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_prod) or decreased skin blood flow (SkBF), 10 nicotine-naïve trained males [37 ± 12 yr; peak oxygen consumption (V̇o_2peak): 66 ± 10 mL·min^-1·kg^-1] completed four trials at 20°C and 30°C following overnight transdermal nicotine (7 mg·24 h^-1) and placebo use in a crossover, double-blind design. They cycled for 60 min (55% V̇o_2peak) followed by a time trial (∼75% V̇o_2peak) during which measures of gastrointestinal (T_gi) and mean weighted skin ([Formula: see text]_sk) temperatures, SkBF, H_prod, and mean arterial pressure (MAP) were made. The difference in ΔT_gi 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]_sk higher during nicotine than placebo trials (0.5 ± 0.5°C, P = 0.02). SkBF became progressively lower during nicotine than placebo trials (P = 0.01) and progressively higher during 30°C than 20°C trials (P < 0.01); MAP increased from baseline (P < 0.01) and remained elevated in all trials. The difference in H_prod between 30°C and 20°C trials was lower during nicotine than placebo (P = 0.01) and became progressively higher during 30°C than 20°C trials with exercise duration (P = 0.03). Mean power output during the time trial was lower during 30°C than 20°C trials (24 ± 25 W, P = 0.02), and although no effect of nicotine was observed (P > 0.59), two participants (20%) were unable to complete their 30°C nicotine trials as one reached the ethical limit for T_gi (40.0°C), whereas the other withdrew due to "nausea and chills" (T_gi = 39.7°C). These results demonstrate that nicotine use increases thermal strain and risk of exertional heat exhaustion by reducing SkBF.NEW & NOTEWORTHY 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 1) populations that face exertional heat strain and demonstrate high nicotine use (e.g., athletes and military, 25%-50%) and 2) study design whereby screening and exclusion for nicotine use or standardization of prior use (e.g., overnight abstinence) is encouraged.