The Association Between Outdoor Temperature During the Past Weeks and Current Fluid Homeostasis

BACKGROUND: It remains unclear why cool temperatures cause more persistent adverse health effects compared with hot weather. Fluid homeostasis may constitute a causal link between past temperatures and adverse health effects. In this study, we investigated the association between past outdoor temperatures and current fluid homeostasis. METHODS: We studied participants from five cohorts during three decades in Sweden (total n = 29,755, age 18-86 years, 50.4% women). We quantified fluid homeostasis through indicators of hormonal regulation (vasopressin biomarker plasma copeptin), urine concentration (urine osmolality), and replenishment of fluid loss (total water intake) and related these parameters to past outdoor temperatures (21 days) using distributed nonlinear lag models. RESULTS: Past temperatures were nonlinearly associated with current copeptin and urine osmolality. Cool temperatures during days and weeks prior contributed to distinct patterns of high copeptin with concomitant influence on urine osmolality. Overall, a scenario of temperatures of 0 °C for 21 days showed 14.9% (95% confidence interval = 11.5%, 18.3%) higher copeptin levels compared with reference temperatures of 14.3 °C for 21 days. Associations between copeptin and warm temperatures were less complex and of shorter duration, linking elevated temperatures within 24 hours with higher copeptin compared with the reference temperature. CONCLUSIONS: Alterations in human fluid homeostasis may partly explain the observed link between moderately cool outdoor temperatures and adverse health effects weeks later. If so, avoiding altered water balance through moderately increased water intake might mitigate the adverse health effects of cool weather.