Physiology of sweat gland function: The roles of sweating and sweat composition in human health

Abstract

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Physiological mechanisms determining eccrine sweat composition

“…If this amount of Ca 2+ were incorporated into a 1 mL sample of sweat collected over a 10 cm 2 surface area (typical absorbent patch size), it would equate to 0.12-0.62 mmol/L of Ca 2+ contaminating the sweat [Ca 2+ ] value. To put this in perspective, the range in sweat [Ca 2+ ] typically reported is 0.2 to 2.0 mmol/L (Baker 2019). Based on these estimations, skin surface contamination could artificially elevate sweat [Ca 2+ ] by two-to fourfold, which is in line with findings from studies comparing cell-rich vs. cell-poor sweat in Supplemental (Boysen et al 1984;Ely et al 2011).…”

“…However, WBW is a meticulous method that requires a wellcontrolled laboratory setting. Therefore, in most studies, sweat is collected from one or more small regions of the body using a myriad of techniques, which have been described in detail elsewhere (Baker 2017(Baker , 2019Taylor and Machado-Moreira 2013). Briefly, regional sweat collection techniques for sweat composition analysis include scraping or dripping methods, filter paper, absorbent patches, sweat pouches, sweat capsules, arm gloves/bags, and sweat collectors (e.g., Macroduct ® /Megaduct).…”

“…Figure 1 provides a general view of eccrine and apocrine sweat glands. For more details on thermoregulatory sweating and sweat gland structure/function, the reader is referred to more comprehensive reviews (Baker 2019;Gagnon and Crandall 2018;Sato 1983). Hereafter, the focus of the review will be sweat composition.…”

“…Furthermore, an increase serum [Na + ] and osmolality with Na + ingestion stimulates renal water reabsorption for more complete rehydration (Evans et al 2017). Individuals with salty sweat (e.g., [Cl − ] and [Na + ] ≥ 70 to 80 mmol/L) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating (Baker 2019;Montain et al 2006). Because of the large inter-individual variability in sweating rate and sweat [Na + ] personalized fluid/Na + replacement strategies are recommended to maintain fluid and Na + balance during exercise (Belval et al 2019;McDermott et al 2017;Sawka et al 2007).…”

“…To understand the potential utility of sweat as a reflection of blood, one must first have a thorough understanding of the physiological mechanisms determining the composition of sweat-from the composition of primary sweat in the secretory coil, to the mechanisms of reabsorption and secretion in the duct, which influence final sweat excreted onto the skin surface. While several excellent classic reviews on sweat composition and mechanisms of sweat gland function have been published (Costill 1977;Robinson and Robinson 1954;Sato 1977;Sato et al 1989), very few recent reviews have provided an update on mechanisms underlying sweat composition in the context of sweat as a potential biomarker of human health (Baker 2019). It is critical to identify what is known and what is not yet known regarding the physiology of sweat composition to generate mechanism-based hypotheses and guide the direction of future research in the area of sweat diagnostics.…”