1 Introduction
2 Eccrine Sweat Glands and Thermoregulatory Sweating
3 Measurement Techniques
3.1 Sweating Rate
3.2 Sweat [Na+]
4 Methodological Sources of Variability in Sweating Rate and Sweat [Na+]
4.1 Sample Collection
4.1.1 Method of Sweat Stimulation
Local SR | Local sweat [Na+] | Comments | |
---|---|---|---|
Sample collection
| |||
Type of sweating | |||
Exercise vs. pharmacological | ↑ | ↑/↓/↔ | Exercise involves central/peripheral and thermal/non-thermal mechanisms of sweating, whereas pilocarpine iontophoresis involves only peripheral cholinergic stimulation of sweat glands |
Method of collection | |||
Local: gravimetry vs. hygrometry | ↓ | NA | Difference primarily early in exercise (e.g. first 20–30 min, i.e. prior to establishing steady-state sweating); microenvironment is created by both methods; gravimetry most practical in field tests |
Local vs. whole-body | ↑ (vs. WBSR) | ↑ (vs. WB sweat [Na+]) | Local typically overestimates WB, but varies with anatomical site |
Skin surface contamination | |||
Scrubbing vs. light cleaning; removal of initial sweat | ? | ↔ | Seems to impact trace minerals more than Na+ and K+
|
Leaching | ? | ↑ | Leaching of electrolytes from stratum corneum into sweat and/or water from sweat into stratum corneum; can be indicated by high sweat [K+] |
Timing | |||
Patch application (before vs. 20–30 min after exercise onset) | ↓ | ↓ | Lower SR at start of exercise vs. after steady-state sweating has been established |
Patch removal | ? | ↔/? | Reported duration of patch time on skin varies from approximately 5 to approximately 90 min exercise; no differences found between 30 and 70 min in one study; more research needed |
Patch saturation | ↓ | ↓ | Moisture accumulation on skin leads to hidromeiosis |
Local SR | Local sweat [Na+] or [Cl−] | Comments | |
Sample storage
| |||
Storage temperature | 3–7 days in storage; some information gleaned from the CF literature (e.g. sweat [Cl−]) | ||
Freezing (−80 °C) | NA | ↓/? | More research needed for sweat [Na+] |
Refrigeration (2–8 °C) | NA | ↑/↔/? | More research needed for sweat [Na+]; CF sweat-testing guidelines recommend 4 °C for a maximum of 3 days in airtight containers |
Room (21–25 °C) | NA | ↑/↔/? | More research needed for sweat [Na+] |
Incubation (32–37 °C) | NA | ↑/↔/? | More research needed for sweat [Na+] |
Local SR | Local sweat [Na+] | Comments | |
Sample analysis
| |||
Analytical technique | NA | IC < ISE < FP ≤ conductivity | General synopsis across multiple studies; more research directly comparing all techniques is needed |
4.1.2 Skin Surface Contamination and Initial Sweat
4.1.3 Timing and Duration of Sweat Collection
4.1.4 Sample Storage
4.1.5 Sample Analysis
5 Intra/Interindividual Sources of Variability in Sweating Rate and Sweat [Na+]
5.1 Intraindividual Variability
5.1.1 Day-to-Day
WBSR | Local SR | Local sweat [Na+] | Comments | |
---|---|---|---|---|
Day-to-day (CVs) | 5–7% | 6–22% | 5–16% (WB: 11–17%) | Includes instrument variability (1–3%) |
Regional differences | ||||
Across body (% difference) | NA | 200–360% | 80–120% | Range includes anatomical sites typically used/accessible in field testing (back, chest, forearm, thigh, and forehead) |
Contralateral sides | NA | ↔ | ↔ | Forearms and scapulas |
Exercise intensity (absolute VO2) | Impacts E
req
| |||
High vs. moderate vs. low | ↑ | ↑ | ↑ | Directly related to metabolic energy expenditure (i.e. metabolic heat production) |
Environmental conditions | ||||
Temperature (↑) | ↑ | ↑ | ↑ | Impacts E
req; ↑ radiant heat gain and therefore ↑ T
c
|
Solar radiation (↑) | ↑ | ↑ | ↑/? | Impacts E
req; ↑ radiant heat gain and therefore ↑ T
c
|
Humidity (↑) | ↑ | ↑ | ↑/? | ↓ Water vapor gradient leads to ↓ evaporation of sweat, which ↑ T
c and the need for higher SR than calculated from E
req, but prolonged exposure can lead to hidromeiosis and decreased SR |
Wind (↑) | ↓ | ↓ | ↓/? | Impacts E
req; ↑ convective/evaporative heat loss and therefore ↓ T
c
|
Body mass | ||||
Larger vs. smaller | ↑ | ? | ? | Related to metabolic heat production and possibly sweating efficiency |
Protective equipment | ↑ | ↑ | ? | ↓ Evaporative and radiant heat loss, ↑ metabolic heat gain and therefore ↑ T
c
|
Sex | ||||
Men vs. women | ↑ | ↑ | ↑/↔ | SR differences related to higher body mass and metabolic heat production of men, rather than sex per se; less wasteful sweating by women in humid heat |
Aging | ||||
Older vs. middle-aged vs. young adult | ↓ | ↓ | ↔/? | Related to decline in fitness (and associated decline in cholinergic sensitivity), rather than aging per se |
Maturation | ||||
Pre vs. post-pubertal | ↓ | ? | ↓ | Related to lower sweat gland sensitivity; SR differences in males only, suggesting testosterone may be involved (although direct evidence is lacking) |
Heat acclimation | ↑ | ↑ | ↓ | ↑ Cholinergic and aldosterone sensitivity; gland hypertrophy; ↑ slope of relation between SR and T
c; ↓ T
c threshold for sweat onset |
Aerobic capacity | ||||
Higher vs. lower VO2max
| ↑ | ↑ | ↔/? | ↑ Cholinergic sensitivity; ↑ slope of relation between SR and T
c; ↓ T
c threshold for sweat onset |
Hydration status | ||||
2–3% BML vs. euhydration | ↓ | ↓ | ↑/? | Hypovolemia ↓ slope of relation between SR and T
c; hyperosmolality ↑ T
c threshold for sweat onset |
Menstrual cycle | ||||
Luteal vs. follicular | ↔ | ↓ | ↓/↔/? | Luteal phase ↑ T
c threshold for sweat onset and ↓ slope of relation between SR and T
c (thus LSR lower at a given T
c); effect lessens with heat acclimation |
Dietary sodium | Studies involved 8–14 days on strictly controlled, modified diets | |||
Change from moderate to high intake (8–9 g Na+) | ↔ | ↔ | ↑ | ↓ Circulating aldosterone |
Change from moderate to low intake (1–2 g Na+) | ↔ | ↔ | ↓ | ↑ Circulating aldosterone |
Exercise duration (↑) | ||||
Low intensity | ↔ | ↔ | ↔ | Studies involved 3–7 h of exercise and low SR |
High intensity | ↓ | ↓ | ↓ | Related to effects of hidromeiosis with prolonged heavy sweating |
Race/ethnicity | ↔ | ↔ | ↔ | Indigenous environmental factors are more important than race or ethnicity per se. Heat habituation (lower, more efficient sweating) may occur in people indigenous to hot or tropical climates |
5.1.2 Regional
5.1.3 Intra- and/or Interindividual Variability
5.1.4 Rate of Evaporation Required for Heat Balance
5.1.5 Structural Differences in Sweat Glands
5.1.6 Central and Peripheral Control of the Sweating Response
5.1.7 Rate of Ductal Na+ Reabsorption
5.1.8 Other Factors
6 Practical Recommendations
Whole-body sweating rate | |
---|---|
Conditions | Test in conditions (environment, intensity, season, equipment, etc.) representative of training/competition |
Conduct multiple tests within athletes to determine sweating rate in various conditions | |
Method | Change in body mass, preferably with athlete nude or wearing minimal clothing |
Calculation | WBSR = [Body massPRE-EX – (Body massPOST-EX – fluid intakeEX + urine outputEX)]/exercise duration |
Additional corrections | Food intake and stool loss (include in the intake and output portion of the above equation, respectively) |
Respiratory water loss and metabolic mass loss, particularly when exercise is >2–3 h, high-intensity, and/or in a dry environment | |
Trapped sweat in clothing/uniform, if not obtaining nude body mass | |
Quality control | Take pre-exercise body mass measurement after athlete voids bladder |
Record any clothing worn during pre- and post-exercise body mass measurements | |
Measure pre- and post-exercise body mass in duplicate | |
Monitor fluid intake/bathroom breaks between pre- and post-exercise body mass measurements—flag data if fluid intake and urine loss are not measured | |
Monitor for spitting/squirting of fluid from drink bottles—flag data if not controlled/prevented; offer a separate bottle of water if athletes want to use it for body cooling purposes (e.g. squirting on face, dumping on head, etc.) |
Local sweat [Na+] | |
---|---|
Conditions | Test during exercise (as opposed to passive heat stress or pharmacologically-induced local sweating) |
Test in conditions (environment, intensity, season, equipment, etc.) representative of training/competition | |
Conduct multiple tests within athletes to determine sweat [Na+] in various conditions | |
Methods | Check for background electrolytes in collection system (e.g. patches, storage tubes, etc.) |
Anatomical location: consider site accessibility and validity compared with whole-body sweat [Na+] (e.g. forearm may be best suited when considering both factors) | |
Clean skin immediately prior to application: alcohol, deionized/distilled water rinse, and dry with sodium-free gauze/towel | |
Apply multiple patches per athlete (e.g. right and left forearm) to have a backup (e.g. in case one patch falls off) | |
Apply patches 20–30 min after the onset of exercise (to establish steady-state sweating prior to sweat collection) | |
Avoid hidromeiosis: prevent patch saturation by limiting patch time on skin, using patches with high absorbent capacity, and/or changing patches frequently | |
Check patches for adherence to skin—flag data if patch becomes detached prematurely | |
Apply multiple patches per session if expecting significant changes in factors that would impact sweating rate (exercise intensity or environment) or if conditions are conducive to whole-body hidromeiosis (e.g. prolonged intense running in humid, still air) | |
Avoid cross-contamination when working with multiple athletes (e.g. use clean forcipes for each patch) | |
Storage | Refrigerate (e.g. approximately 4 °C) for up to approximately 3–5 days in airtight (e.g. Parafilm-M® sealed) containers |
Analysis | IC or ICP-MS in the laboratory; ISE in the field |
Analysis in the field recommended if sample storage duration and conditions during transportation cannot be well-controlled | |
Corrections | Use regression equations to predict whole-body sweat [Na+] from local sweat [Na+] |
Quality control | Flag samples that meet the following criteria: |
• Sweat sample volume suggestive of saturated patch (volume depends on specific patch type and size) | |
• Sweat [Na+] <10 mmol/L or >90 mmol/L | |
• Sweat [K+] <2 mmol/L or >10 mmol/L |