related to physical exertionare difficult to quantify.

Widdowson eta/ [5] measured daily energy intake and energy output of 12 cadetsleading a rigorously controlled life at a military academy. When individual daily intakeand expenditure were compared over the short-term 1-2 days), no relationship wasfound between the two variables. However, the statistical means of the individual intakesand expenditures over 14 days of observation were well matched. This suggests thatshort-term matching of food intake to increased physical activity in man may beimprecise, whereas extended exercise may more precisely couple food intake to energyexpenditure.

McGowan eta/ [6] examined the effects of altering energy expenditure (no exercise,


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the subjects remained sedentary. This could be interpreted as a mismatch of food intakewith energy expenditure in obese individuals. However, it is unclear whether the subjectsconsciously restricted food intake in an effort to promote weight loss.

Durrant eta/. [8] increased energy expenditure in four lean and 12 obese subjects with100 kcal per day of cycling. d libitum energy intake was measured over 6 days usingan automated food dispensing machine. During the non-exercising period, obese andlean subjects showed no overall significant difference in energy intake or eating patterns.However, the obese subjects consumed 18 kcal per day less and the lean subjects ate

155 kcal per day more during the exercise period. The authors interpreted their findingsas evidence that lean subjects tend to regulate energy intake more accurately than doobese subjects in response to a moderate increase in physical activity. However, it isdifficult to ascribe the small 'adaptive' changes in caloric intake to the moderate increasein physical activity because of the large day-to-day variation of caloric intake withinindividuals.

In our estimation, the most impressive human work examining the response o food

intake to energy expenditure has been performed by Woo and colleagues [9,10]. Theadvantage of their study designs relates to the duration of the experiments and thecontrolled environment that permitted spontaneous assessment of food intake behavior.In the first study [9], they examined the effects of increased physical activity on thecaloric content of food intake in six obese women during three 19-day sessions: onesedentary session and two sessions with treadmill exercise which increased daily energyexpenditure to 100 (mild) and 125 (moderate) of sedentary energy expenditure.Individual daily energy expenditures and ad libitum intakes were determined by activitydiaries and covert monitoring of food intake, respectively. Negative energy balances wereobserved with mild and moderate exercise, because subjects did not increase their foodintakes to correspond to the increases in physical activity. In obese women, moderatelevels of physical activity do not appear to influence food intake. In a follow-up study,Woo and Pi-Sunyer [10] engaged five nonobese women in sedentary, mild and moderatelevels of physical activity. The lean women matched food intake with energy expenditureduring a sedentary period, with a compensatory increase in food intake in response toboth physical activity regimens. The authors speculated that high levels of body fat maycontribute to the unresponsiveness o obese women to match food intake with expenditure in response to physical activity.

Although there is substantial evidence that exercise influences food intake (at least in


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269ffect physical activity and cold exposure on energy balance

cise. They proposed that exercise may exert control over food intake either by itsinfluence on the body fat reserves or by altering the source of fuel oxidation.

Collectively, the goal of individuals engaged in prolonged wilderness activities shouldbe to maintain energy balance by matching caloric intake to energy expenditure. Insuring

n adequate energy flow to meet the metabolic demands of an increase in physicalactivity may not be a simple task. Coupling food intake to energy expenditure appears torelate to body fat and gender, as well as to the intensity and duration of theexercise challenge. Because the level of energy expenditure ultimately determines thelevel of caloric intake, it is important to appreciate the biological and behavioralcontributions to the matching (or mismatching) of intake to expenditure and theirinfluence on energy balance. The reader is referred to other reviews that have consideredthe effects of physical activity on food intake in human subjects [15,16].

Effects of physical activity on resting metabolic rate

Several cross-sectional studies have reported that the 'trained state' is associated with ahigher resting metabolic rate. In young men, Tremblay et a/ [ 7] found that restingmetabolic rate was highest ( - 11 ) in a group of highly trained athletes, relative tomoderately trained and inactive men. This effect persisted even when resting metabolicrate was normalized per kg of fat-free weight. This study raised the possibility that, tleast in young men, very high levels of aerobic training are a prerequisite to increasing theresting metabolic rate. Tremblay et a/ [18] later confirmed their original findings in alarger sample size in which resting metabolic was found to be 11 higher in a cohort of59 individuals composed of 20 trained and 39 non-trained subjects.

Poehlman et a/ [19] also demonstrated this effect. They found a 10 higher restingmetabolic rate in a group of trained men, even after comparing them to an inactive groupthat was matched for fat-free weight and body fat. This suggests that differences in bodycomposition cannot solely account for the difference in resting metabolic rate in trainedand untrained men. Although thyroid hormones carry a major responsibility forregulating resting metabolic rate, no differences in basal plasma levels of triiodothyronineand thyroxine were noted. The additional energy expenditure associated with a higherresting metabolic rate can be extrapolated to an increase in daily energy expenditure of170 kcal per day and would result in a loss of approximately 8 kg over one year if caloricintake was not increased accordingly. Thus, high levels of food intake are a prerequisite


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Table 1 Resting metabolic rate (RMR) of untrained, moderately trained and highly-trained males.

Variable Untrained Moderately trained Highly trainedN= 9) N= 11) N = 8 )

RMR (kcal min- 1 ) 1.24 0.05 1.13 0.03 1.31 0.05 H > M; 0.01RMR (kcal kg- 1 h- 1 ) 0.93 0.02 0.92 0.02 1.11 0.03 H > U; 0.01

H > M; 0.05RMR (kcal FFW- 1 h- 1 ) 1.09 0.03 1.06 0.03 1.20 0.04 H > U; 0.05

H > M; 0.05

Values are means SEM; U =untrained, M =moderate ly trained, H =highly trained.From Poehlman et al [20].

significant differences between inactive and active men [26). A more detailed discussion

of reasons contributing to discrepant results has been published [2].Prudent interpretation of the relationship between resting metabolic rate and the'trained state' is warranted. A cause and effect relation between physical activity andresting metabolic rate cannot be inferred from cross-sectional studies. Individuals whoselect themselves for and maintain vigorous activity may differ constitutionally fromthose who do not participate in physical exercise. That is it is unclear whetherparticipation in exercise-training preceded the higher metabolic rate or whether it wasalready present in individuals predisposed to participate in vigorous physical activity.

Several exercise intervention studies found a higher resting metabolic rate afterexercise training. Tremblay eta/. [27] submitted eight moderately obese women to an 11week program of physical activity, which included 5 h of aerobic exercise per weekperformed at a mean intensity of 50 2 max. They found that exercise traininginduced a significant increase in resting metabolic rate, normalized per kg of fat-freeweight. These results suggested that vigorous exercise is not required to enhance restingmetabolic rate and that the metabolic rate of obese and lean individuals may respond toan increase in physical activity in a similar manner.

The studies of Lawson eta/. [28] and Lennon eta/. [29) support an increase in restingmetabolic rate after exercise training in females. In the study of Lawson eta/. six obesefemales increased their resting metabolic rate following a 10-week program of jogging.


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state of energy balance. Woo eta/ [30] tested this concept in a recent experiment. A'high energy turnover' state was created by overfeeding six men whose energy expenditure was increased proportionally by participation in long-duration aerobic exercise tomaintain energy balance. Resting metabolic rate was higher after exercise training, but nochanges were found in plasma thyroid hormones. These results suggest that besides thewell-known adaptations caused by energy surplus and deficit conditions on restingmetabolic rate, the 'flux of energy' through the system may play a role in altering restingmetabolic rate.

Effect of physical activity on energy expenditure during submaximal exercise

The level of physical fitness, as well as age, influences man's capacity to performsubmaximal activity in wilderness activities. It is well established that oxygen consumption during maximal exercise (V 0 2 max), which is considered the best index ofaerobic fitness, declines with age [ 31] and increases following endurance exercise training

given that the exercise stimulus is of sufficient frequency, intensity and duration tostimulate cardiovascular adaptations. t has been suggested that persons who exerciseregularly may not only have a higher vo max than their sedentary counterparts, butmay also attenuate the age-related decline in physical fitness [32]. In contrast, littleinformation is known about the effects of age and physical fitness on energy expenditureduring submaximal exercise. Because most outdoor and wilderness activities areperformed at a submaximallevel of energy expenditure, small changes in the efficiency ofenergy utilization may influence the regulation of body weight and ultimately the abilityto sustain physical activity. We will discuss several studies that have examined theinfluence of age and physical fitness on submaximal energy expenditure.

Gardner et al [33] reported a longitudinal decrease of 5.9 in total energy expendedduring 10 min of cycling at 100 watts in a cohort of middle-aged men examined in 1969and reexamined in 1985. Although the cause for this phenomenon could not bedetermined, it was speculated that it resulted from either: (1) a reduction in the restingmetabolic rate due to aging [34], (2) a greater proportion of energy derived fromanaerobic sources, or (3) a change in the optimal pedaling frequency for a given poweroutput with increasing age.

In another study, Gardner eta . [35] examined the influence of chronic endurancetraining and energy expenditure during submaximal exercise. Twenty younger (31.2


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[37] trained 28 people with an average age of 68 years for a nine week period. Thesubjects were placed into high-intensity and low-intensity exercise groups. A reduction inV 2 during submaximal exercise between 60 and 75 of V 2 max was found in bothgroups; the authors attributed this to enhanced mechanical efficiency of pedaling.

Stuart eta/ [38] compared energy expenditures of five sprint runners with those offive long distance runners via bicycle ergometry at power outputs of 30, 60, 90, 120 and150 watts. Except at 30 watts, sprinters expended more energy at all power outputs thandid long distance runners. The authors hypothesized that differences in muscle fiberrecruitment following chronic exercise may have contributed to the change in exerciseeconomy. Fast twitch fibers, which predominate in sprinters, utilize more energy duringmuscular contraction than do slow-twitch fibers. The greater recruitment of fast-twitchmuscle fibers in the sprinters would increase submaximal exercise energy expenditurebecause of the muscles' lower contraction-coupling efficiency relative to the muscleconfiguration of distance runners, in which slow-twitch fibers predominate [39].

A number of studies have found no change in submaximal energy expenditure during

exercise related to age or training. Astrand eta/. [31] found no significant change withage across a span of 21 years in energy expenditure during submaximal exercise on abicycle ergometer. However, their subjects were younger than those of Gardner eta/[33], many of whom were not beyond the age of 45 by the completion of the study. Onecould speculate that a decrease in muscle mass would not have been as likely to haveoccurred in the younger men and thus did not contribute to alterations in gross energyexpenditure due to changes in body composition. Similarly, Dill et a/ [40] showed thatsubmaximal vo2 at a given running pace on a treadmill showed no mean change overapproximately 20 years in subjects.

Girandola and Katch [ 41] trained 33 young males (22 1.9 years) and found nochange in steady-state V 2 at a power output of 1080 kpm min- 1 on a bicycle ergometer.This finding was supported by Saltin eta/. [42], who had 42 people between the ages of34 50 years run two miles once per week and intermittently twice per week for eight to10 weeks. These subjects did not change their V

2during a bicycle ergometer test at

power outputs of 300, 600, and 900 kpm min.- 1 Thus, it appears that short-term trainingprograms do not influence energy expenditure during submaximal exercise in young andmiddle-aged individuals, but may reduce energy expenditure in older subjects.

I f participation in regular physical activity results in a decreased energy expenditureduring submaximal exercise, this suggests conservation of energy that accompanies an


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and vaporization of water. Healthy humans are able to maintain a relatively constantbody temperature despite large changes in environmental temperature with continuousfine adjustment of heat production and loss. The homeostatic control of body temperature is coordinated by reflex responses that originate in the hypothalamus.

Figure 2, based on the data of GroHman [43], demonstrates three salient features ofthe effects of ambient temperature on heat production under resting conditions. First,when ambient temperature is below core temperature, resting heat production increasesto replace heat lost to the colder environment. Additional heat production under restingconditions utilizes involuntary muscular activity shivering). Under non-resting conditions, additional heat can be produced via voluntary muscular activity e.g., footstomping, fidgeting). In addition, heat losses are minimized by involuntary e.g.,increases in cutaneous vasoconstriction, horripilation) and voluntary actions.

The second feature of the response to ambient temperature depicted in Fig. 2 is thatheat production declines until it reaches a nadir at the thermoneutral zone, which isdefined as the region of temperature changes that do not affect heat production. At

thermoneutral temperatures, heat losses are negligible and heat production is truly basal.The third feature of Fig. 2 is that at a certain temperature, heat production begins to risewith greater efficiency of metabolic processes at temperatures slightly higher than bodytemperature.

The metabolic adaptations displayed during cold exposure are related to bodycomposition [44] and physical activity [45], and are gender-specific [46]. In general, fatterindividuals are able to maintain body temperature more efficiently than are leanindividuals, due to the insulatory properties of subcutaneous fat. However, McArdle etal demonstrated that women possessing the same relative fatness as men were lessefficient at maintaining body temperature at rest and when exposed to cold waterbetween 20C and 28oC [46]. This was explained by greater heat losses due to the largersurface area to body mass ratio in women compared to men, as well as a lowerthermogenic response in women.

Under non-resting conditions, the metabolic adaptation to cold exposure becomesmore complex. McArdle et a/ [45] showed that submaximal exercise arm and legergometry at 36 watts for 1 h) during water immersion at 20- 28oC was beneficial inmaintaining temperature regulation. This effect was more pronounced in women, a

z 2 5 0 0 . - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -0


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274 Poehlman Gardner nd Goran

phenomenon that was explained by gender related differences in the distribution ofsubcutaneous fat over the active musculature [ 45]. Such prevention of body cooling by

exercise has not been reported at water temperatures below 20C. At rest, muscle is animportant tissue in limiting heat loss from the body, whereas during exercise, muscle doesnot retain heat as efficiently. Consequently, during exercise, the responsibility for heatinsulation is shifted to subcutaneous fat and skin [ 4 7].

Additional complications arise if body heat loss becomes greater than metabolic heatproduction, causing a fall in core temperature (e.g., body cooling). Under theseconditions, the energy cost of exercise increases 10-40 for every 0 . 5 - l S C fall in coretemperature, and the vo max is reduced by 10-30 [48]. When core temperature falls,there is a quicker onset of anaerobic metabolism, due to the combined effects of thehigher energy cost of activity and the reduced capacity to deliver oxygen.

Milan and Rodahl described a study of food intake and energy expenditure in menworking in the Antarctic [ 49]. Caloric intake was recorded by weighing the food itemsthat were self-selected at meal times and by self-recording of food intake between meals.

Energy expenditure was measured by the factorial method, which involves determiningthe caloric cost of various activities using respiratory gas collections and indirectcalorimetry, and recording the amount of time spent in various physical activities.Determinations of energy balance were taken over periods of 5 to 6 days. Five menstudied in the autumn were in a slight negative energy balance, with energy expenditureaveraging 3800 kcal per day, compared to an average intake of 3400 kcal per day.However, four men studied in the winter were in positive energy balance by 1000 kcal

per day (3400 kcal per day expenditure vs 4400 kcal per day intake); these men gained3.7 kg of body weight over 4 months. A third study of four other men conductedduring the spring demonstrated a state of energy balance, but at a higher energy flux(4200 kcal per day expenditure, 4300 kcal per day intake). The authors suggested thatpositive energy balance occurred in the winter because the men overate through boredom. They concluded that effects of exposure to low temperatures are minimal comparedto the effects produced by the greater physical activity required to live under suchextreme conditions.

Acheson eta/. [50] examined longitudinal changes in caloric intake in a group of menwho were working in the Antarctic for 1 year. Dietary intake was examined by threedifferent methods (food weighing, dietary recall and bomb calorimetry of duplicatemeals). Mean food intake for periods of 5 to 6 days was 3210 kcal per day, with

f f


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In the same group of men studied by Acheson eta/. no overall changes in body weightor fat as assessed from skinfold thickness) were observed [55]. This implies that on

average, the workers maintained energy balance. However, there w s a large degree ofindividual variation in the longitudinal changes in body weight and body fat content. fthe 12 men studied, four gained weight in winter and then lost weight in summer, threesteadily lost weight, three steadily gained weight, and two lost weight and maintained theloss. This variation in the type of response, together with no overall group change, hasbeen observed in healthy individuals on fixed caloric intakes 56].

Wilson eta/. [57] have studied the effects of cold exposure upon resting metabolic ratein a group of exercising men who slept in rooms kept at near-freezing temperatures without blankets at night with a group of exercising men that slept in more comfortable andwarmer conditions. No difference in resting metabolic rate was found between the twogroups when resting metabolic rate was re-measured at a more comfortable temperature.This study suggests that there is no chronic effect of cold exposure on resting metabolicrate.

A relatively stable core temperature can be achieved despite wide fluctuations inambient temperature, due to the homeostasis between heat production and loss. Manystudies agree that total energy expenditure under cold conditions is very similar to that ofmen working at similar activity levels at comfortable temperatures. Increased restingmetabolic rate during cold exposure does not appear to be directly related to coldadaptation, but may be related to increases in physical activity. Because humans do notappear to have a superlative mechanism for non-shivering thermogenesis, adaptation to

the cold is shifted towards behavioral mechanisms aimed at reducing heat loss.

Summary

Regular participation in physical activity influences energy intake and output in humans.A short-term increase in physical activity does not appear to influence food intake innormal weight individuals, whereas long-term participation in physical activity mayregulate caloric intake. Resting metabolic rate may be increased by regular participationin physical activity, although this issue remains unsettled. Physically active individualsmust maintain a higher level of food intake to maintain energy balance. Data about theeffect of physical activity on energy expenditure during submaximal energy expenditureare conflicting. Recent findings support a conservation of energy during submaximal


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the Impact of Physical Activity and Cold Exposure

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