Moderate-Intensity Intermittent Work in the Heat Results in Similar Low-Level Dehydration in Young and Older Males

Abstract

Older individuals may be more susceptible to the negative thermal and cardiovascular consequences of dehydration during intermittent work in the heat. This study examined the hydration, thermal, and cardiovascular responses to intermittent exercise in the heat in 14 Young (Y, Mean ± SE; 25.8 ± 0.8 years), Middle-age (MA, 43.6 ± 0.9 years), and Older (O, 57.2 ± 1.5 years) healthy, non-heat acclimated males matched for height, mass, body surface area, and percent body fat. Rectal temperature (T_re), heart rate (HR), local sweat rate (LSR), and hydration indices were measured during 4 × 15-min moderate to heavy cycling bouts at 400 W heat production, each followed by a 15-min rest period, in Warm/Dry (35°C, 20% relative humidity [RH]) and Warm/Humid (35°C, 60% RH) heat. No differences were observed between the age groups for T_re, T_re change, HR, LSR, mass change, urine specific gravity, and plasma protein concentration in either condition, irrespective of the greater level of thermal and cardiovascular strain experienced in the Warm/Humid environment. Plasma volume changes (Dry Y: −5.4 ± 0.7, MA: −6.2 ± 0.9, O: −5.7 ± 0.9%, Humid Y: −7.3 ± 1.0, MA: −7.9 ± 0.8, O: −8.4 ± 1.0%) were similar between groups, as were urine specific gravity and plasma protein concentrations. Thus, physically active Young, Middle-age, and Older males demonstrate similar hydration, thermal, and cardiovascular responses during moderate- to high-intensity intermittent exercise in the heat.

Keywords::

• heat stress
• aging
• core temperature
• dry and humid environments

ACKNOWLEDGMENTS

This project was funded by a research grant provided by the Workplace Safety and Insurance Board (Ontario) and Canada Foundation for Innovation-Leaders Opportunity Fund (held by Dr. Glen P. Kenny). The authors wish to acknowledge Drs. M.B. DuCharme, A.J. Seely, and J.E. Yardley as part of the grant-writing team. Dr. Glen Kenny was supported by a University of Ottawa Research Chair Award. Dr. Heather Wright was supported by a MITACS-Accelerate fellowship (Vale Ltd.). The authors wish to thank the participants for their time and effort and Dr. Francois Haman for the use of his laboratory freezer.