Age effects on body fluid distribution during exercise in the heat.

Background: The purpose was to study the effects of age on body fluid distribution during exercise and exercise combined with heat stress.

Methods: Ten young (Y; 23.2 +/-0.7 yr) and eight older (O; 65.3 +/- 1.0 yr) men performed two 80-min intermittent exercise bouts on a cycle ergometer at 50% VO2peak, one in a thermoneutral (N), 22 degrees C environment, the other in a hot (H), 40 degrees C environment. For each condition, changes in serum protein, albumin, and electrolytes (Cl-, Na+, K+); total body water (TBW); plasma volume (PV); and interstitial (ISF), extracellular (ECF), and intracellular (ICF) fluids were determined.

Results: During exercise, [Cl-] responses increased more in O than Y (p < or = 0.05) while albumin (g) increased in Y and decreased in O (p < or = 0.05). There were no age-related differences in total protein changes nor any of the other blood parameters. PV decreased similarly for all subjects in both conditions with larger decreases in the H environment (241.8 +/- 40.3 vs. 478.3 +/- 46.0 mL). Loss of TBW was exacerbated in the H condition and significantly different (p < 0.05) between the groups (N: Y = -1.03 +/- 0.076 L, O = -0.88 +/- 0.12 L; H: Y = -1.65 +/- 0.12 L, O = -1.85 +/- 0.14 L). The O group lost more ICF (p < 0.05) (N: Y = -510.4 +/- 86.8 mL, O = -631.7 +/- 115.1 mL; H: Y = -529.3 +/- 118.0 mL, O = -928.6 +/- 118.9 mL) and less ISF (p < 0.05) (N: Y = -295.5 +/- 101.1 mL, O = 15.8 +/- 77.6 mL; H: Y = -684.6 +/- 134.5 mL, O = -397.9 +/- 165.5 mL) in both conditions.

Conclusions: There was an interactive effect between age and heat stress in TBW loss. In addition, older individuals lost more ICF and less ISF than younger individuals during prolonged exercise. These findings suggest that the utilization of ICF, rather than ISF, to preserve PV during cycling exercise at or near 50% VO2max may be an age-related compensation.