Rates of appearance (Ra) and disappearance (Rd) of glucose
and glycerol were calculated according to the Steele equation
(38), which was modified for use with stable isotopes according
to Romijn et al. (35), because the amount of tracer infused
is no longer considered negligible. Enrichment and concentration
data were fitted to curves by use of spline fitting (35);
then the kinetics were calculated as described in the previous
paragraph. The volume of distribution was assumed to be
100 ml/kg for glucose and 230 ml/kg for glycerol (35). We
attempted to minimize the changes in enrichment by increasing
the infusion rate in step increments when exercise was
initiated, as previously described (4).
Statistical analysis. The physical characteristics of the
participants and the hormone concentrations were analyzed
using a two-way analysis of variance (ANOVA). All other
data were analyzed using a three-way ANOVA, with gender
being the between-factor variable, condition (PRE, ABS, and
REL) being the first within-factor variable, and time (t 5 0,
30, 60, 75, 90 min) being the second within-factor variable.
When significance was obtained, the location of the difference
was determined using Newman-Keuls post hoc analysis. The
level of significance was set at P # 0.05. Values are presented
as means 6SE.