JOURNAL OF APPLIED PHYNOLOGY
Vol. 40, No. 2, February
1976. Printed
Physiological
to five-hour
in U.S.A.
adjustments
desert walks
of young
D. B. DILL, L. F. SOHOLT,
AND IB ODDERSHEDE
Laboratory
of Environmental
Patho-Physiology,
Desert Research
University
of Nevada System, Boulder City, Nevada 89005
Institute,
unteers are found in Table 1. The subjects were acclimatized except for LM who came from the cooler climate of
Albuquerque 2 days earlier. Three of the subjects were
exercising regularly: TD, swimming; IO, running; and
LM was in a jogging program and took part frequently
as a subject in desert and altitude studies. Subjects JC
and TD played football and basketball in high school
The other subjects were not in competitive athletics but
were relatively fit as indicated by values for their aerobic capacity and body fat (Table 1).
METHODS
Prior to beginning the walk the subject showered,
and
nude weight was recorded. A rectal
probe
(Yellow
Springs type) was taped in place, and a plastic glove for
hand sweat was secured with medical tape. Prior to
positioning the electrodes for heart rate (HR), areas of
skin were roughened with household scouring powder,
cardiovascular
adjustments
and temperature
regulation
in de- washed, dried, and sprayed with an antiperspirant.
Lead electrodes were positioned with surrounding foam
sert walks; heart rate; blood pressure; rectal and skin temperapads and were secured with millipore
tape. Walking
ture; blood; extracellular
fluid; thirst; osmotic pressure; sweat
chloride; hand and body sweat; sweat suppression
attire was shorts, socks, and shoes chosen by the subject; shorts and socks were laundered beforehand.
Each subject walked at 100 mlmin around a desert
course
of 711 m. The walk began late in the forenoon
THERE ARE INSTANCES
of death from heat exposure every
and
continued
for 5 h with brief interruptions.
The HR
summer in southern Nevada. Commonly death results
each lap using a transmitter and rewhen an attempt is made to “walk out” without water to was monitored
drink. In a comparison of men and women (7) we have ceiver made by Parks Electronics (Beaverton, Oregon).
found that most fit men, young and old, can walk 12 km Skin temperatures (T,) were observed each hour by a
surface probe at each of four sites: forehead, neck, foreat 100 m/min even without water, but that most fit
young women are unable to do so with or without water arm, and thigh (l), The average of these values was
to drink. In the current study seven young men under- considered mean T,.
After the first lap and each hour thereafter
the subject
took to walk 30 km with cool tap water to drink each
stepped
on
the
treadmill;
it
was
in
the
sun
along the
hour. A question of principal concern was: is sweat
at 100 m/min.
While the walk
suppression seen in desert walks? Second, we sought for course and was operated
continued on the treadmill, T,, was observed, blood
a relation between the saltiness of sweat and the
amount of water required to satisfy thirst. In general, pressure was noted, and a sample of blood was drawn
we sought from objective
findings
and subjective re- from an antecubital vein. While this was going on, the
subject’s metabolic rate was measured
in the usual way
ports,
clues to impending
breakdown.
Observations
using
a
mixing
chamber
and
Parkinson-Cowan
meter.
made included environmental conditions, the sources
Samples
of
expired
air
were
analyzed
on
the
Haldane
and amount of water used for evaporation, records of
rectal and skin temperature, heart rates and blood pres- apparatus. The subject then stepped off the treadmill,
sures, osmotic pressure of blood, and chloride content of was weighed on a Fairbanks scale (under ideal conditions this scale is accurate to 210 g but because of
sweat from hand and from body.
incomplete shielding from the wind the error was nearer
-+25 g) and proceeded around the course: at the end of
SUBJECTS
each hour the subject stepped on the treadmill for the
Pertinent observations on the seven male subject vol- observations described above. In addition the glove was
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DILL, D. B., L. F. SOHOLT,
AND IB ODDERSHEDE.P~~S~~Z~~~cal adjustments
of young men to five-hour
desert walks, CT.
Appl. Physiol. 40(2): 236-242.
1976-Seven
young men undertook a desert walk of 30 km at a rate of 100 mlmin. Six
finished; the seventh stopped after 24 km. Each satisfied his
thirst with cool tap water each hour. Periodic observations
included metabolic rate, blood pressure, heart rate, rectal and
skin temperature,
body weight, and volume of water drunk.
Hand sweat was collected each hour and body sweat residues
on the skin were collected at the end of the walk. Subjective
reports revealed portents of breakdown:
aching muscles, painful joints, hot or blistered feet, hunger,and
boredom. Cardiovascular adjustment
and temperature
regulation
maintained
tolerable conditions. The volumes of water evaporated by the
5-h walkers were about the same. Wet bulb temperatures
were
below 25°C; all sweat evaporated and was available for temperature regulation.
The volume of water drawn from body reserves was closely correlated with concentration
of chloride in
body sweat; the volume of water that satisfied thirst maintained osmotic pressure.
men
FIVE-HOUR
DESERT
237
WALKS
TABLE
1. Physical characteristics and
aerobic capacity of subjects
Subj
Age, yr
Ht, cm
Wh kg
38
19
18
15
17
26
29
189
165
176
180
189
176
177
79.6
58.2
68.0
69.6
71.1
70.2
52.3
LM
DW
RH
TD
JC
IO
LS
* Treadmill
sidual volume.
walking
test.
t
Surface,
m*
2.07
1.65
1.84
1.89
1.96
1.87
1.66
From
underwater
vo2 tnax’
ml/kg.min
Body Fat,?
9%
47.5
41.8
51.0
55.3
59.0
55.4
41.2
14
13
13
6
8
9
10
weight
and
re-
RESULTS
The following are excerpts from subjective accounts of
the walks.
1) By subject LM: “. . . the hourly drinking intervals
were responsible for my continued feeling of well being;
my desire for water became progressively less during
the walk. After about 3 h I began to feel the stress of
working in the heat: it manifested itself when I stopped
to be weighed (by) feelings of oncoming syncope. During
the last hour I experienced muscle weakness (and) chill
sensations. . . . the voluntary
quitting
point was
reached during the last hour; only the motivating force
generated by the experiment kept me going. Following
the walk I had no interest in another drink of water and
I even found it necessary to lie down on the floor to avoid
fainting while passively standing in the shower. General weakness remained for a few hours. I didn’t experience great thirst for more than an hour after the walk.”
2) By subject DW: “The difficulties I encountered
arose mainly from the soreness of my feet rather than
from the effects of dehydration. I welcomed the chance
to satisfy my thirst at the end of each 1-h period. At the
endof5h I felt fine except for a tiredness in my legs and
the soreness of my feet. I believe that with proper footwear I could have continued for at least two more
hours.”
3) By subject RH: “I did not find the walk particularly
difficult. My feet were sore and I felt a little exhausted
afterwards but this did not last long. I felt fit enough to
Environmental Conditions and
Mean Evaporation Rates
Mean ambie nt temperatures, T,, and blackbody temperatures, Tbb, for days when the six men walked 5 h are
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removed, volume of collected sweat was measured, and
a sample retained. The other hand was washed and a
clean glove taped in place. The subject then was
weigh .ed and was allowed to drink water at 5-10°C ad
lib.: the volume drunk was noted and he resumed his
walk. One subject (IS) carried the container of water
with him, drinking as he walked, and returned the
container at the end of the lap. Six subjects completed 5
h; a seventh 9LS 3was compelled to stop after the fourth
hour as reported in RESULTS.
At the end of the walk, after observations on the
treadmill, the subject was weigh .ed clothed and then
nude, and wash ed down for sweat collection and analysis (5, 6) . Most of the anal ytical procedures including
meteorological observations have been described by Dill
et al. (7). Osmolality of plasma was determined with a
vapor pressure osmometer (Model 5120, Wescor, Inc.,
Logan, Utah).
play basketball that night after the walk. . . . at the end
of the fourth hour (after failure of venepuncture) I became nauseated and almost fainted. That hour I could
take only a sip of water. The pattern of my thirst was
perplexing. My desire for water was greatest in the
middle of the hour. But when the time came for me to
drink, the thirst subsided. I drank as much as I thought
I could without becoming sick. I was refreshed most by
the coolness of the water. At the end of each hour I was
the center of attention but during the hour I felt deserted and bored. If it had not been possible to judge the
passing of time by counting laps, it would have been a
rougher ordeal.”
4) By subject TD: “I didn’t feel the effects of dehydration as much as hunger; I appreciated the water each
hour. The weather was very hot, with occasional gusts
of wind which (were) very uncomfortable with the sand
in my face. Except for the heat and aches I didn’t mind
the walk.”
5) By subject JC: “I’ve lived in the desert almost all of
my life, so the heat of the sun didn’t bother me. I’ve
never been able to get used to the dry mouth and throat
that I always got toward the end of the walk. Boredom
was another factor. It (became) monotonous walking
around the same old track for 5 h. After the third hour
my legs began to tire. I could have walked longer, but I
was happy I didn’t have to. After drinking all the juice I
could and taking a long cool shower I was almost back to
normal. ”
6) By subject IO: “The sunshine and the fact that the
skin remained dry despite considerable evaporation,
made (the walk) quite pleasant. The hourly measurements broke the routine, so that no boredom was experienced. My thirst generally was easily satisfied. After 3
h, however, the mouth and throat became very dry.
Drinking relieved only this feeling for a short period of
time. A manifestation of fatigue was some soreness in
the feet, which became evident after 4 h. Toward the
end of the walk some hunger (was) experienced; I finished in good shape, and could easily have continued for
another hour. Following the exercise a strong urge to
drink persisted for more than an hour, at which time
most of the weight loss had been replaced.”
7) By subject LS: “I never felt excessively dehydrated
or heat stressed during the walk; I did look forward to
the water at the end of each hour. I drank slowly as I
walked and never felt as though I was forcing myself
either to drink more or less than was comfortable. After
the first hour I had the feeling of sweating ‘copiously’
from the forehead. Only once or twice was the water
sloshing in my stomach even mildly uncomfortable. The
fourth hour satisfied my senseof pride; during that hour
I was completely unable to maintain the pace. The effort
of lifting my feet made me decide to quit at the end of
that hour. At the end of the walk I drank what I felt like
drinking without any problems. My legs were wobbly
from exhaustion but recovered fairly quickly.”
238
DILL,
shown in Fig. 1. Subject LS walked 4 h only: on that day
T, was unusually
low so environmental
conditions and
his evaporation
rate for that day are plotted separately
in Fig. 1. The general trend was a rise in temperatures
during the first or second hour with nearly constant
values during the last 3 h. Winds usually were from 2 6 mph with an increase generally occuring in mid afternoon. In this figure, mean evaporation rates are shown
for the six men. The unit chosen, ml/m’*min,
renders
comparison
convenient
with many studies from this
laboratory
with sweat rate defined by the same unit.
The mean rate after the first hour was about nine,
which
is indicative
of considerable
environmental
stress. The highest evaporation rate for subject LS with
T, below 36OC was below 5.
Rectal and Skin
SOHOLT,
36.0r
AND
ODDERSHEDE
+
Temperatures
HOUR
CI c-4.0
---LS
P-
C-
-----
b
4
a
OF WALK
FIG. 2. Rectal
and skin temperatures
during
the walks.
Mean
values are shown by dotted lines. While the trend of skin temperatures is down and of rectal temperatures
up, there are great individual variations
especially
in skin temperature.
Note the remarkable
decrease in skin temperature
of LM who had the steepest sustained
rise in rectal temperature.
increased 0.5”C in the first hour and another 0,5OC in
the next 4 h, reaching 38.5”C. Subject LS, who was
unable to walk the fifth hour, was not handicapped by
temperature regulation: his T,, reached only 37.8OC
Values for T, varied widely within the range 32.6”C 36°C. The mean trend was downward while the mean
T,, was increasing. The most dramatic contrast was
between the two records on LM whose T,, increased
steadily from 37.1”C to 38.9”C while his skin temperature dropped from 35.2”C to 33OC. See his subjective
reactions in mmmrs.
Heart Rate and Blood Pressure
HOUR OF
WALK
1. Mean evaporation
rates by the subjects and mean ambient
conditions.
Subject LS walked
for 4 h and since ambient
temperature
was moderate
that day, values for his walk are plotted
separately.
Note that temperatures
tended to increase
during
the first 3 h and
generally
varied
but little
in the last 2 h. Mean evaporation
rate
reached its highest value in the fourth hour and declined
about 7% in
the last hour.
Heart rate (HR) was telemetered from the walking
subject. Blood pressure was observed as the subject
continued walking at the same rate on the treadmill.
The initial observations recorded in Fig. 3 were made
after the first lap on the course, i.e., after walking for
about 7 min. At that time the range in HR was from 95
in10 to 136 in RH. It increased in all subjects to the end,
more rapidly in some than in others. The increase was
minimal in 10: 10 beats. It was maximal in DW: 40
beats to a final HR of 160 indicating that he was approaching his cardiovascular limit.
Both systolic and diastolic blood pressures changed
slightly but uniformly from the sixth minute of the walk
to the end. The mean change in systolic pressure was
from 130 to 134 mmHg and in diastolic, from 62 to 54
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Individual records of rectal temperature,
T,,, and skin
temperature,
T,, are shown in Fig. 2. No one reached an
intolerable
body temperature.
The maximal
T,, was
39.7”C in TD in the third hour, after which it declined to
39.O”C at the end of the fifth hour. In one subject, RH,
T,, was constant throughout.
Mean T,, for the six men
FIVE-HOUR
DESERT
239
WALKS
r
I60
CL
m
Tn
0
JC
160
t
FIG.
values
2
3
HOUR
OF
3. Heart
rates, systolic
and
are shown by dotted lines.
4
5
WALK
diastolic
blood
pressures.
Mean
mmHg. These are at the levels expected in young men
in this grade of work and in a comfortable environment.
In the case of D W who had the greatest increase in HR,
40 beats, systolic blood pressure increased only from 140
to 144 mmHg and diastolic dropped from 68 to 60 mmHg:
these changes are similar to the mean changes of all the
subjects who walked for 5 h.
Water Exchange
Of greatest interest are the records of water exchange
in Fig. 4. All subjects depended on body water in the
first hour. Two patterns were evident: subjects JC and
RH evaporated about 400 ml/m2 while the other four
men evaporated from 500 to 550. In subsequent periods
all subjects satisfied their thirst with cool water (510°C) but the amounts of water drunk by subjects LM
and10 (middle section of Fig. 4) were much less than by
the other four men. The bottom section of this figure
shows hourly evaporation, ml/m2, by each subject. Since
all sweat was evaporated it follows that, with the possible exception of LM, each drank enough each period to
bring his total evaporation to the level required for
temperature regulation. As pointed out above, only one
subject, LM, had a steady rise in T,, and he was also the
only one to show a continuing decline in hourly evaporation. Subject TD showed a large decline in total evaporation in the last hour. Subject DW showed a small decline
while JC showed no change and RH and 110 showed
small increases. The large decline in the case of TD was
accompanied by a decline in T,, (Fig. 2) and evidently
reflected change in environmental stress in the last
hour. An examination of the weather records reveals
that only on the day subject TD walked was there a
significant change. A “cold” front passed through with a
drop from the fourth to the fifth hour of 5°C in T,, 10°C
I
I
1
I
I
I
2
3
4
5
HOUR
OF WALK
FIG. 4. Total
water
evaporated,
water ingested,
and body water
evaporated.
Note that IO and LM drew heavily
on body reserves;
the
patterns
of total evaporation
were about the same after the second
hour.
in Tbb, and an increase in mean wind speedsfrom 2 to 13
mph. Including TD the mean decline in evaporation in
the last hour was 7%; excluding him it was less than 2%.
Properties of the Blood: Red Cells, Plasma
Changes in hematocrit give some clue to loss of water
from plasma. In subjects JC and TD there was no
change, in D W an increase from 53.5 to 54.1, in RH from
47.8 to 49.0, in IO from 38.7 to 41.7, and in LM 48.9 to
54.1. Changes in protein content of plasma also give a
clue to loss of water from plasma. Neither clue is infallible: red cells and plasma are not uniformly mixed
throughout the bloodstream and protein can move in
and out of the bloodstream. If one assumes that no
change occurs in blood mixing and that the amount of
protein in plasma does not change the percentage
changes in plasma volume calculated from the two assumptions are, respectively: LM, -15, -21; DW, 0, -9;
RH, -5, -4; TD, 0, -7; JC, 0, -5; ZO, -12, -7; LS,
+lO, 0. These results point to hemodilutions in LS and
hemoconcentration in all the others: this was relatively
small in four subjects, large in10 and larger in LM. The
discrepancies between the two methods of calculating
movement of water in and out of plasma point to the
unreliability of one or both of the assumptions.
Evidence of another kind is useful in determining
what happens to the water content of red cells. Since
hemoglobin cannot escape from the red cells, its concentration, measured by the relation of hemoglobin content
of blood to the hematocrit, gives a measure of change in
water content of red cells. This ratio, g of Hblml of red
cells, proved remarkably constant in five of the six men
who walked for 5 h: the mean change was from 0.315 to
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I
240
DILL,
0.310; the range of changes in the five was from +0.002
to -0.007. The mean change of -0.005 corresponds
to a
2% increase in red cell volume. In subject 3C it changed
from 0.324 to 0.308 indicating
a 7% increase in his red
cell volume. The finding that red cells did not surrender
but rather gained a little water suggests that the same
was true of cells of other tissues and that extracellular
fluid was the principal
if not the only source of body
water used for sweat formation.
Properties
of Blood:
Osmotic
Pressure
This story is simply told by the data of Table 2: with
the possible exception of subject DW there was no consistent change in osmotic pressure. The mean value for
the six 5-h subjects was 282 initially and 282 after 5 h.
This indicates that thirst
was satisfied
by drinking
enough to maintain osmotic pressure.
Concentration
in Sweat
Values for concentration
of chloride in sweat are collected in Table 3. As described above, hand sweat was
collected in a rubber glove each hour. Sweat residues
accumulated on the skin and in the clothing during the
entire walk were collected by wash down. Total sweat
volume was calculated from the values for total evaporation (plotted in Fig. 4) corrected for water expired, water
loss by diffusion, and the difference in weight between
CO, expired and 0, used. Illustrative
of the great range
in volume of sweat from the hands, subject RH produced
too little sweat to be measured while in the other subjects the hourly volume of hand sweat varied from less
than 1 ml to a maximum
of 35 ml in subject LM. The
maximum volume of sweat was collected in the second
hour in four subjects, and in the fourth hour in two
subjects. The volume was least in the first hour in two
subjects, and in the fifth hour in four subjects. This variability was not related to variations
in total evaporation: these reflected variations
in body size and environmental stress. Clearly the subjects with the possible
exception ofLM had enough active sweat glands to yield
the amount of sweat needed to regulate body temperature but the distribution
of the glands varied. There
must have been very few on the hands of subject RH.
There are two significant
features of the findings in
Table 3: the wide range in saltiness of body sweat, 9
TABLE 2. Osmotic
Subj
pressure
of plasma,
mosmolll
min
lh
2h
3h
4h
5h
LM
DW
RH
TD
JC
IO
287
272
284
285
280
281
289
273
290
287
284
279
292
274
285
288
282
281
285
278
285
281
281
286
279
286
288
278
283
290
282
277
285
275
285
288
278
285
288
280
282
Mean
LS
IO*
282
278
279
284
279
282
284
281
283
283
280
281
283
283
284
282
283
286
283
7
Mean-5
h
* In this experiment
the subject was weighed
nude, washed down,
and donned clean clothing
each hour. This involved
an interruption
of about 12 min each hour.
AND
ODDERSHEDE
3. Concentration
of chloride
in sweat, hand, and body
TABLE
Sweat
Chloride,
Hand,
meq/I
Subj
LM
DW
RH
TD
JC
IO
LS
IO,*
IO,*
-
Hand
Body
lh
2h
3h
4h
5h
Mean
47
21
67
30
68
34
76
48
67
49
65
36
34
16
28
31
35
14
44
31
55
18
56
31
52
18
69
39
75
21
98
50
17
59
33
36
24
36
23
28
31
30
29
26
23
31
26
Sweat C hloride,
Body,
meq/l
45
32
17
21
9
35
28
* In this experiment
the subject was weighed
nude, washed down,
and donned clean clothing
each hour. This involved
an interruption
of about 12 min each hour.
meq/l in JC to 45 meq/l in LM, and the hourly increase
in saltiness of hand sweat. The mean value for chloride
in hand sweat was greater than in body sweat in every
subject, ranging from + 12%’ in D W to more than 100% in
TD. The greatest increase in chloride of hand sweat was
insubject IO, 28 to 98. He was the first subject and in his
case the same hand was used throughout
whereas subsequently alternate hands were used. To shed light on this
phenomenon,
subject IO did another 5-h walk: the results and explanatory
details are found in Table 3. With
a wash down each hour involving
an hourly interruption of about 12 min, chloride concentration
did not
increase in hand sweat nor in body sweat: the mean
concentration
was only slightly higher in hand sweat
than in body sweat. The mean concentration
of chloride
in his body sweat was somewhat
lower with the five
interruptions
and hourly wash down than in his other
walk. While the conclusion must be tentative it appears
that the hourly wash down including 12-min recuperation permitted the sweat glands of the hands to return to
their initial state. This does not seem to have occurred
in most of the subjects who continued walking
even
though their hands were gloved alternately.
The wide range in saltiness of sweat in the six men
bears a relation to the amount of body water utilized:
the saltier the sweat the smaller the intake of chloridefree tap water and the greater the use of body water.
This is evident from Fig. 5 where concentration
of chloride in body sweat is plotted against the volume of body
water used. The best fitting equation for that relationship is, y = 33.7~ + 200 where y = body water evaporated in 5 h in ml/m2 of body surface and x = chloride
concentration
in body sweat, meq/l. The correlation between x andy is 0.93 with a P value of 0.01.
Metabolic
Measurements
After the first lap and at the end of each hour with the
subject walking
on the treadmill,
metabolic measurements were made on six of the seven subjects. Subject
LM was hyperventilating
during metabolic measurements with respiratory
ratios high, some even above
unity. In DW, RH, TD, and IO, mean respiratory
ratio
declined from an initial value of 0.90 to a final mean of
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Chloride
SOHOLT,
FIVE-HOUR
DESERT
CHLORIDE
IN SWEAT-m
Eq/l
5. Relation
of chloride
concentration
in body
subjec t to his loss of body water expressed
in mllm2.
FIG.
241
WALKS
sweat
of each
DISCUSSION
One of the questions for which an answer was sought
in this study was: is sweat suppression seen in desert
walks? The pioneering study of this topic, first called
fatigue
of the sweat gZands, was conducted by Gerking
and Robinson (8). They undertook the study because
there had been indication’s of sweat suppression in investigations by Johnson et al. (9) and Pitts et al. (10) in
exercise lasting several hours. Following this lead,
Gerking and Robinson had six subjects walk 6 h in a
climatic room, stopping 10 min/h for weight, for urination, and for drinking enough 0.1% saline to balance the
weight loss. In one series the men wore only socks,
shoes, and shorts. In another they added an army tropical uniform of poplin. The two environments were humid (32-38°C db, 95-91 rh), and dry (40-50°C db, 31-34°C
wb, and 38-H rh). In the clothed subjects the sweat rate
measured by weight loss in kg/h, taking into account
water intake and urine output, decreased in every subject from the first to the sixth hour. This also was true
for those wearing shorts; their mean decline in kg/h was
from 1.26 to 0.76 in humid heat and from 1.41 to 1.20 in
dry heat. The metabolic rate in nearly all their walks
was 190 cal/m’*h, slightly less than the 200 cal/m2*h of
our walks. They did not report T,, but with the subjects
all able to walk for 6 h the conditions must have permitted equilibrium in temperature regulation. The lo-min
recovery period each hour was an advantage. It is safe to
assume that sweat rate in the sixth hour was adequate
for temperature regulation. It seems therefore that
sweat volume in excess of that amount in earlier periods
was wasted runoff. Clearly sweat suppression is a more
descriptive term than fatigue of the sweat glands; perhaps sweat conservation
would be an even better term.
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0.79, reflecting a shift in fuels from one-third fat to twothirds
fat. There was a related
increase
in vo2,
ml/kg.min,
from 17.8 to 18.8. This metabolic rate was
not a large fraction of the aerobic capacity, ranging from
45% in DW and LS to 32% in JC. In subject LM with
high respiratory ratios the percentage was 34: it is
unlikely that he was in metabolic acidosis during the
few minutes on the treadmill; it is certain the 5-h walk
was completely aerobic for all the subjects.
Another term that has been suggested is hidromeiosis.
In our subjects with the possible exception of unacclimatized LM there was no sweat suppression. This also
was true in 2-h walks previously reported (7). For some
reason when one undertakes a desert walk with wb
temperatures between 20 and 23°C as in this study, rate
of sweating soon reaches a level adequate for temperature regulation. The control mechanism whatever it
may be prevents excess sweating; under the conditions
of our walks higher sweat rates would have entailed
higher evaporation rates and body cooling.
Since the Gerking-Robinson study many others have
reported cases of sweat suppression. In a 1975 review
Cabanac (2) concludes that in sustained exposure to
heat, sweat secretion increases to a maximum and then
decreases. He reports evidence that a liquid film on the
skin inhibits sweating. In gloved hands, however, when
the skin is left constantly wet the volume of sweat
varies from hour to hour and, as pointed out above, may
increase in some subjects. In the light of the evidence,
we conclude that sweat suppression is rarely seen in
desert walks; the control mechanism that regulates the
rate of sweating in desert walks comes into action as the
walk begins. Further study is required to reveal the
mechanism.
In a study of water balance in man and dog in the
desert, Dill et al. (4) found that the dog replaced nearly
all the water lost by distillation from the tongue and
airway while man drank far less water than he lost in
sweat This was explained by the dog’s conservation of
salt and man’s loss of salt in sweat. This implies that
when one drinks tap water the saltier the sweat the less
he drinks and the greater the amount of body water he
evaporates. This led to the suggestion that in desert
walks the stimulus to drink is increased osmotic pressure (3). In our 5-h walks the subjects drank as much
cool tap water as thirst dictated. As shown in Table 2,
osmotic pressure of blood was well regulated. The mean
milliosmolal pressure of blood in the 5-h walk was 282 at
the beginning and at the end. In the six subjects the
range in chloride concentration in body sweat was from
9 to 45 meq/l. The range in body water evaporated
during the walk expressed in ml/m2 of body surface was
from 580 to 1,700 ml. Pairs of values for the six subjects
plotted in Fig. 5 show a high correlation. This seems to
confirm that in such desert walks dominant control of
water intake is exercised by osmotic pressure.
The above objective observations give some leads to
impending breakdown, especially in the case of unacclimatized LM, but the subjective reports (see RESULTS)
provide a fuller understanding of the course of events.
These reports reveal that LS quit after 4 h because of
“complete inability to maintain the pace.” Subject LM
drank the least water: he found “the desire for water
became progressively less.” He had a steadily rising T,,
(Fig. 2); “only the motivating force generated by the
experiment kept me going.” He remarked on intervals
of “chill sensations”; note the drop in his T,, Fig. 2. He
was unique in another respect: hyperventilation
during
the metabolic measurements. Subject TD didn’t mind
the walk “except for the heat and aches.” Subject IO
“finished in good shape and could easily have continued
DILL,
242
AND
ODDERSHEDE
so he estimated that he could have walked two more
hours.
In summary it appears that fit young men can walk 30
km at 100 m/min in desert heat provided cool water is
available. Signs of breakdown
are evident to the walker
-aching
muscles and joints, sore feet, perhaps sunburn. Objective evidence indicated that with the possible exception of the one unacclimatized
subject all sweat
enough from the start but not too much: there was no
sweat suppression.
The amount of water drunk was less
the saltier the sweat, suggesting that water intake was
adjusted to the level required for maintaining
a constant osmotic pressure.
We are indebted
to those who underwent
the ordeal of the long
desert walks.
Besides the two junior
authors,
they were Loren G.
Myhre,
David Wolfenbarger,
Randy
Hastings,
Tom Drost, and John
Connolly.
When not subjects
they doubled
as laboratory
assistants.
This study was supported
by National
Institutes
of Health
Grant
2ROl HD
05626-04Al
and National
Science
Foundation
Grant
BMS74-04861.
Received
for publication
20 June
1975
REFERENCES
1. ALDRICH,
L. B. A study
of body radiation.
Smithsonian
Inst.
Misc. Collections.
8: 8-12, 1930.
2. CABANAC,
M. Temperature
regulation.
Ann. Rev. PhysioL.
37:
415-439,
1975.
3. DILL, D. B. Life, Heat, and Altitude.
Cambridge,
Mass,: Harvard Univ.
Press, 1938, pa 51-72.
4. DILL, D. B., A. V. BOCK, AND H. T. EDWARDS.
Mechanisms
for
dissipating
heat in man and dog. Am. J. PhysioL.
104: 36-43,
1933.
5. DILL, D. B., F. G. HALL,
AND H. T. EDWARDS.
Changes
in
composition
of sweat during
acclimatization
to heat. Am. J.
Physiol.
123: 412-419,
1938.
6. DILL, D. B., F. G. HALL, AND W. VAN BEAUMENT.
Sweat chloride
concentration:
sweat
rate, metabolic
rate,
skin temperature,
and age. J. Appl. PhysioL.
21: 99-106,
1966.
7. DILL, D. B., M. K. YOUSEF, AND J. D. NELSON. Responses
of men
and women
to two-hour
walks
in desert heat. J. Apple. Physiol.
35: 231-235,
1973.
8. GERKING,
D. S., AND S. ROBINSON.
Decline in the rates of sweating of men working
in severe heat. Am. 3. Physiol.
147: 370-378,
1946.
9. JOHNSON,
R. E., G. C. PITTS, AND F. C. CONSOLAZIO,
Factors
influencing
chloride
concentration
in human sweat. Am. J. Physiol. 141: 575-589,
1944.
10. PITTS, G. C., R. E. JOHNSON, AND F. C. CONSOLAZIO.
Work in the
heat as affected
by intake
of water,
salt and glucose.
Am. J.
Physiol.
142: 253-259,
1944.
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for another hour.”
Subject JC “could have walked
longer but I was happy I didn’t have to.” Subject RH
“did not find the walk particularly
difficult.”
He played
basketball that evening. Subject DW believed that with
proper footwear
he could have continued at least two
more hours before hunger forced him to stop.
Only in the case of LA4 do the objective observations
point to an early breakdown-steadily
rising T,,, remarkably
low T,, a high chloride concentration
in sweat
and a large deficit in body water; on the other hand his
cardiovascular
system responded excellently.
He was
the only unacclimatized
subject, which may account for
some of his difficulties.
Subject IO had salty sweat also
and drew heavily on body water but seemed to suffer no
ill effects from those events: his cardiovascular
responses were excellent as was his temperature
regulation None of the measurements
made gave any clue to
aching muscles and joints, experienced
to varying extents by all subjects. There was evidence from records of
Fig. 2 that T,, was rising in most subjects, pointing to an
eventual failure of temperature
regulation.
Also HR
was rising in all subjects, reaching 160 in DW, but even
SOHOLT,