Surface contamination artificially elevates initial sweat mineral concentrations

Abstract

Several sweat mineral element concentrations decline with serial sampling. Possible causes include reduced dermal mineral concentrations or flushing of surface contamination. The purpose of this study was to simultaneously sample mineral concentrations in transdermal fluid (TDF), sweat, and serum during extended exercise-heat stress to determine if these compartments show the same serial changes during repeat sampling. Sixteen heat-acclimated individuals walked on a treadmill (1.56 m/s, 3.0% grade) in a 35°C, 20% relative humidity (RH), 1 m/s wind environment 50 min each hour for 3 h. Mineral concentrations of Ca, Cu, Fe, K, Mg, Na, and Zn were measured each hour from serum, sweat from upper back (sweat pouch) and arm (bag), and TDF from the upper back. Sites were meticulously cleaned to minimize surface contamination. Mineral concentrations were determined by spectrometry. TDF remained stable over time, with exception of a modest increase in TDF [Fe] (15%) and decrease in TDF [Zn] (-18%). Likewise, serum and pouch sweat samples were stable over time. In contrast, the initial arm bag sweat mineral concentrations were greater than those in the sweat pouch, and [Ca], [Cu], [Mg], and [Zn] declined 26-76% from initial to the subsequent samples, becoming similar to sweat pouch. Nominal TDF mineral shifts do not affect sweat mineral concentrations. Arm bag sweat mineral concentrations are initially elevated due to skin surface contaminants that are not removed despite meticulous cleaning (e.g., under fingernails, on arm hair), then decrease with extended sweating and approach those measured from the scapular region.

Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability

“…Several studies have shown that when serial samples are collected during exercise, initial sweat often has higher mineral concentrations than subsequent samples [99, 108–111]. This is likely because initial sweat mixes with minerals trapped in the sweat pore or residing in the epidermis [112–115].…”

“…The decrease in mineral concentrations observed throughout exercise is likely due to flushing of surface contamination. As Ely et al [99] have shown, meticulously cleaning the skin (with distilled water, soap, and a surgical scrub brush) prior to collection results in stable sweat mineral concentrations during 3 h of exercise (using the Parafilm-M ® pouch technique on the back).…”

“…iron [Fe], zinc [Zn], copper [Cu], magnesium [Mg], and calcium [Ca]) [99, 108–111, 114, 115]. Studies have shown no change [108] or an increase [99] in sweat [Na + ] and [K + ] throughout exercise, which may be explained in part by the significantly higher concentrations of Na + and K + in sweat (i.e. less impact on the signal-to-noise ratio) compared with trace minerals; however, more work is needed to confirm this hypothesis.…”

“…This commonly includes wiping the skin surface with alcohol and/or rinsing with distilled water. Meticulous cleaning, such as shaving and/or scrubbing with a surgical brush [99], may only be necessary when conducting laboratory research, using the arm-bag technique (to remove dirt and other material under finger nails), or when measuring trace mineral concentrations in sweat (see Table 1 for more information).…”

“…forehead > chest > scapula > forearm > thigh) [55]. In addition, the arm bag or rubber glove technique tends to overestimate sweat electrolyte concentrations to a greater extent than other local methods [88, 98, 99]. Nonetheless, studies have shown that local sweat [Na + ] is highly and significantly correlated with WBW sweat [Na + ], thus regression equations are available to estimate WBW sweat [Na + ] from local sweat [Na + ] using absorbent patches [54] and Parafilm-M ® pouches [55].…”