Abstract
BackgroundThe independent effects of hypohydration and hyperthermia on cognition and mood is unclear since the two stresses often confound each other. Further, it is unknown if obese individuals have the same impairments during hyperthermia and hypohydration that is often observed in non-obese individuals.MethodsThe current study was designed to assess the independent and combined effects of mild hypohydration and hyperthermia on cognition, mood, and mental task load in obese and non-obese females. Twenty-one healthy females participated in two passive heating trials, wherein they were either euhydrated or hypohydrated prior to and throughout passive heating. Cognition (ImPACT), mental task load (NASA-TLX), and mood (Brunel Mood Scale; BRUMS) were measured before and after a 1.0 °C increase in core temperature (TC).ResultsAfter a 1.0 °C TC elevation, hypohydration resulted in greater (p < 0.05) body mass loss (−1.14 ± 0.48 vs −0.58 ± 0.48 kg; hypohydrated and euhydrated, respectively) and elevation in serum osmolality (292 ± 4 vs 282 ± 3 mOsm; p < 0.05) versus euhydration. Hypohydration, independent of hyperthermia, did not affect mental task load or mood (p > 0.05). Hyperthermia, regardless of hydration status, impaired (∼5 A.U) measures of memory-based cognition (verbal and visual memory), and increased mental task load, while worsening mood (p < 0.05). Interestingly, obese individuals had increased mental task load while hyperthermic compared to the non-obese individuals (p < 0.05) even while euhydrated. Hypohydration did not exacerbate any heat-related effects on cognition between obese and non-obese females (p > 0.05).ConclusionThese data indicate that hyperthermia independently impairs memory-based aspects of cognitive performance, mental task load, and leads to a negative mood state. Mild hypohydration did not exacerbate the effects of hyperthermia. However, obese individuals had increased mental task load during hyperthermia.
Highlights
Military personnel, construction workers, and firefighters are often exposed to occupational heat stress
After a 1.0 ◦C TC elevation, hypohydration resulted in greater (p < 0.05) body mass loss (−1.14 ± 0.48 vs −0.58 ± 0.48 kg; hypohydrated and euhydrated, respectively) and elevation in serum osmolality (292 ± 4 vs 282 ± 3 mOsm; p < 0.05) versus euhydration
Subjects arrived with a body mass loss of only 0.02 ± 1.19%, a serum osmolality (SOSM) of 283 ± 2 mOsm, urine osmolality (UOSM) of 331 ± 198 mOsm, and a urine specific gravity (USG) of 1.011 ± 0.005
Summary
Construction workers, and firefighters are often exposed to occupational heat stress. Some investigators have shown that mild hypohydration, with approximately 1–3% body mass loss by fluid restriction, causes slower psychomotor processing speed, impaired visual and spatial working memory, increased errors and skill impairments (Wilson & Morley, 2003; Petri, Dropulic & Kardum, 2006; Patel et al, 2007; Suhr et al, 2010; Smith, Newell & Baker, 2012; Lindseth et al, 2013) Mood ratings, such as feelings of tiredness and reduced vigor are observed with hypohydration (Shirreffs, 2003; Szinnai et al, 2005; Patel et al, 2007; Pross et al, 2013). Obese individuals had increased mental task load during hyperthermia
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