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Body Composition and Physical Performance 1992. Pp. 195-206. Washington, D.C. National Academy Press 1 ~ Body Composition and Performance in Relation to Environment Roy ;J. Shephar`1 DEFINITION OF OBESITY Clinicians commonly define obesity as a body mass that exceeds the actuarial "ideal" value by a specified margin such as 10 or 15 pounds (5 to 7 kg). However, this is not an entirely appropriate basis of assessment from an ergonomic point of view. First, the goal of the clinician when diagnos- ing obesity is to detect an increased vulnerability to chronic disease (Table 12-11. Thus, groups such as the Society of Actuaries (1959) have expressed the mortality from a variety of chronic conditions and diseases as a percent- age of the "standard" mortality values observed in subjects of the same age and sex who had an "ideal" body mass relative to their height. Notice that a substantial increase of vulnerability develops only when there is a major excess of body mass (and by inference a major excess of body fat). More- over, loss of production from morbid obesity and the resultant chronic dis- orders has a relatively minor impact upon the performance in the young adults of a military labor force. In contrast to the threshold of fatness required for a clinical diagnosis of obesity, most aspects of occupational performance tend to be continuous functions of body composition. Further- more, the energy cost of most tasks depends not only on total body mass, but also on the nature of this mass (muscle or fat) and if the person is burdened by an excess of fat on its distribution (deep or superficial). A possible method for defining body fatness is to compare hydrostatic estimates of body fat content with those observed in a person of ideal body 195
96 ROY J. SHEPHARD TABLE 12-1 Mortality of Obese Subjects, Classified by Condition or Disease, and Expressed as a Percentage of Standard Values for Subjects of the Same Gender, Aged 15 to 69 years. Percent of Standard Values of Mortality of Matched Controls Men Women Excess Weight Excess Weight Condition >+24 kg 2 +33 kg >+28 kg 2 +37kg Diabetes 179 385 270 242 Cerebrovascular disease 136 183 143 142 Heart/circulation 131 155 175 178 Pneumonia/flu 128 193 148 110 Digestive diseases 147 197 140 200 Kidney diseases 146 230 93 122 Accident/homicide 109 126 85 98 Suicide 71 104 47 All causes 123 145 130 138 SOURCE: Based on data of the Society of Actuaries (1959). Previously published and reprinted by permission of Greenwood Publishing Group, Inc., Westport, CT, from Physiology and Biochemistry of Exercise by Roy J. Shephard (1982). mass. One study of military recruits arbitrarily set the upper limit of ideal values at 14 percent body fat in men (Amor, 1978), although the same author's data apparently suggested that 16.8 percent fat would correspond with the upper limit of the actuarial ideal body mass. Taking the 14 percent ideal figure for the men, and the 18 percent ideal body fat for the women, this would imply respective ideal fat masses of 10 and 11 kg in the two genders. Applying the clinical criterion of obesity, those men with a 5 kg excess of fat (the obese) would have 21 percent or more fat, and the women would have 27 percent or more fat. Some 50 percent of the male soldiers studied by Amor (1978) exceeded the actuarial ideal of body mass. The proportion of those who exceeded his arbitrary criterion of obesity (18 percent of body mass in men) increased with age (Table 12-2) but was unrelated to the physical demands of employment (Table 12-31. Those whom he classified as obese nevertheless tended to have a poor maximal oxygen transport, particularly if this was expressed in ml/kg x minute (Table 12-41. Thus, for military purposes an obesity threshold of 18 per- cent fat in men and perhaps 24 percent fat in women seems preferable to the clinical criteria of 21 percent and 27 percent fat in the two genders. A second potential method of identifying the obese is to measure skin
PERFORMANCE IN RELATION TO ENVIRONMENT TABLE 12-2 Relation of Chronological Age of Military Personnel to Obesity in the British Army Age in Years (n)* Percentage Obese (> 18 percent body fat) 17-19 (702) 20-24 (1,190) 25-29 (805) 30-34 (255) 35-39 ( 1 1 8) 32 39 46 56 62 *(n) = number of personnel surveyed. SOURCE: Amor ( 1978). 197 fold thicknesses, again comparing the actual readings with the values ob- served in a person meeting actuarial standards of ideal body mass (Shep- hard, 1982~. Average readings are about 10 mm in a male and 14 mm in a woman of ideal mass. Assuming that a double fold of the skin per se accounts for 4 mm of the total skinfold reading (Shepherd, 1991), there is a superficial layer of some 5,400 cm3 of fat in the ideal man with a body surface of 1.8 m2 (4.8 kg of fat, assuming a density of 0.9) and 8,000 cm3 of fat in the ideal woman with a body surface of 1.6 m2 (7.2 kg of fat). Assuming also a 50 percent increase of subcutaneous fat in a person who is clinically obese, the clinical threshold of obesity would be an average skinfold reading of 13 mm in a man and 19 mm in a woman. However, if the military threshold of obesity were to be set at the ideal body mass, TABLE 12-3 Relation of Job Intensity to Prevalence of Obesity among Military Personnel in the British Army Percentage Obese Job Intensity (n)* (>18 percent fat) Sedentary (353) 53 Light ( 1,389) 42 Moderate ( 1,269) 38 Heavy (59) 51 *(n) = number of personnel surveyed. SOURCE: Amor ( 1978).
98 ROY J. SHEPHARD TABLE 12-4 Relation of Percentage of Body Fat to Percentage of British Army Personnel with Poor Aerobic Fitness Percentage with Poor Aerobic Fitness Body Fat Percent (n)* (maximal oxygen intake <35 ml/kg x min) <10 (154) 10-14 (728) 14-18 (912) 18-22 (642) 22-26 (390) >26 (244) * (n) = number of personnel surveyed. SOURCE: Amor (1978). 2 5 9 21 32 51 as proposed by Amor (1978), the ceiling of militarily acceptable skinfold reading would average about 11 mm in a man and 14 mm in a woman. In summary, the standards for judging obesity in military personnel should be more rigorous than those adopted for clinical purposes; both body fat (limits of 18 percent in men and 24 percent in women) and skinfold readings (limits of 11 mm in men and 14 mm in women) should correspond to those observed in a person of ideal body mass. PERFORMANCE IN COMFORTABLE CLIMATES Physical Performance Physical performance may be classified simply into endurance activi- ties, well exemplified by prolonged marching with a backpack, and lifting tasks that are commonly the limiting factor in the front-line employment of military personnel (for example, the ability to lift a mass of 36 kg from the ground to a height of 1 10 cm; Nottrodt and Celentano, 1984~. The metabol- ic load imposed by any given task reflects the sum of resting metabolism plus the energy cost of the required activity (Shepherd, 19741. Resting Metabolism Because of the effects of body surface upon heat loss, resting metabo- lism is a power function of body mass M (Shepherd, 19821: VO2 = (Mid 7s However, a large part of the fat cell is occupied by metabolically inert stored triglyceride. Thus, the resting metabolism per unit of body mass is
PERFORMANCE IN RELATION TO ENVIRONMENT 199 greater in a muscular than in a fat individual. Obesity also affects respira- tion through mass loading of the chest; the obese person shows decreases of lung volume, chest wall, and lung compliance that can precipitate a classi- cal Pickwickian syndrome of hypercapnia and hypoxia (Burwell et al., 19564. In partial compensation for the added respiratory work, the respiratory cen- ters of the obese individual may show an increased sensitivity to hypoxia (Burki and Baker, 1984) and in moderate but not in severe obesity there is an increased sensitivity to carbon dioxide (Emirgil and Sobel, 1973; Nish- ibayashi et al., 19871. Endurance Activities Givoni and Goldman (1971) and Pandolf et al. (1977) developed vari- ous equations for the prediction of the energy cost of marching in fit young recruits. In general, these authors found that the metabolic cost is a linear function of body mass and the mass of any backpack that is being carried. Thus, a heavier person will spend more energy when marching, regardless of whether the added body mass is attributable to muscle or fat. If the normal expectation is that a 70-kg recruit will carry 30 kg of equipment, then a person who is 10 kg heavier will immediately have a 10 percent handicap of endurance performance (Givoni and Goldman, 19711. If the added burden is muscle, a heavy person may show some compen- satory increase in their absolute maximal oxygen transport, and because the active muscles are stronger, it may also be possible for the individual con- cerned to operate at close to their maximal oxygen intake for a sustained period, so that their endurance performance may approach that of a lighter person. However, if the extra body mass is fat, there is certainly no com- pensatory development of maximal oxygen intake; indeed, oxygen transport is often poorer than in a lighter person, so that endurance performance is at least correspondingly limited. In moderate obesity, there is no change of mechanical efficiency, so that the oxygen cost of walking per kg of body mass is unchanged. However, if the obesity is extreme, a combination of heavier limbs, awkward or impeded body movements, and increased respi- ratory loading may give rise to a low mechanical efficiency, with a further restriction of potential performance (Dempsey et al., 1966~. Lifting and Carrying The current Canadian military requirement is that recruits be able to lift 18 kg regularly and 36 kg occasionally to shoulder height (Nottrodt and Celentano, 19844. The load normally lifted is thus about 20 percent of body mass, and if objects are to be picked up from the ground, the resultant displacement of body mass is a major fraction of the overall task. Brown
200 ROY J. SHEPHARD (1966) suggested that the oxygen cost of most occupational tasks could be described by an equation of the type VO2 = A + B(M)n where A and B are constants, and M is body mass raised to an exponent n that varied from 0.75 to 1.0. Godin and Shephard (1973) suggested that there might be merit in a three-term equation of the type: VO2 = A + B(M)0 75 + C(M)n where A, B. and C are constants, M is body mass, and n is an exponent varying from 0.1 to 0.2 in very light arm work to near 1.0 in heavy physical tasks involving displacement of much of the body mass. The second term in this last equation distinguishes the influence of body mass on resting metabolism, a particular advantage in situations where a heavy, muscular Person is performing relatively light industrial work. Another consideration is that the obese person is often characterized by insulin resistance and difficulty in mobilizing fatty acids (Pacy et al., 1986; Scheen et al., 19831. If an endurance task must be sustained for a long period, the function of such an individual may be impaired by a depletion of glycogen reserves. With a lifting task, the factor limiting performance is usually muscular strength rather than maximal oxygen intake; thus, if body mass is increased, it becomes critical whether the added load is due to muscle that can provide a compensating increase of strength or to fat, which merely increases the overall mass of the system. Underwater Activity Underwater activity is a special case where it is an advantage to be somewhat obese, both from the viewpoint of thermal insulation and also because of the resultant increase in buoyancy. Heavy, muscular individuals often have substantial difficulty swimming over long distances, because they must exert much greater effort to remain afloat, and a less horizontal leg position also decreases the mechanical efficiency of their swimming (Shepherd et al., 1973~. However, the person who lacks a normal amount of body fat can compensate for this handicap by keeping the lungs relatively well filled with air while swimming. Size Problems A final consideration is that many military work stations, such as air- craft cockpits, tanks, or submarines, have limited space for the human oper
PERFORMANCE IN RELATION TO ENVIRONMENT 201 ator. A grossly obese person may be handicapped when working in such a situation because body size exceeds the available clearances. Poor Health Minor sickness and absenteeism are common sources of poor perfor- mance in all kinds of occupation. Among government employees, 10 of 220 working days are commonly lost through absenteeism each year, and an unpredictable need for well-trained replacements adds some 8 percent to payroll costs (Shepherd, 19861. Given the well-recognized actuarial associ- ation between obesity, chronic disease, and premature death, possible rela- tionships between obesity, absenteeism and increased illness should be ex- amined in military personnel. Bardsley (1978) has commented on a substantial cost to the armed services from "diseases of choice," where risk is influenced by lifestyle- conditions such as myocardial infarction, bronchitis, emphysema and alco- holism (Table 12-51. As shown in this table, in 1973, the Canadian forces expended $5.9M for replacement of the dead, $5.8M for replacement of the released, $12.4M for those who were hospitalized, and $1.5M for those who were on sick leave due to "diseases of choice." However, much of this expense is related to the adverse health effects of smoking and alcohol abuse rather than to the adverse consequences of obesity; a substantial ex- cess body mass (20 to 30 kg) is needed for an appreciable increase of morbidity from back problems and of deaths from such diseases as coronary atherosclerosis and diabetes (Society of Actuaries, 19591. Moreover, the economic impact of obesity-related morbidity and mortality would be great- est in older members of the labor force, after the normal time of retirement TABLE 12-5 Estimated Monetary Costs Incurred by the Canadian Forces in 1973 (1973 Canadian dollars) Due to Diseases Associated with Choice of an Adverse Personal Lifestyle Cost ($ million) Replacement of the dead Replacement of the released Hospitalization + lost wages Wages of those on sick leave Total $ 5.9 5.8 12.4 1.5 25.6* *This cost is spent on a labor force of about 80,000 military personnel. SOURCE: Bardsley (1978).
202 ROY J. SHEPHARD from the armed services. Finally, many of the absences from work among younger individuals are attributable to causes other than organic disease, and such personnel would be unlikely to respond to the correction of obesi- ty or indeed to any other form of medical treatment (Williamson and Van Nieuwenhuijzen, 19741. While there remains scope for more detailed anal- yses of the economic impact of "diseases of choice," at first inspection it thus seems much more important to correct smoking and an excessive con- sumption of alcohol than to attempt a reduction of body fat in military personnel. OTHER IMPLICATIONS OF OBESITY Image is an important aspect of effectiveness in many organizations, including the armed services (Shepherd, 1986~. Obese personnel do not fit the public image of a soldier, and it seems logical that for this reason they will weaken the military effectiveness of a unit, although there has been no experimental examination of this point. Baun et al. (1986) have further commented on an association between achievement orientation and personal fitness. By selecting personnel who meet specified standards of body composition and physical fitness, a unit may be enriched by the recruitment of premium personnel. HOT AND COLD ENVIRONMENTS Because heat exchange is proportional to body surface area, tolerance of hot and cold environments may be influenced somewhat by the differenc- es of body surface area between a tall, thin ectomorph and a short, fat endomorph. Indeed, at one time anthropologists sought to explain the colo- nization of hot and cold regions in terms of body linearity, the so-called "rules" of Bergmann (1847) and Allen (1877~. However, the effects of body form are at most of secondary importance in a normal working popu- lation. The main impact of obesity on thermal balance comes from the added insulation of subcutaneous fat (Shepherd, 1985), although an increased con- striction of subcutaneous blood vessels may further augment the insulation of a fat person (Jequier et al., 1974), and there are also effects of body fat stores upon cold-induced thermogenesis. It has been shown that a 5 kg accumulation of fat may add 1.5 mm to the subcutaneous fat layer of a man and 2.5 mm to that of a woman. The functional value of this thin layer of fat can be gauged from the importance that distance swimmers attach to covering their skins with a few mm of grease. At rest, the thermal gradient across a layer of superficial fat is only about 0.15°C/mm, but the insulating effect is proportional to heat flux. Thus, when a person is working at 10
PERFORMANCE IN RELATION TO ENVIRONMENT 203 times the basal metabolic rate, there will be a 10-fold increase of thermal gradient, to about 1.5°C/mm of superficial fat (Pugh et al., 1960~. Assum- ing an energy expenditure of 10 METS (the ratio of observed to basal metabolism) and a uniform pattern of heat loss, a woman who had accumu- lated 5 kg of body fat, with a resultant 2.5 mm increase of the subcutaneous fat layer, would have a subcutaneous temperature some 3.8°C higher than a person of ideal body composition. It might be thought that a thick layer of subcutaneous fat would be an advantage when adapting to a very cold climate. Certainly skin tempera- tures drop to a lower level in the obese before their metabolism is stimulat- ed (Wyndham et al., 1968), perhaps because of an altered setpoint in the hypothalamic thermal regulators (Zahorska-Markiewicz and Straszkiewicz, 1987), although this change has adverse consequences for manual dexterity. The very low skinfold readings of traditional Inuit (Rennie, 1963; Shep- hard, 1978) is one strong argument against the view that fat accumulation has any great adaptive value in the cold. It is only with acculturation to "western" civilization that body fat has accumulated (Rode and Shephard, 19841. The main problem of the fat person who is living in the arctic is that the extent of insulation cannot be reduced once physical work is begun; in fact, because of the increased heat flux, the effectiveness of any insulation is actually increased during work. Body temperature thus rises to the point where sweating is initiated; this wets the clothing from within, largely de- stroying its insulation, and the individual rapidly becomes hypothermic when a rest-break must be taken. In contrast, a traditional Inuit hunter with a very thin layer of subcutaneous fat is able to conserve body heat by using skillfully fashioned clothing that provides up to 11 CLOT units of insulation (Burton and Edholm, 1969~. One fascinating feature shown by military personnel who have recently arrived in the arctic is an increased metabolism of fat. Both field observa- tions and laboratory crossover trials have shown that a given amount of activity produces a fat loss in the cold that is not mirrored in a warm or temperate environment (O'Hare et al., 1978, 1979~. Reasons for this fat loss are still not entirely clear. Contributing factors include energy lost in a substantial ketosis, small increases in the energy cost of movement due to the weight and hobbling effect of arctic clothing, and a possible cold-in- duced stimulation of resting metabolism; it remains unclear whether this last response occurs through residual brown fat (Huttunen et al., 1981) or 1 The CLO unit was originally defined as the insulation provided by British indoor clothing. It is now defined in terms of the thermal gradient from the skin to ambient air (Ts - Ta, °C), the body surface area (m2) and the heat loss (kJ/hour); therefore, CLO units = 0~75 (Ts - Ta) m2 / kJ per hour.
204 ROY J. SHEPHARD TABLE 12-6 Effects of a 2-mm Increase in Subcutaneous Fat (a 4-mm Increase in Skinfold Readings) on the Performance of Physical Work in the Heat Intensity of Work (kJ/min) Core Temperatures Work Rate (°C)* (percent)" Light (<14) +0.8 -11 Moderate (<23) 1.2 -17 Heavy (<38) 2.0 -28 *If work-rate is unchanged. tIf core temperature is unchanged. NOTE: It is assumed there is an initial thermal gradient of 7°C from the body core to the environment. SOURCE: Based on a concept of Pugh et al. (1960). the initiation of futile metabolic cycles in other tissues. Interestingly, it seems easier to produce the fat loss in obese men than in those who are initially slim (O'Hare et al., 1978), and it is also more readily developed in men than in women (Murray et al., 19861. Perhaps fat stores in women are more stable, in order to meet the demands of pregnancy and lactation. Obese subjects have difficulty undertaking vigorous work in a hot envi- ronment, partly because they must expend more energy to complete a given task and partly because insulation is increased by the thicker layer of super- ficial fat. One potential method of restoring thermal balance in the obese person is a greater relative increase of skin blood flow during exercise. However, if blood is directed to the skin, it cannot be directed elsewhere- to the working muscle and the brain. The peak oxygen transport and peak power output are thus reduced in the heat. The obese person works more slowly than a slimmer peer, or if pace is maintained, collapse occurs earlier than in a thin subject. One recent calculation suggested that with an initial thermal gradient of 7°C from the body core to the environment, the core temperature would rise by an additional 0.8 to 2.0°C if a person with an additional 2 mm of subcutaneous fat undertook industrial work (Shepherd, 1987; Table 12-61; conversely, if the rise of core temperature were to be avoided by a slower rate of working, it would be necessary to reduce the work-rate by 1 1 to 28 percent. CONCLUSIONS AND RECOMMENDATIONS In young adults who make up the bulk of military personnel, the main argument for controlling the burden of body fat is a deterioration of physi
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