Moderate-intensity intermittent work in the heat results in similar low-level dehydration in young and older males.

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 (Tre), 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 Tre, Tre 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.