AUTHOR=Zurbuchen Anouck , Lanzi Stefano , Voirol Ludovic , Trindade Cybele Barboza , Gojanovic Boris , Kayser Bengt , Bourdillon Nicolas , Chenevière Xavier , Malatesta Davide 

TITLE=Fat Oxidation Kinetics Is Related to Muscle Deoxygenation Kinetics During Exercise

JOURNAL=Frontiers in Physiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2020.00571

DOI=10.3389/fphys.2020.00571

ISSN=1664-042X

ABSTRACT=<sec><title>Purpose</title><p>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.</p></sec><sec><title>Methods</title><p>Eleven cyclists [peak oxygen uptake (<inline-formula><mml:math id="INEQ1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover accent="true"><mml:mi>V</mml:mi><mml:mo>.</mml:mo></mml:mover><mml:mi>O</mml:mi><mml:msub><mml:mi /><mml:mrow><mml:mn>2</mml:mn><mml:mo>⁢</mml:mo><mml:mi>p</mml:mi><mml:mo>⁢</mml:mo><mml:mi>e</mml:mi><mml:mo>⁢</mml:mo><mml:mi>a</mml:mi><mml:mo>⁢</mml:mo><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula>): 64.9 ± 3.9 mL⋅kg<sup>–1</sup>⋅min<sup>–1</sup>] and 11 active individuals (<inline-formula><mml:math id="INEQ2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover accent="true"><mml:mi>V</mml:mi><mml:mo>.</mml:mo></mml:mover><mml:mi>O</mml:mi><mml:msub><mml:mi /><mml:mrow><mml:mn>2</mml:mn><mml:mo>⁢</mml:mo><mml:mi>p</mml:mi><mml:mo>⁢</mml:mo><mml:mi>e</mml:mi><mml:mo>⁢</mml:mo><mml:mi>a</mml:mi><mml:mo>⁢</mml:mo><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula>: 49.1 ± 7.4 mL⋅kg<sup>–1</sup>⋅min<sup>–1</sup>) performed a maximal incremental cycling test to determine <inline-formula><mml:math id="INEQ3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover accent="true"><mml:mi>V</mml:mi><mml:mo>.</mml:mo></mml:mover><mml:mi>O</mml:mi><mml:msub><mml:mi /><mml:mrow><mml:mn>2</mml:mn><mml:mo>⁢</mml:mo><mml:mi>p</mml:mi><mml:mo>⁢</mml:mo><mml:mi>e</mml:mi><mml:mo>⁢</mml:mo><mml:mi>a</mml:mi><mml:mo>⁢</mml:mo><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> 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<sub>max</sub>) 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 (<italic>a</italic><sub>1</sub>) and the breakpoint ([HHb]<sub>BP</sub>) were determined.</p></sec><sec><title>Results</title><p>MFO (<italic>p</italic> = 0.01) and absolute fat oxidation rates between 20 and 65% <inline-formula><mml:math id="INEQ4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover accent="true"><mml:mi>V</mml:mi><mml:mo>.</mml:mo></mml:mover><mml:mi>O</mml:mi><mml:msub><mml:mi /><mml:mrow><mml:mn>2</mml:mn><mml:mo>⁢</mml:mo><mml:mi>p</mml:mi><mml:mo>⁢</mml:mo><mml:mi>e</mml:mi><mml:mo>⁢</mml:mo><mml:mi>a</mml:mi><mml:mo>⁢</mml:mo><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> were higher in cyclists than in active participants (<italic>p</italic> &lt; 0.05), while Fat<sub>max</sub> occurred at a higher absolute exercise intensity (<italic>p</italic> = 0.01). <italic>a</italic><sub>1</sub> was lower in cyclists (<italic>p</italic> = 0.02) and [HHb]<sub>BP</sub> occurred at a higher absolute intensity (<italic>p</italic> &lt; 0.001) than in active individuals. <inline-formula><mml:math id="INEQ5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover accent="true"><mml:mi>V</mml:mi><mml:mo>.</mml:mo></mml:mover><mml:mi>O</mml:mi><mml:msub><mml:mi /><mml:mrow><mml:mn>2</mml:mn><mml:mo>⁢</mml:mo><mml:mi>p</mml:mi><mml:mo>⁢</mml:mo><mml:mi>e</mml:mi><mml:mo>⁢</mml:mo><mml:mi>a</mml:mi><mml:mo>⁢</mml:mo><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> was strongly correlated with MFO, Fat<sub>max</sub>, and [HHb]<sub>BP</sub> (<italic>r</italic> = 0.65–0.88, <italic>p</italic> ≤ 0.001). MFO and Fat<sub>max</sub> were both correlated with [HHb]<sub>BP</sub> (<italic>r</italic> = 0.66, <italic>p</italic> = 0.01 and <italic>r</italic> = 0.68, <italic>p</italic> &lt; 0.001, respectively) and tended to be negatively correlated with <italic>a</italic><sub>1</sub> (<italic>r</italic> = -0.41, <italic>p</italic> = 0.06 for both).</p></sec><sec><title>Conclusion</title><p>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 O<sub>2</sub> delivery and consumption supporting higher fat oxidation rates.</p></sec>