Christiano A. Machado-Moreira

Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans

Literature from the past 168 years has been filtered to provide a unified summary of the regional distribution of cutaneous water and electrolyte losses. The former occurs via transepidermal water vapour diffusion and secretion from the eccrine sweat glands. Daily insensible water losses for a standardised individual (surface area 1.8 m2) will be 0.6–2.3 L, with the hands (80–160 g.h−1) and feet (50–150 g.h−1) losing the most, the head and neck losing intermediate amounts (40–75 g.h−1) and all remaining sites losing 15–60 g.h−1. Whilst sweat gland densities vary widely across the skin surface, this same individual would possess some 2.03 million functional glands, with the highest density on the volar surfaces of the fingers (530 glands.cm−2) and the lowest on the upper lip (16 glands.cm−2). During passive heating that results in a resting whole-body sweat rate of approximately 0.4 L.min−1, the forehead (0.99 mg.cm−2.min−1), dorsal fingers (0.62 mg.cm−2.min−1) and upper back (0.59 mg.cm−2.min−1) would display the highest sweat flows, whilst the medial thighs and anterior legs will secrete the least (both 0.12 mg.cm−2.min−1). Since sweat glands selectively reabsorb electrolytes, the sodium and chloride composition of discharged sweat varies with secretion rate. Across whole-body sweat rates from 0.72 to 3.65 mg.cm−2.min−1, sodium losses of 26.5–49.7 mmol.L−1 could be expected, with the corresponding chloride loss being 26.8–36.7 mmol.L−1. Nevertheless, there can be threefold differences in electrolyte losses across skin regions. When exercising in the heat, local sweat rates increase dramatically, with regional glandular flows becoming more homogeneous. However, intra-regional evaporative potential remains proportional to each local surface area. Thus, there is little evidence that regional sudomotor variations reflect an hierarchical distribution of sweating either at rest or during exercise.

The cholinergic blockade of both thermally and non-thermally induced human eccrine sweating.

Thermally induced eccrine sweating is cholinergically mediated, but other neurotransmitters have been postulated for psychological (emotional) sweating. However, we hypothesized that such sweating is not noradrenergically driven in passively heated, resting humans. To test this, nine supine subjects were exposed to non‐thermal stimuli (palmar pain, mental arithmetic and static exercise) known to evoke sweating. Trials consisted of the following four sequential phases: thermoneutral rest; passive heating to elevate (by ∼1.0°C) and clamp mean body temperature and steady‐state sweating (perfusion garment and footbath); an atropine sulphate infusion (0.04 mg kg−1) with thermal clamping sustained; and following clamp removal. Sudomotor responses from glabrous (hairless) and non‐glabrous skin surfaces were measured simultaneously (precursor and discharged sweating). When thermoneutral, these non‐thermal stimuli elicited significant sweating only from the palm (P < 0.05). Passive heating induced steady‐state sweating ranging from 0.20 ± 0.04 (volar hand) to 1.40 ± 0.14 mg cm−2 min−1 (forehead), with each non‐thermal stimulus provoking greater secretion (P < 0.05). Atropine suppressed thermal sweating, and it also eliminated the sudomotor responses to these non‐thermal stimuli when body temperatures were prevented from rising (P > 0.05). However, when the thermal clamp was removed, core and skin temperatures became further elevated and sweating was restored (P < 0.05), indicating that the blockade had been overcome, presumably through elevated receptor competition. These observations establish the dependence of both thermal and non‐thermal eccrine sweating from glabrous and non‐glabrous surfaces on acetylcholine release, and challenge theories concerning the psychological modulation of sweating. Furthermore, no evidence existed for the significant participation of non‐cholinergic neurotransmitters during any of these stimulations.

Sweat secretion from palmar and dorsal surfaces of the hands during passive and active heating

INTRODUCTION It is generally accepted that the palmar (volar) and dorsal surfaces of human hands display different sudomotor responses to mental or thermal stimuli. We tested the hypothesis that, during thermal stimulation, secretion from the dorsal surfaces would always exceed that from the volar aspect of the hand. METHODS Sweat secretion from 10 hand sites and the forehead was examined (ventilated capsules) in 10 subjects during passive heating (climate chamber: 36 degrees C, 60% relative humidity, water-perfusion suit: 40 degrees C) immediately followed by incremental cycling to volitional fatigue. RESULTS This treatment significantly increased core temperature (39.3 degrees C), heart rate (178 bpm), and sweat rate at all sites. Mean sweat secretion during exercise was greater at the forehead (2.90 mg x cm(-2) x min(-1); +/- 0.19) than the hand (1.49 mg x cm(-2) min(-1); +/- 0.27). While no significant differences in sweating were observed among dorsal sites, a nonuniform secretion pattern was observed across the volar surface, with sweating at the palm being the lowest, and that from the volar aspect of the distal phalanges being equivalent to the dorsal hand. These differences became more evident as exercise progressed. Mean hand sweat rate during exercise was 41.7 ml x h(-1), with sweating from the palm accounting for only about 6% of sweat secretion. CONCLUSION Sweat secretion from both the palmar and dorsal surfaces of the hand increases during exercise in the heat, although this occurs in a nonuniform fashion. It is possible that a greater sweat gland density on the fingers may account for variations across the volar surface. However, higher dorsal sweating with lower gland counts (high glandular flow) may be attributable to either larger sweat glands, or to a greater cholinergic sensitivity of these glands.

Hidratação durante o exercício: a sede é suficiente?

The present work proposes a review about exercise fluid replacement and a discussion whether, during exercise, the fluid ingested according to thirst is sufficient to maintain hydration. Exercise sweat loss, mainly in the heat, can cause dehydration, can alter the hidroelectrolyte balance, disturb thermoregulation, presenting a health risk and/or impairing the athletic performance. It has been asserted that athletes do not drink, spontaneously, the sufficient fluid volume to prevent dehydration during the physical activity. Thus, international recommendations to fluid replacement during physical activities have been proposed. According to the American College of Sports Medicine (ACSM), about 500 mL of fluid on the two hours before the exercise must be ingested. During exercise, they propose that athletes should start fluid replacement since the beginning in regular periods and should drink enough fluid to restore all the sweating losses or ingest the maximal volume tolerated. The National Athletic Trainer’s Association (NATA) proposes the following recommendations: ingestion of 500 to 600 mL of water two or three hours before exercise or other sport drink and ingestion of 200 to 300 mL 10 to 20 minutes before exercise starting. During exercise, the fluid replacement should match the sweating and urine losses and at least should maintain hydration status reaching maximal body weight losses of 2%. After the exercise, fluid replacement must restore all the fluid losses accumulated. In addition, ACSM and NATA asserted about fluid temperature and palatability, beverage carbohydrate and electrolyte additions according to exercise duration and intensity and recommended hydration schedules to provide easier access to fluid ingestion. However, other authors contest the use of hydration schedules based on predetermined fluid volumes and suggest that fluid replacement according to thirst is enough to maintain body homeostasis.ABSTRACTExercise fluid replacement: is thirst enough? The present work proposes a review about exercise fluid repla-cement and a discussion whether, during exercise, the fluid in-gested according to thirst is sufficient to maintain hydration. Exer-cise sweat loss, mainly in the heat, can cause dehydration, canalter the hidroelectrolyte balance, disturb thermoregulation, pre-senting a health risk and/or impairing the athletic performance. Ithas been asserted that athletes do not drink, spontaneously, thesufficient fluid volume to prevent dehydration during the physicalactivity. Thus, international recommendations to fluid replacementduring physical activities have been proposed. According to theAmerican College of Sports Medicine (ACSM), about 500 mL offluid on the two hours before the exercise must be ingested. Dur-ing exercise, they propose that athletes should start fluid replace-ment since the beginning in regular periods and should drinkenough fluid to restore all the sweating losses or ingest the max-imal volume tolerated. The National Athletic Trainer’s Association(NATA) proposes the following recommendations: ingestion of 500to 600 mL of water two or three hours before exercise or othersport drink and ingestion of 200 to 300 mL 10 to 20 minutes be-fore exercise starting. During exercise, the fluid replacement shouldmatch the sweating and urine losses and at least should maintainhydration status reaching maximal body weight losses of 2%. Af-ter the exercise, fluid replacement must restore all the fluid loss-es accumulated. In addition, ACSM and NATA asserted about fluidtemperature and palatability, beverage carbohydrate and electro-lyte additions according to exercise duration and intensity and rec-ommended hydration schedules to provide easier access to fluidingestion. However, other authors contest the use of hydrationschedules based on predetermined fluid volumes and suggest thatfluid replacement according to thirst is enough to maintain bodyhomeostasis.

Sudomotor responses from glabrous and non-glabrous skin during cognitive and painful stimulations following passive heating

Aim:  It is widely accepted that thermal and psychological sweating are independently controlled and respectively restricted to non‐glabrous (hairy) and glabrous skin. These assumptions were evaluated in six experiments conducted across eight body segments, in which 38 glabrous and non‐glabrous skin surfaces were investigated.

Psychological sweating from glabrous and nonglabrous skin surfaces under thermoneutral conditions.

Recent experiments revealed psychological sweating to be a ubiquitous phenomenon in passively heated individuals. Since heating potentiates sweating, and since most research into psychological sweating was not conducted in this thermal state, these observations required thermoneutral verification. Thermoneutral subjects performed mental arithmetic (at 26(o) C) with psychological sweating evaluated from nine sites (ventilated capsules, skin conductance). Discharged sweating was evident from three glabrous sites (P < .05). However, significant sweating was evident from two nonglabrous surfaces (P < .05), and skin conductance increased at the volar and dorsal finger surfaces (P < .05). Each of these changes occurred while core and skin temperatures remained stable (P > .05). These thermoneutral observations further refute the proposition that psychological sweating in humans is restricted to the glabrous skin surfaces.

Design data for footwear: sweating distribution on the human foot

Purpose – The purpose of this paper is to provide footwear designers, manikin builders and thermo‐physiological modellers with sweat distribution information for the human foot.Design/methodology/approach – Independent research from two laboratories, using different techniques, is brought together to describe sweat production of the foot. In total, 32 individuals were studied. One laboratory used running at two intensities in males and females, and measured sweat with absorbents placed inside the shoe. The other used ventilated sweat capsules on a passive, nude foot, with sweating evaluated during passive heating and incremental exercise to fatigue.Findings – Results from both laboratories are in agreement. Males secreted more than twice the volume of sweat produced by the females (p<0.01) at the same relative work rate. Both genders demonstrated a non‐uniform sweat distribution, though this was less variable in females. Highest local sweat rates were observed from the medial ankles (p<0.01). The dorsal f...

Temporal and thermal variations in site-specific thermoregulatory sudomotor thresholds: precursor versus discharged sweat production.

Temporal and thermal differences between the initiation of precursor, eccrine sweat and its surface discharge were investigated during passive heating. Sudomotor activity was evaluated using electrodermal (precursor) and ventilated sweat capsule measurements (dorsal fingers, dorsal hand, forehead, forearm). Passive heating significantly elevated auditory canal (0.5 degrees C) and mean body temperatures (0.9 degrees C). At each site, the precursor sudomotor thresholds occurred at a lower mean body temperature (P < .05), with an average elevation of 0.35 degrees C (SD 0.04). However, discharged thresholds were delayed until this temperature had risen 0.53 degrees C (SD 0.04), producing significant phase delays across sites (mean: 4.1 min [SD 0.5]; P < .05). It is concluded that precise sudomotor threshold determinations require methods that respond to sweat accumulating within the secretory coil, and not discharged secretions, reinforcing the importance of electrodermal techniques.

Changes in eccrine sweating on the glabrous skin of the palm and finger during isometric exercise

Aim:  The goals of this study were to investigate changes in the sweating and cutaneous vascular responses on the palm and the volar aspect of the index finger during sustained static exercise of increasing intensity and to determine whether the former can be attributed to altered sweat gland activity.

Thermogenic and psychogenic recruitment of human eccrine sweat glands: Variations between glabrous and non-glabrous skin surfaces

Human eccrine sweat-gland recruitment and secretion rates were investigated from the glabrous (volar) and non-glabrous hand surfaces during psychogenic (mental arithmetic) and thermogenic stimuli (mild hyperthermia). It was hypothesised that these treatments would activate glands from both skin surfaces, with the non-thermal stimulus increasing secretion rates primarily by recruiting more sweat glands. Ten healthy men participated in two seated, resting trials in temperate conditions (25-26°C). Trials commenced under normothermic conditions during which the first psychogenic stress was applied. That was followed by passive heating (0.5°C mean body temperature elevation) and thermal clamping, with a second cognitive challenge then applied. Sudomotor activity was evaluated from both hands, with colourimetry used to identify activated sweat glands, skin conductance to determine the onset of precursor sweating and ventilated sweat capsules to measure rates of discharged sweating. From glandular activation and sweat rate data, sweat-gland outputs were derived. These psychogenic and thermogenic stimuli activated sweat glands from both the glabrous and non-glabrous skin surfaces, with the former dominating at the glabrous skin and the latter at the non-glabrous surface. Indeed, those stimuli individually accounted for ~90% of the site-specific maximal number of activated sweat glands observed when both stimuli were simultaneously applied. During the normothermic psychological stimulation, sweating from the glabrous surface was elevated via a 185% increase in the number of activated glands within the first 60s. The hypothetical mechanism for this response may involve the serial activation of additional eccrine sweat glands during the progressive evolution of psychogenic sweating.