Not Eating Enough Overcoming Underconsumption of Military Operational Rations (1995) / Chapter Skim
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14 When Does Energy Deficit Affect Soldier Physical Performance?
14 When Does Energy Deficit Affect Soldier Physical Performance?
Pages 253-284
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From page 253... ...
of severely restricted intakes (Consolazio et al., 1967, 1979~. This finding corroborated the conclusions from longer-term studies conducted at the University of Minnesota that an energy deficit resulting in less than 10 percent loss of body weight ~ Karl E
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Changes in the oxidative capacity of muscle, the oxygen carrying capacity of the blood, and the mass of metabolically active tissue probably account for most of the observed decrease in aerobic capacity, which in turn, explains reduced stamina and physical work capacity (Keys et al., 1950; Spurr, 19863. Loss of skeletal muscle, changes in muscle biochemistry, and changes in the balance of muscle fiber types produce reductions in dynamic strength (Henriksson, 1990; Taylor et al., 1957~.
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A certain proportion of lean mass, usually about one-third of the total weight change, is lost or gained along with changes in storage fat (Forbes, 1993) , but as fat stores become increasingly scarce with a severe weight loss, this proportion increases.
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The first point to be made from this example is that relative weight losses cannot be meaningfully compared unless the initial nutritional status is also considered. The soldiers at Kut were already at reduced weight from earlier fighting and a long march to Kut.
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Cureton provided compelling evidence that the loss of excess fat weight can indeed enhance various types of physical performance, most notably run time (Cureton, 1 992~. 2 Percent body fat in these studies was measured by whole body scan using duel-energy x-ray absorptiometry.
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This conclusion is based, in part, on a superior nutritional status in the form of larger gender-appropriate fat stores. A typical, young female soldier weighing 60 kg with 30 percent body fat carries sufficient storage fat to survive an energy deficit for approximately twice as long as her male counterpart.
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ASSESSING PERFORMANCE Work Capacity and Energy Expenditure The importance of adequate intake to worl< productivity (i.e., voluntary energy expenditure) can be demonstrated when energy deficits are high or prolonged, specifically because additional calories will be readily consumed when offered, and productivity will increase (for extensive review, see Consolazio, 1983; Spurr, 1986, 1990~.
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From page 260... ...
Although other factors such as mood and morale may contribute to these phenomena, the reduction in physical work capacity is most probably related to reduced energy stores, including low body fat and perhaps inadequately replenished muscle glycogen (Karlsson and Saltin, 1971~. Ranger students present a variation where a sustained physical effort is not optional, demonstrating behavioral and metabolic adaptations to accommodate the reduction in energy intake while maintaining the required level of effort.
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(b) 261 INCA verage Energy Expenditure ~Average Energy Intake Rangers Rangerl FIGURE 14-1 Daily intakes (A)
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However, the early responses to restricted energy intake in a normal subject do not include a change in resting metabolic rate; instead there is an increased utilization of body fat, reflected in weight loss and a change in the respiratory quotient. Note that in all of these more extreme examples, including wartime coal miners, undernourished Columbian children, Annoy Ranger students, and volunteers in the Minnesota Starvation Study, all would have willingly consumed more calories, if they were offered.
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Purer tests of physical capacity, which at least differentiate muscle energy sources, such as a dynamic lift test for strength, the Wingate test (30 seconds of maximal cycling exercise against a relative resistance) for anaerobic power, and a treadmill test to exhaustion for maximal aerobic capacity, can be carefully monitored and reproduced in a standardized way (see Vogel, 1994~.
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The maximal lift test and grip strength demonstrated better stability (Teves et al., 1986~. Testing strength with an isokinetic dynamometer can be very useful because specific muscle groups caI1 be targeted, for example, representing upper body, lower body, and trunk muscle strength.
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Even the soldier with the most extreme weight loss of 23 percent of body weight in 8 weeks demonstrated no decline in grip strength performance. Other studies examining large weight losses and grip strength in ..
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Thus, grip strength appears to be well preserved until nutritional status is severely compromised. The maximal lift test has been used in several nutrition studies, and a large Army data base exists from the testing performed by Marilyn Sharp and others (Sharp and Vogel, 1992~.
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In Ranger-I (where grip strength demonstrated no decrease) , maximal lift declined by 24 percent (the jump test was not performed)
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EFFECTS OF ENERGY DEFICIT ON AEROBIC CAPACITY Aerobic capacity has been examined (by treadmill testing) for soldiers in several nutrition studies involving energy deficits (Table 14-2~.
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269 ;^ ce ._ cq o cry ce x o ct ._ x ce ._ cd a: ._ cn ct ._ ~ A_ 5~ Cal ct a: .
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For example, in basic training for the Danish army, men scoring the highest aerobic capacities at the start of training demonstrated significant declines 3 months later, while the men starting in poorer condition improved significantly (Harding, 1975~. Thus, standard military training usually does not meet the more specialized training requirements to maintain physical performance levels in the best-trained soldiers.
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Reductions in aerobic capacity could be important because physical work capacity is largely determined by maximal aerobic capacity (Spurr, 1986~. Physical tasks in the Army may involve short bursts of high-intensity work but more typically involve work at less than 40 percent of maximal aerobic capacity (Table 14-3~.
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In the first half-hour cycle, crew members worked at nearly 50 percent of their peak aerobic capacity, but by their last cycle of loading, firing missions were completed in a shorter period of time, and work was achieved at less than 40 percent of neak aerobic capacity. This result suggests that to have a significant r~~ ~ ~~ rid or -- -- - - - impact on this performance, maximal oxygen uptake must be markedly reduced, to a magnitude greater than that observed in any of the voluntary consumption studies (i.e., > 20 percent reduction)
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From page 273... ...
These results lead to the inescapable conclusion that the changes in VO2 maX that have been measured in various nutrition studies are of little consequence to soldier physical performance. SHORT-TERM STUDIES WITH HIGH-ENERGY EXPENDITURES A different problem of underconsumption occurs in short-tenn (less than 1 week)
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From page 274... ...
and a 5-d scenario also involving upper body exertion such as load carriage (Murphy et al., 1984~; in both cases, with minimal weight losses, upper body muscular strength and endurance significantly decreased. In another 8-d field artillery scenario with somewhat lower work levels, no decrements were observed (Patton et al., 1989~.
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From page 275... ...
This conclusion was attributed to the effects of sleep deprivation. In other, more-controlled laboratory studies of short-term high-energy deficit, tests of power, such as the Wingate test, and tests of isometric strength, including grip strength, have usually not been affected (Consolazio et al., 1967; Henschel et al., 1954; Hickner et al., 1991~.
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probably has little effect on physical performance, as evidenced by the work of Consolazio, Johnson, and others. The primary concern during weight loss involves loss of muscle
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Women have not been studied in the context of physical performance and weight loss in field conditions with operational rations. Such studies may provide a quite different relationship, since aerobic and upper body strength capacities are usually lower to begin with, and female soldiers are more prone to iron deficiency.
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Pp. 31-S6 in Body Composition and Physical Performance, B.M.
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Roemmich, and D.L. Costill 1991 Test development for the study of physical performance in wrestlers following weight loss.
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Kerrigan-Brown 1989 Physical performance and carbohydrate consumption in CF commandos during a 5-day field trial. Technical Report 89-RR-48, AD A217 204.
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Westphal, and K.E. Friedl 1994 The physical performance of female Army basic trainees who pass and fail tl~e Army body weight and percent body fat standards.
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during a 30-day field training exercise. Technical Report T6-95.
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Friedl, and M.A. Sharp 1995 Relationship between iron status and physical performance in female soldiers during 15 U.S.
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The outcome of military nutrition research must be rations that are readily consumed in quantities sufficient to fuel maximum intellectual and physical performance. Thus, the question of whether underconsumption is reliably associated with measurable degradation in intellectual behavior is of considerable concern to developers of military rations.
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