To identify the mechanism underlying the exaggerated hyperglycemia dur
ing exercise in the heat, six trained men were studied during 40 min o
f cycling exercise at a workload requiring 65% peak pulmonary oxygen u
ptake (VO2peak) on two occasions at least 1 wk apart. On one occasion,
the ambient temperature was 20 degrees C [control (Con)], whereas on
the other, it was 40 degrees C [high temperature (HT)]. Rates of gluco
se appearance and disappearance were measured by using a primed contin
uous infusion of [6,6-H-2]glucose. No differences in oxygen uptake dur
ing exercise were observed between trials. After 40 min of exercise, h
eart rate, rectal temperature, respiratory exchange ratio, and plasma
lactate were all higher in HT compared with Con (P < 0.05). Plasma glu
cose levels were similar at rest (Con, 4.54 +/- 0.19 mmol/l; HT, 4.81
+/- 0.19 mmol/l) but increased to a greater extent during exercise in
HT (6.96 +/- 0.16) compared with Con (5.45 +/- 0.18; P < 0.05). This w
as the result of a higher glucose rate of appearance in HT during the
last 30 min of exercise. In contrast, the glucose rate of disappearanc
e and metabolic clearance rate were not different at any time point du
ring exercise. Plasma catecholamines were higher after 10 and 40 min o
f exercise in HT compared with Con (P < 0.05), whereas plasma glucagon
, cortisol, and growth hormone were higher in HT after 40 min. These r
esults indicate that the hyperglycemia observed during exercise in the
heat is caused by an increase in liver glucose output without any cha
nge in whole body glucose utilization.