Fat Oxidation Kinetics Is Related to Muscle Deoxygenation Kinetics During Exercise

Abstract

Purpose: The present study aimed to determine whether whole-body fat oxidation and muscle deoxygenation kinetics parameters during exercise were related in individuals with different aerobic fitness levels.

Methods: Eleven cyclists [peak oxygen uptake ( $\stackrel{.}{V}O{}_{2peak}$ ): 64.9 ± 3.9 mL⋅kg^-1⋅min^-1] and 11 active individuals ( $\stackrel{.}{V}O{}_{2peak}$ : 49.1 ± 7.4 mL⋅kg^-1⋅min^-1) performed a maximal incremental cycling test to determine $\stackrel{.}{V}O{}_{2peak}$ and a submaximal incremental cycling test to assess whole-body fat oxidation using indirect calorimetry and muscle deoxygenation kinetics of the vastus lateralis (VL) using near-infrared spectroscopy (NIRS). A sinusoidal (SIN) model was used to characterize fat oxidation kinetics and to determine the intensity (Fat_max) eliciting maximal fat oxidation (MFO). The muscle deoxygenation response was fitted with a double linear model. The slope of the first parts of the kinetics (a ₁) and the breakpoint ([HHb]_BP) were determined.

Results: MFO (p = 0.01) and absolute fat oxidation rates between 20 and 65% $\stackrel{.}{V}O{}_{2peak}$ were higher in cyclists than in active participants (p < 0.05), while Fat_max occurred at a higher absolute exercise intensity (p = 0.01). a ₁ was lower in cyclists (p = 0.02) and [HHb]_BP occurred at a higher absolute intensity (p < 0.001) than in active individuals. $\stackrel{.}{V}O{}_{2peak}$ was strongly correlated with MFO, Fat_max, and [HHb]_BP (r = 0.65-0.88, p ≤ 0.001). MFO and Fat_max were both correlated with [HHb]_BP (r = 0.66, p = 0.01 and r = 0.68, p < 0.001, respectively) and tended to be negatively correlated with a ₁ (r = -0.41, p = 0.06 for both).

Conclusion: This study showed that whole-body fat oxidation and muscle deoxygenation kinetics were both related to aerobic fitness and that a relationship between the two kinetics exists. Individuals with greater aerobic fitness may have a delayed reliance on glycolytic metabolism at higher exercise intensities because of a longer maintained balance between O₂ delivery and consumption supporting higher fat oxidation rates.

Keywords: Fatmax; NIRS; aerobic fitness; breaking point; cycling; indirect calorimetry.

FIGURE 1

Schematic illustration of the three independent variables of the SIN model (dilatation [d], symmetry [s], and translation [t]) and their impact on the whole-body fat oxidation kinetics. Basic symmetric SIN curve with d = 0, s = 1, and t = 0 (A). Changes in d (B), s (C), and t (D) and corresponding modifications in the basic symmetric SIN curve (dotted lines). MFO, maximal fat oxidation; $\stackrel{.}{V}O{}_{2peak}$, peak oxygen uptake.

FIGURE 2

Whole-body fat oxidation kinetics in relative (A) and absolute (B,C) values and muscle deoxygenation kinetics as a function of relative (D) and absolute (E) exercise intensity in cyclists (black line) and active people (black dots). Δ[HHb], variation in deoxygenated hemo- and myoglobin concentration; MFO, maximal fat oxidation; $\stackrel{.}{V}O{}_{2peak}$, peak oxygen uptake. ^∗Significant difference between the cyclist and active group (p ≤ 0.05).

FIGURE 3

Schematic representation of the correlations among aerobic fitness [peak oxygen uptake ($\stackrel{.}{V}O{}_{2peak}$), ventilatory threshold (VT), and respiratory compensation point (RCP)], whole-body fat oxidation kinetics [maximal fat oxidation (MFO) and exercise intensity at which MFO occurs (Fat_max) and the symmetry variable], and muscle deoxygenation kinetics [the slopes of the first (a₁) and second (a₃) parts of the curve of the double linear model].

FIGURE 4

Example of the whole-body fat oxidation (A) and muscle deoxygenation (B) kinetics of a cyclist participant determined during the submaximal incremental test. Δ[HHb], variation in deoxygenated hemo- and myoglobin concentration; MFO, maximal fat oxidation; $\stackrel{.}{V}O{}_{2peak}$, peak oxygen uptake.