Influence of Fluid Delivery Schedule and Composition on Fluid Balance, Physiologic Strain, and Substrate Use in the Heat

Abstract

Introduction-Wildfire suppression is characterized by high total energy expenditure and water turnover rates. Hydration position stands outside hourly fluid intake rates. However, dose interval remains ambiguous. We aimed to determine the effects of microdosing and bolus-dosing water and microdosing and bolus-dosing carbohydrate-electrolyte solutions on fluid balance, heat stress (physiologic strain index [PSI]), and carbohydrate oxidation during extended thermal exercise. Methods-In a repeated-measures cross-over design, subjects completed four 120-min treadmill trials (1.3 m⋅s-1 , 5% grade, 33 • C, 30% relative humidity) wearing a US Forest Service wildland firefighter uniform and a 15-kg pack. Fluid delivery approximated losses calculated from a pre-experiment familiarization trial, providing 22 doses⋅h-1 or 1 dose⋅h-1 (46±11, 1005±245 mL⋅dose-1). Body weight (preand postexercise) and urine volume (pre-, during, and postexercise) were recorded. Heart rate, rectal temperature, skin temperature, and steady-state expired air samples were recorded throughout exercise. Statistical significance (P<0.05) was determined via repeated-measures analysis of variance. Results-Total body weight loss (n=11,-0.6±0.3 kg, P>0.05) and cumulative urine output (n=11, 677±440 mL, P>0.05) were not different across trials. The micro-dosed carbohydrate-electrolyte trial sweat rate was lower than that of the bolus-dosed carbohydrate-electrolyte, bolus-dosed water, and microdosed water trials (n=11, 0.

Workshift Changes in Hydration Status During Wildfire Suppression

“…WLFFs on assignment are capable of maintaining euhydration status (via USG and void frequency measurements) and serum electrolytes (sodium and potassium) during an extended fireline workshift within previously reported work intensities (accelerometry derived) 3–5,7,9 and ambient temperatures 1,2,7,8,15–17 . Currently provided fluid and food provisions can amount to quantities sufficient to maintain euhydration status, suggesting that while on a workshift, consumption of an adequate total fluid volume may be more important than the interval at which it is consumed 14 . However, current dietary intake recommendations for long-duration moderate-intensity exercise 31 indicate that observed carbohydrate intakes during wildfire suppression are semi-deficient.…”

“…8 Similarly, net hydration status appears to be unaffected by fluid ingestion schedule following 2 hours of exercise under thermal laboratory conditions. 14 During a workshift, consuming an adequate total hourly volume of fluid may be more crucial for hydration status than the frequency at which it is delivered. The present study did not standardize fluid delivery systems, document drink frequency, or clamp drink frequency during the workshift.…”

“…1,2,7,8,[15][16][17] Currently provided fluid and food provisions can amount to quantities sufficient to maintain euhydration status, suggesting that while on a workshift, consumption of an adequate total fluid volume may be more important than the interval at which it is consumed. 14 However, current dietary intake recommendations for long-duration moderate-intensity exercise 31 indicate that observed carbohydrate intakes during wildfire suppression are semi-deficient. Supplementary food items contribute 31 AE 17% of ingested workshift carbohydrates.…”

“…Matching fluid ingestion to fluid output minimizes hypohydration, which can otherwise contribute to increases in heart rate and core temperature that correspond with elevated physiologic strain levels 11–13 when not moderated by work:rest ratios 14 . Fluid ingestion on the fireline has been observed as both sufficient 6,7,15,16 and insufficient 6,8,15,16 for the total workshift needs, as evidenced by pre- to post-workshift changes in body weight, urine specific gravity (USG), and urine or plasma osmolality.…”

“…Matching fluid ingestion to fluid output minimizes hypohydration, which can otherwise contribute to increases in heart rate and core temperature that correspond with elevated physiologic strain levels [11][12][13] when not moderated by work:rest ratios. 14 Fluid ingestion on the fireline has been observed as both sufficient 6,7,15,16 and insufficient 6,8,15,16 for the total workshift needs, as evidenced by pre-to post-workshift changes in body weight, urine specific gravity (USG), and urine or plasma osmolality. Together, previous stable isotope tracer 1,2,5,6 and fluid ingestion studies 6,8,[15][16][17] on WLFFs have been able to quantify the total fluid intake needs in addition to total ingested fluid volumes throughout an entire workshift.…”