Nicotine exacerbates exertional heat strain in trained men: a randomized, placebo-controlled, double-blind study

To determine whether using nicotine exacerbates exertional heat strain through an increased metabolic heat production (H ) or decreased skin blood flow (SkBF), 10 nicotine-naïve trained males [37 ± 12 yr; peak oxygen consumption (V̇o ): 66 ± 10 mL·min ·kg ] completed four trials at 20°C and 30°C following overnight transdermal nicotine (7 mg·24 h ) and placebo use in a crossover, double-blind design. They cycled for 60 min (55% V̇o ) followed by a time trial (∼75% V̇o ) during which measures of gastrointestinal (T ) and mean weighted skin ([Formula: see text] ) temperatures, SkBF, H , and mean arterial pressure (MAP) were made. The difference in ΔT between nicotine and placebo trials was greater during 30°C (0.4 ± 0.5°C) than 20°C (0.1 ± 0.7°C), with [Formula: see text] higher during nicotine than placebo trials (0.5 ± 0.5°C, = 0.02). SkBF became progressively lower during nicotine than placebo trials ( = 0.01) and progressively higher during 30°C than 20°C trials ( < 0.01); MAP increased from baseline ( < 0.01) and remained elevated in all trials. The difference in H between 30°C and 20°C trials was lower during nicotine than placebo ( = 0.01) and became progressively higher during 30°C than 20°C trials with exercise duration ( = 0.03). Mean power output during the time trial was lower during 30°C than 20°C trials (24 ± 25 W, = 0.02), and although no effect of nicotine was observed ( > 0.59), two participants (20%) were unable to complete their 30°C nicotine trials as one reached the ethical limit for T (40.0°C), whereas the other withdrew due to "nausea and chills" (T = 39.7°C). These results demonstrate that nicotine use increases thermal strain and risk of exertional heat exhaustion by reducing SkBF. In naïve participants, acute nicotine use exerts a hyperthermic effect that increases the risk of heat exhaustion during exertional heat strain, which is driven by a blunted skin blood flow response. This has implications for ) populations that face exertional heat strain and demonstrate high nicotine use (e.g., athletes and military, 25%-50%) and ) study design whereby screening and exclusion for nicotine use or standardization of prior use (e.g., overnight abstinence) is encouraged.