The effects of smoking and nicotine ingestion on exercise heat tolerance

Smoking has a thermogenic effect and is associated with low physical performance. Nevertheless, a direct, quantitative effect of acute smoking on exercise heat tolerance has not been reported. Sixteen healthy young male volunteers, eight cigarette smokers, and eight non-smokers participated in the study. All subjects performed a maximal oxygen consumption test (VO_2max) and a standardized heat tolerance test (HTT) after at least 12 h without smoking under the following conditions: no nicotine exposure, 10 min after nicotine exposure (2 mg nicotine lozenge), and 10 min after smoking two cigarettes (0.8 mg nicotine in each cigarette, smokers only). There was no significant effect of nicotine exposure on physiological performance and heat tolerance in the non-smokers group. In the smokers group, cigarette smoking, but not nicotine ingestion, resulted with higher heart rate (by 9±9 bpm) at the end of the HTT (p<0.05). Moreover, both smoking and nicotine ingestion increased smokers' rectal temperature at the end of the HTT (by 0.24±0.16°C and 0.21±0.26°C, respectively, p<0.05) and were associated with higher sweat rate during the HTT (by 0.08±0.07 g/h and 0.06±0.08 g/h, respectively, p<0.05). Heart rate variability (HRV) analysis also revealed a higher LF/HF (low frequency/high frequency) ratio after exposure to nicotine and smoking in the smokers group compared with no exposure (2.13±2.57 and 2.48±2.76, respectively, p<0.05), indicating a higher sympathetic tone. According to this preliminary study, cigarette smoking and nicotine ingestion increase the physiological strain during a HTT in smokers. Acute smoking may, therefore, increase heat intolerance and the risk to heat injuries.