1Department of Health, Athletic Training, Recreation, and Kinesiology, Longwood University, 201 High St, Farmville, VA 23909 USA
Lipids as a fuel source for energy supply during submaximal exercise originate from subcutaneous adipose tissue derived fatty acids (FA), intramuscular triacylglycerides (IMTG), cholesterol and dietary fat. These sources of fat contribute to fatty acid oxidation (FAox) in various ways. The regulation and utilization of FAs in a maximal capacity occur primarily at exercise intensities between 45 and 65% VO2max, is known as maximal fat oxidation (MFO), and is measured in g/min. Fatty acid oxidation occurs during submaximal exercise intensities, but is also complimentary to carbohydrate oxidation (CHOox). Due to limitations within FA transport across the cell and mitochondrial membranes, FAox is limited at higher exercise intensities. The point at which FAox reaches maximum and begins to decline is referred to as the crossover point. Exercise intensities that exceed the crossover point (~65% VO2max) utilize CHO as the predominant fuel source for energy supply. Training status, exercise intensity, exercise duration, sex differences, and nutrition have all been shown to affect cellular expression responsible for FAox rate. Each stimulus affects the process of FAox differently, resulting in specific adaptions that influence endurance exercise performance. Endurance training, specifically long duration (>2 h) facilitate adaptations that alter both the origin of FAs and FAox rate. Additionally, the influence of sex and nutrition on FAox are discussed. Finally, the role of FAox in the improvement of performance during endurance training is discussed.
Lipids are the substrate largely responsible for energy supply during submaximal exercise [1–3]. However, the definitive role of lipid contribution during cellular respiration has yet to be fully elucidated. Subcutaneous adipose tissue, intramuscular triacylglycerides (IMTG), cholesterol, and dietary fat all contribute to fatty acid oxidation (FAox) . Moreover, the energy contribution from lipid oxidation during submaximal exercise is in addition to carbohydrate oxidation (CHOox) . However, as exercise intensity increases, the contribution of carbohydrate oxidation increases in proportion to the decrease in lipid oxidation . Nonetheless, the oxidation of lipids is the predominant fuel source (%) during submaximal exercise intensities (<65%VO2max) [1, 2, 5]. Increases in exercise intensity that exceed 65% of VO2max produces a shift in energy contribution favoring CHOox. A term used to describe the point when lipid oxidation reaches maximum is maximal fat oxidation (MFO). Exercise intensities that exceed MFO oxidize CHO in greater proportion [2, 4, 5].