Cardiac output and skin blood flow in lean and obese individuals during exercise in the heat.

Five obese (% body fat greater than or equal to 27%) and five relatively lean (% body fat less than 20%) men performed upright exercise on a cycle ergometer at intensities of 30, 50, and 70% of their maximal aerobic power [VO2max (ml X kg fat-free wt-1 X min-1)] in both a thermoneutral [dry bulb temperature (Tdb) = 22 degrees C, wet bulb temperature (Twb) = 14 degrees C] and a hot (Tdb = 38 degrees C, Twb = 20 degrees C) ambient environment. Cardiac output (Q) was measured by CO2 rebreathing and forearm blood flow (FBF) was measured by venous occlusion plethysmography. Esophageal temperature (Tes) was measured by a thermocouple placed in the esophagus at approximately heart level, and mean skin temperature (Tsk) was calculated from the average of thermocouple readings from six skin sites. When the exercise intensity was normalized for metabolic body mass by dividing by fat-free weight, Q was similar between lean and obese at all exercise intensities and in both ambient environments. No differences between the two groups were found in Tes and Tsk under all conditions. The obese had significantly lower FBF compared with the lean during the higher exercise intensities in the hot ambient environment. In both ambient environments, the slope of the FBF-Tes relationship was significantly less in the obese group. It was concluded that body composition may alter the balance between the two opposing sets of cutaneous vascular reflexes (baroreceptor-induced vasoconstriction and thermoregulatory vasodilation) that regulate the competition for blood flow between the skin and working muscle during exercise in the heat.