Cutaneous microvascular function is reduced in winter compared with summer in healthy young adults.

Seasonal differences in temperature and ultraviolet radiation (UVR) exposure may alter NO-mediated vasodilation in the cutaneous microvasculature. We assessed seasonal variation in cutaneous microvascular function in 10 healthy (23 ± 2; 6 men, 4 women) adults in the summer/early-fall and winter/early-spring. Microvascular (intradermal microdialysis and local skin heating [42°C]) endothelial function was assessed at two visits separated by ~6 months. Mean daytime (sunrise to sunset) outdoor temperature and 2-h peak UV Index were recorded for the 30 days prior to each experimental visit. Serum vitamin D concentrations [25(OH)D] were measured for each visit. Multivariate regression analysis evaluated the factors (temperature, UV Index, and [25(OH)D]) that best explained variance in microvascular endothelial function. The local heating response (74.37 ± 12.21 %CVCmax vs. 55.89 ± 15.50 %CVCmax, p = 0.009) and the NO contribution to that response (61.73% ± 9.67% vs. 38.13% ± 13.26%, p = 0.004) were lower in winter/early-spring compared to summer/early-fall. The best-fit regression model suggested that 30-day average outdoor temperature and 2-h peak UV Index were positively and negatively associated with NO-dependent cutaneous vasodilation, respectively. Healthy adults exhibit lower NO-mediated cutaneous vasodilation in the winter/early-spring compared with summer/early-fall, which is partially explained by seasonal differences in outdoor temperature.