Physiological mechanisms determining eccrine sweat composition

Abstract

Purpose: The purpose of this paper is to review the physiological mechanisms determining eccrine sweat composition to assess the utility of sweat as a proxy for blood or as a potential biomarker of human health or nutritional/physiological status.

Methods: This narrative review includes the major sweat electrolytes (sodium, chloride, and potassium), other micronutrients (e.g., calcium, magnesium, iron, copper, zinc, vitamins), metabolites (e.g., glucose, lactate, ammonia, urea, bicarbonate, amino acids, ethanol), and other compounds (e.g., cytokines and cortisol).

Results: Ion membrane transport mechanisms for sodium and chloride are well established, but the mechanisms of secretion and/or reabsorption for most other sweat solutes are still equivocal. Correlations between sweat and blood have not been established for most constituents, with perhaps the exception of ethanol. With respect to sweat diagnostics, it is well accepted that elevated sweat sodium and chloride is a useful screening tool for cystic fibrosis. However, sweat electrolyte concentrations are not predictive of hydration status or sweating rate. Sweat metabolite concentrations are not a reliable biomarker for exercise intensity or other physiological stressors. To date, glucose, cytokine, and cortisol research is too limited to suggest that sweat is a useful surrogate for blood.

Conclusion: Final sweat composition is not only influenced by extracellular solute concentrations, but also mechanisms of secretion and/or reabsorption, sweat flow rate, byproducts of sweat gland metabolism, skin surface contamination, and sebum secretions, among other factors related to methodology. Future research that accounts for these confounding factors is needed to address the existing gaps in the literature.

Keywords: Amino acids; Ammonia; Bicarbonate; Biomarker; Cytokines; Electrolytes; Ethanol; Glucose; Lactate; Urea.

Fig. 1

Physiological and methodological factors impacting the composition of final sweat collected from the skin surface

Fig. 2

Mechanisms of water, ion, and metabolite passage into the secretory coil. ACh acetylcholine, AQP-5 aquaporin-5, Best2 bestrophin 2, CFTR cystic fibrosis transmembrane conductance regulator (note that chloride secretion via CFTR in the clear cells is activated by beta-adrenergic stimulation, which is not depicted), ENaC epithelial Na channel, GLUT2 glucose transporter 2, NHE1 Na⁺/H⁺ exchanger isoform 1, SGLT3 sodium-dependent glucose transporter 3, SGLT4 sodium-dependent glucose transporter 4, TMEM16A transmembrane member 16A