Thermoregulatory responses to desert heat: age, race and sex.

Sixty-nine whites (38 men and 31 women) aged 17 to 88 years and 48 blacks (19 men and 29 women) aged 17 to 61 years were studied. Each person walked in desert heat for 1 hour at a rate requiring 40% of aerobic capacity. Observations were recorded on their rectal temperature (Tre), skin temperature (Tsk), heart rate (HR), blood pressure, and sweat rate (SR). Older men and women of both races were able to complete their walks without any ill effects. Age, per se, did not significantly reduce elderly individuals' ability to tolerate the combined stress of dry heat and exercise. Men of both races had higher sweat rate and lower heart rate and rectal and skin temperature than women working at the same percentage of aerobic capacity. Success of thermoregulation at 40% of aerobic capacity of blacks and whites was equal, but in both races men thermoregulated more successful than women. Our data suggest that thermoregulatory capacity of humans under desert conditions differs between sexes and is not influenced significantly by age or race except for differences in aerobic capacity.

Volume and composition of hand sweat of White and Black men and women in desert walks.

Many investigators have sought, but failed to find, ethnic differences in the number and regional distribution of active sweat glands. In this study measurements have been made of sweat secreted on one hand and also on the whole body of Whites and Blacks walking in desert heat. Whites numbered 31 men and 27 women, ages 30 to 88 years; there were 21 Black men and 31 Black women, ages 16 to 61 years. Each walked on three occasions for 1 hour at a rate that required an oxygen consumption of about 40% of aerobic capacity. Ambient temperature ranged from 32 to 44 degrees C in 1979 and 1980; means were 38.4 degrees C in 1979 and 36.7 degrees C in 1980. There was no sweat in the gloves of many Blacks; this was true of only a few Whites. Volume of body sweat increased in both races with rate of walking; volume of hand sweat increased more in Whites than in Blacks. The Mann-Whitney test revealed that volumes of hand sweat were significantly greater for Whites than for Blacks. It was concluded that in desert walks most Whites and few Blacks sweat freely on their hands. In samples of hand sweat, Na+, K+, and Cl- were determined. Concentrations of each ion varied widely in both races, and were unrelated to race. Concentrations of Na+ and Cl- generally are somewhat higher in hand sweat than in body sweat; concentrations of K+ are much higher. It follows that the values for concentration of Na+ and Cl- reported in Table 3 probably are somewhat higher than would have been found in body sweat, and concentrations of K+ are probably much higher.

Metabolic observations on Caucasian men and women aged 17 to 88 years.

Aerobic capacity (VO2 max) and body fat were measured indoors and VO2 was measured at about 36 to 42 degrees C in desert walks or runs in 69 adults aged 17 to 88 years. Eleven were athletic youths, and many of the older adults had participated in jogging programs. Body fat increased and VO2 max decreased with age, although there were notable exceptions. Rates of walking and running were planned to require about 40% of VO2 max. Midway in each walk VO2 (ml O2/horizontal m . kg) served as a measure of skill; skill was high in 10 of 11 youths who did many walks or runs. Each older adult did three walks; skill improved. In four groups with some persons aged 50 and older with body fat up to 40%, skill in their third walk matched that of youths. The least fit women and the least fit men did not attain that level of skill.

Vital capacity, exercise performance, and blood gases at altitude as related to age.

Vital capacity (VC) rarely may decrease 35-60% in healthy mountain climbers associated with high-altitude pulmonary edema (HAPE). In the age range 58-71 yr, five of six men during a week or more on White Mt. in 1962 had decreases in VC from 20 to 32% without frank symptoms of HAPE, Dill, one of the five, had decreases in VC again on White Mt. in 1977 and 1978. Yet none of 11 young climbers on White Mt. studied by Hultgren (personal communication) had a significant decrease in VC. Dill's arterial O2 saturation at age 87 at 485 Torr was about 79% in rest and 74% when VO2 was 0.74 ml/min.kg. His aerobic capacity at age 87 yr was 18 ml/min.kg at 695 Torr and 15 at 485 Torr.

Cardiac output during rest and exercise in desert heat.

Cardiac output and stroke volume were measured in two environments and at metabolic rates ranging from rest to the maximum rate that could be sustained for 25 minutes. One environment was indoors at about 23 degrees C, the other outdoors in desert sunshine and low water vapor pressure. The age range of the one female and four of the male subjects was from 19 to 40; the fifth male subject was 85 years old. Cardiac output was the same in the two environments; stroke volume was less at higher metabolic rates in the heat. The cardiac output for the old man was about one-tenth less and stroke volume about 20 ml less than that observed for the same work 50 years earlier.

Wheel running of kangaroo rats, Dipodomys merriami, as related to food deprivation and body composition.

Kangaroo rats deprived of food ran themselves to death in 48 h in wheel cages. Despite the loss of 14.5% of body weight the ratio of water to protein was the same after the run as it was in control rats. Metabolic measurements at rest and in the running wheel and weight loss in the 48-h run were used to estimate fuels used and water expended. Two-thirds of the initial amount of fat and 9% of the protein were metabolized. The terminal mean percentage of body fat was about twice that observed in rats trapped in the spring of 1967, when seed production was low: death in the 48-h run could not have been due to depletion of body fat alone. The powerful activity drive seen in hungary kangaroo rats presumably is intensified in dry years when food is scarce and may deplete their reserves enough to result in death from starvation.

Capacity of young males and females for running in desert heat.

Tolerance for sustained activity in the desert at about 40 degrees C was assessed on high school students, mostly athletically oriented and scholastically superior. The 14 males compared with the 12 females had an aerobic capacity greater by about one-half and a percentage of body fat smaller by about one-half. Each sex attained about the same percentage of aerobic capacity in their maximal sustained effort. This involved an increase in metabolic rate of 3 to 5 fold in females and 6 to 8 fold in males. In maximal sustained effort responses of males and females were alike in respect to rectal and skin temperatures and heart rate. At a rate at which nearly all walked for one hour, 100 m/min, there were no significant differences in metabolic rate, sweat rate nor in composition of sweat. Running at 120 m/min required maximal effort by most females; their maximal sweat rates ranged from 7.4 to 14.2 ml/m2.min. Most males were able to run at 160 m/min for one-half hor to one hour; their maximal sweat rates ranged from 11.3 to 14.6 m/m2.min. Superior capacity of males over females for sustained exercise in desert heat is related to their higher aerobic capacity and not to a difference in capacity for thermoregulation.