Fluid deficits during prolonged overnight fasting in young healthy adults

Background: Overnight fasting is often prolonged before scheduled surgery, and the extent of perioperative fluid replacement may influence outcome. In clinical practice, basic requirements are estimated at 1.2-2.0 mL·kg⁻¹·h⁻¹, but there is little contemporary clinical data on what deficits result from complete fasting. This prospective preclinical study was designed to determine total fluid loss during overnight fasting, prolonged during daytime. Methods: Twenty (10 female) healthy adult volunteers, aged 24 (range 21-46) years, fasted from 22:00 until 16:00, and had their body weight and urine output measured at predefined time intervals. Results: The median (interquartile range) fluid deficits were 0.82 (0.73-1.00) kg, corresponding to 1.26 (1.11-1.41) g·kg⁻¹·h⁻¹ for the initial overnight fasting period, 0.59 (0.40-0.70) kg and 0.99 (0.83-1.31) g·kg⁻¹·h⁻¹ for the consecutive daytime period, and 1.47 (1.27-1.64) kg and 1.19 (1.05-1.28) g·kg⁻¹·h⁻¹ for the total period of fasting. Urine output accounted for 52% of total weight loss and was 36% of the baseline hourly level during the last four-hour period of fasting. Conclusions: Ten hours of overnight fasting in young adults induces fluid deficits at the lower limit of estimated intervals referred to in clinical practice, and hourly weight loss gradually decreases further during prolonged daytime fasting. These findings indicate that current routine procedures do slightly overestimate fluid deficits resulting from prolonged fasting in perioperative clinical practice.