Age and regulation of fluid and electrolyte balance during repeated exercise sessions

A common response after only 3-4 days of repeated exercise in younger individuals is an expansion of plasma volume (PV); however, it is not known if older individuals have a similar response. In this study, six older (O) (67 ± 1 yr/and six younger (Y) men (24 ± 2 yr) cycled for 4 successive days at 50% maximal oxygen consumption (V̇O(2max)) for 90 min in a warm environment [30°C temperature dry bulb (T(db)), 24°C temperature wet bulb (T(wb))]. On day 4, PV was increased (P < 0.05) in Y (10.0 ± 1%) but not (P > 0.05) in O (1.7 ± 2%). The increased PV was associated with a greater (P < 0.05) daily fluid intake during the exercise period in Y (45 ± 3 ml · day^{- 1} · kg body wt^-1) compared with O (32 ± 2 ml · day^-1 · kg body wt^{- 1}) and an increase (P < 0.05) in the total circulating protein (TCP) content in Y (0.23 ± 0.1 g/kg body wt) but not in O (0.10 ± 0.1 g/kg body wt). Throughout the 4-day exercise period there were similar reductions in 24-h urine flow rate (UV̇) and urinary sodium excretion (U(Na)V̇) in Y and O. Additionally, acute renal clearance measures made during exercise on days 1 and 4 showed similar (P > 0.05) reductions in U(Na)V̇ between Y (-55 ± 10%) and O (-44 ± 6%). However, during exercise in O there were no changes (P > 0.05) in UV̇ (2 ± 12%) and urine osmolality (U(Osm)) (-12 ± 6%) from resting values compared with Y, where UV̇ was decreased (P < 0.05) by 41 ± 9% and U(Osm) was increased (P < 0.05) by 39 ± 8%. Therefore, the inability of the older subjects to increase PV after repeated days of exercise is not related to an impaired renal fluid and Na⁺ conservation ability, despite a reduced urine concentrating ability during exercise, but to other factors (e.g., fluid intake and TCP) that appear necessary for the hypervolemic response.