Metabolic flexibility is termed
the body’s ability to adapt fuel oxidation to fuel availability and demand.
Metabolic inflexibility, whereby the capacity to switch between fat and glucose
oxidation in response to a meal is impaired and preference is for glucose
oxidation in the fasted state has implications for metabolic diseases such as
obesity and type 2 diabetes.
Purpose: To assess the effect of acute aerobic exercise on
metabolic flexibility using a novel methodology.
Methods: Following ethical approval, six subjects (age, 30.7±11 yrs; height,
174±9 cm; weight, 73.7±12.7) agreed to participate in the study. Baseline Dual
X-Ray absorptiometry (DXA) (body fat, 21.23±10.02%) and maximal oxygen uptake
(VO2max, 49.5±10.2 ml.kg.min-1) measurements were
performed. During an evening visit subjects underwent either 45 mins of seated
rest (REST) or treadmill exercise at 70% VO2max (EX) followed by a
standardized mixed meal. The next morning, 10hrs postprandial, subjects
consumed another mixed meal containing 30% of daily BMR calculated using the
Harris Benedict equation. Serial blood and respiratory gas samples were
collected pre, 30, 60, 90, and 120 mins post meal. Individual variances for
insulin and RQ in response to the meal were determined and used to establish
metabolic flexibility, whereby, higher variance in RQ and lower variance in
insulin represent greater metabolic flexibility.
Results: The variance in RQ (REST, 0.00016±0.0011; EX, 0.0016±0.0008;
p=0.753) and insulin (REST, 14761±9542; EX, 11961±8293; p=0.345) in response to
the mixed meal did not differ significantly between the REST and EX conditions.
Similarly, no significant differences were observed between REST and EX
conditions for fasting RQ (REST, 0.79±0.05; Ex, 0.79±0.03; p=0.6) or Area Under
Curve for Insulin (REST, 19667±9680; EX, 15304±6331; p=0.173).
Conclusions: We hypothesized that an enhanced metabolic flexibility in response
to a mixed meal would result from a previous exercise bout when compared to
rest. This was defined as increased variance in RQ and improved insulin
sensitivity i.e. a decrease in variance and/or AUC for insulin. As there was no
difference between conditions in these variables we conclude that metabolic
flexibility, as measured in this study, is not enhanced after acute aerobic
exercise.