Explaining variation in sweat sodium concentration: effect of individual characteristics and exercise, environmental, and dietary factors

The index of maximum sweat ion reabsorption rates of sweat glands does not correlate to whole body sweat sodium concentration in exercising young healthy men

Factors explaining individual variations in whole body sweat sodium ion concentration ([Na+]) during exercise are not fully understood. Galvanic skin conductance (GSC) reflects the electrical properties of the skin influenced by sweat rate (SR) and the presence of ions. Initiation of increases in this response to elevating sweating may reflect exceeding the maximal capacity of sweat ion reabsorption in sweat glands. We investigated whether the SR threshold for increasing GSC, an indirect measure of maximum ion reabsorption rates of sweat glands, explains the variations in whole body sweat [Na+]. Thirty young healthy males cycled for 90 min at incremental exercise intensities of 30, 45, and 60% peak oxygen uptake (30 min each) in the heat (32°C, 50% relative humidity). Whole body sweat [Na+] was measured using a whole body washdown technique. The SR threshold for increasing GSC was determined from the relationship between the local SR (ventilated capsule) and GSC on the forearm and chest. The average whole body sweat [Na+] was 42.8 ± 18.9 (range: 14.4-81.0) mmol L-1, and the SR threshold for increasing GSC was 0.29 ± 0.20 (range: 0.02-0.62) and 0.35 ± 0.30 (range: 0.01-1.40) mg cm-2 min-1 for the forearm and chest, respectively. Whole body sweat [Na+] was not correlated with the SR threshold for increasing GSC in the forearm or chest (r2 ≤ 0.001, P ≥ 0.921). We conclude that the SR threshold for increasing GSC at the forearm and chest does not explain the individual variation in whole body sweat [Na+] during exercise in the heat.NEW & NOTEWORTHY Galvanic skin conductance is influenced by sweat rate and sweat ions, and the sweat rate at which this response begins to increase may reflect the exceeding capacity of sweat ion reabsorption in sweat glands. However, we show that this indirect measure of the sweat gland's capacity of ion regulation on the forearm and chest does not correlate with whole body sweat sodium concentration during exercise, excluding its role as a determinant of systemic sweat sodium loss.

Assessing the performance of a robust multiparametric wearable patch integrating silicon-based sensors for real-time continuous monitoring of sweat biomarkers

The development of wearable devices for sweat analysis has experienced significant growth in the last two decades, being the main focus the monitoring of athletes health during workouts. One of the main challenges of these approaches has been to attain the continuous monitoring of sweat for time periods over 1 h. This is the main challenge addressed in this work by designing an analytical platform that combines the high performance of potentiometric sensors and a fluidic structure made of a plastic fabric into a multiplexed wearable device. The platform comprises Ion-Sensitive Field-Effect Transistors (ISFETs) manufactured on silicon, a tailor-made solid-state reference electrode, and a temperature sensor integrated into a patch-like polymeric substrate, together with the component that easily collects and drives samples under continuous capillary flow to the sensor areas. ISFET sensors for measuring pH, sodium, and potassium ions were fully characterized in artificial sweat solutions, providing reproducible and stable responses. Then, the real-time and continuous monitoring of the biomarkers in sweat with the wearable platform was assessed by comparing the ISFETs responses recorded during an 85-min continuous exercise session with the concentration values measured using commercial Ion-Selective Electrodes (ISEs) in samples collected at certain times during the session. The developed sensing platform enables the continuous monitoring of biomarkers and facilitates the study of the effects of various real working conditions, such as cycling power and skin temperature, on the target biomarker concentration levels.

Non-faradaic capacitive cation sensing under flow

The ability to continually monitor target ion species in real-time is a highly sought-after endeavour in the field of host-guest chemistry, given its direct pertinence to medical and environmental applications. Developing methodologies which support sensitive and continuous ion sensing in aqueous media, however, remains a challenge. Herein, we present a versatile and facile, proof-of-concept electrochemical sensing methodology based on non-faradaic capacitance, which can be operated continuously with high temporal resolution (≈1.4 s), in conjunction with custom-designed integrated microfluidics. The potential of this method is demonstrated for cation sensing at a chemically simple benzo-15-crown-5-based molecular film (B15C5SAM) as a representative redox-inactive, receptive interface. Detection limits as low as 4 μM are obtained for Na+ by these entirely reagentless analyses, and are additionally characterised by exceptional baseline stabilities that are able to support continuous sensing over multiple days. The platform performs well in artificial sweat across physiologically relevant spans of sodium concentration, and provides meaningful dose-dependent responses in freshwater samples. Finally, the high assay temporal resolution affords an ability to resolve both the kinetics of binding (association/dissociation) and notably characteristic fingerprints for different alkali metals which may be diagnostic of different interfacial ion binding modes.

Reliability and validity of the MX3 portable sweat sodium analyser during exercise in warm conditions

PURPOSE

Accurately measuring sweat sodium concentration ([Na+]) in the field is advantageous for coaches, scientists, and dieticians looking to tailor hydration strategies. The MX3 hydration testing system is a new portable analyser that uses pre-calibrated biosensors to measure sweat [Na+]. This study aimed to assess the validity and reliability of the MX3 hydration testing system.

METHODS

Thirty-one (11 females) recreationally active participants completed one experimental trial. During this trial, participants exercised at a self-selected pace for 45 min in a warm environment (31.5 ± 0.8 °C, 63.2 ± 1.3% relative humidity). Sweat samples were collected from three measurement sites using absorbent patches. The samples were then analysed for sweat [Na+] using both the MX3 hydration testing system and the Horiba LAQUAtwin-NA-11. The reliability of the MX3 hydration testing system was determined following two measurements of the same sweat sample.

RESULTS

The mean difference between measurements was 0.1 mmoL·L-1 (95% limits of agreement (LoA): - 9.2, 9.4). The analyser demonstrated a coefficient of variation (CV) of 5.6% and the standard error of measurement was 3.3 mmoL·L-1. When compared to the Horiba LAQUAtwin-NA-11, there was a mean difference of - 1.7 mmoL·L-1 (95% LoA: - 0.25 X ¯ , 0.25 X ¯ ) and the CV was 9.8%.

CONCLUSION

The MX3 hydration testing system demonstrated very good single-trial reliability, moderate agreement and a very good CV relative to the Horiba LAQUAtwin-Na-11. To further validate its performance, the MX3 hydration testing system should be compared with analytical techniques known for superior reliability and validity.

Personalized Hydration Strategy to Improve Fluid Balance and Intermittent Exercise Performance in the Heat

Sweat rate and electrolyte losses have a large inter-individual variability. A personalized approach to hydration can overcome this issue to meet an individual's needs. This study aimed to investigate the effects of a personalized hydration strategy (PHS) on fluid balance and intermittent exercise performance. Twelve participants conducted 11 laboratory visits including a VO2max test and two 5-day trial arms under normothermic (NOR) or hyperthermic (HYP) environmental conditions. Each arm began with three days of familiarization exercise followed by two random exercise trials with either a PHS or a control (CON). Then, participants crossed over to the second arm for: NOR+PHS, NOR+CON, HYP+PHS, or HYP+CON. The PHS was prescribed according to the participants' fluid and sweat sodium losses. CON drank ad libitum of commercially-available electrolyte solution. Exercise trials consisted of two phases: (1) 45 min constant workload; (2) high-intensity intermittent exercise (HIIT) until exhaustion. Fluids were only provided in phase 1. PHS had a significantly greater fluid intake (HYP+PHS: 831.7 ± 166.4 g; NOR+PHS: 734.2 ± 144.9 g) compared to CON (HYP+CON: 369.8 ± 221.7 g; NOR+CON: 272.3 ± 143.0 g), regardless of environmental conditions (p < 0.001). HYP+CON produced the lowest sweat sodium concentration (56.2 ± 9.0 mmol/L) compared to other trials (p < 0.001). HYP+PHS had a slower elevated thirst perception and a longer HIIT (765 ± 452 s) compared to HYP+CON (548 ± 283 s, p = 0.04). Thus, PHS reinforces fluid intake and successfully optimizes hydration status, regardless of environmental conditions. PHS may be or is an important factor in preventing negative physiological consequences during high-intensity exercise in the heat.

Sudoscan as substitute for quantitative sudomotor axon reflex test in composite autonomic scoring scale and its correlation with composite autonomic symptom scale 31 in type 2 diabetes

OBJECTIVE

This study aims to evaluate the feasibility of substituting electrochemical skin conductance measurement using SUDOSCAN for sudomotor function testing in the Composite Autonomic Scoring Scale (CASS) and to correlate the results with the Composite Autonomic Symptom Scale 31 (COMPASS 31) among patients with type 2 diabetes mellitus (T2DM).

METHODS

Fifty patients with T2DM underwent cardiovascular autonomic function testing and the SUDOSCAN test and completed the COMPASS 31 questionnaire. We developed a SUDOSCAN-based sudomotor subscore as a substitute for the original sudomotor subscore (based on the quantitative sudomotor axon reflex test [QSART]). The modified CASS score (SUDOSCAN-based sudomotor subscore combined with the adrenergic and cardiovagal subscores) and the original CASS score without suomotor assessment (sum of the adrenergic and cardiovagal subscores) were obtained according to the results of the cardiovascular autonomic function and SUDOSCAN tests.

RESULTS

The total COMPASS 31 score was significantly correlated with the modified CASS score (p = 0.019 and 0.037 for the raw and weighted scores, respectively) but not with the CASS score without sudomotor assessment. After adding the SUDOSCAN-based sudomotor subscore, the number of patients identified as having diabetic autonomic neuropathy (DAN) increased from 24 (48 %, based on the CASS score without sudomotor assessment) to 35 (70 %, based on the modified CASS score). The modified CASS score enhances the accuracy of assessing autonomic function and improves the diagnosis of diabetic autonomic neuropathy (DAN) among patients with T2DM. In medical settings where QSART is not accessible, SUDOSCAN testing offers a practical and efficient alternative.

Assessing the Feasibility of Using Electrochemical Skin Conductance as a Substitute for the Quantitative Sudomotor Axon Reflex Test in the Composite Autonomic Scoring Scale and Its Correlation with Composite Autonomic Symptom Scale 31 in Parkinson's Disease

The Composite Autonomic Scoring Scale (CASS) is a quantitative scoring system that integrates the sudomotor, the cardiovagal, and the adrenergic subscores, and the Composite Autonomic Symptom Scale 31 (COMPASS 31) is based on a well-established comprehensive questionnaire designed to assess the autonomic symptoms across multiple domains. We tested the hypothesis that electrochemical skin conductance (Sudoscan) can be a substitute for the quantitative sudomotor axon reflex test (QSART) in the sudomotor domain and assessed its correlation with COMPASS 31 in patients with Parkinson's disease (PD). Fifty-five patients with PD underwent clinical assessment and cardiovascular autonomic function tests and completed the COMPASS 31 questionnaire. We compared the modified CASS (integrating the Sudoscan-based sudomotor, adrenergic, and cardiovagal subscores) and CASS subscores (the sum of the adrenergic and cardiovagal subscores). The total weighted score of COMPASS 31 was significantly correlated with both the modified CASS and the CASS subscore (p = 0.007 and p = 0.019). The correlation of the total weighted score of COMPASS 31 increased from 0.316 (CASS subscores) to 0.361 (modified CASS). When we added the Sudoscan-based sudomotor subscore, the case numbers for autonomic neuropathy (AN) increased from 22 (40%, CASS subscores) to 40 (72.7%, modified CASS). The modified CASS not only better reflects the exact autonomic function, but also improves the characterization and quantification of AN in patients with PD. In areas in which a QSART facility is not easily available, Sudoscan could be a time-saving substitution.