Physiological factors that regulate the use of endogenous fat and carbohydrate fuels during endurance exercise

Abstract

Exercise causes a dramatic increase in energy requirements because of the metabolic needs of working muscles. Exercise-dependent factors regulate fuel use. Absolute exercise intensity determines the exercise-induced increase in energy demands, whereas exercise intensity relative to an individual's maximal aerobic capacity (VO 2 max) determines the proportional contribution of different fuel sources (i.e. plasma glucose, plasma fatty acids, muscle glycogen and intramuscular triacylglycerols). Endurance training increases aerobic capacity in muscle and the oxidation of fat during exercise. In addition, exercise-independent factors, such as diet composition, sex, age, and body composition also influence substrate use during exercise. The present review discusses the regulation of substrate use during exercise in human subjects, with a focus on the role of exercise-independent factors. Energy requirement: Fat metabolism: Carbohydrate metabolism: Endurance exerciseAbbreviations: IMTG, intramuscular triacylglycerols; NEFA, non-esterified fatty acids; PCr, phosphocreatinine; Ra, rate of appearance; Rd, rate of disappearance; TG, triacylglycerols; VO 2 max, maximal O 2 consumption.

Sexual Dimorphism in Human Lipid Metabolism

“…Evaluating lipid kinetics in men and women during exercise is particularly difficult because it is affected by body composition, aerobic fitness, and training status, age, menstrual cycle phase (during high-intensity exercise), and possible other less explored factors such as muscle fiber-type composition (24,25). This may explain in part the often controversial results in the literature.…”

“…This may explain in part the often controversial results in the literature. For example, several studies found that the rate of fat oxidation was higher in both untrained and trained women than men, whereas others found that women use more fat and less carbohydrate than men; still others were unable to report differences between the sexes (25). To avoid those confounding influences, we recently examined lipid metabolism during moderate-intensity endurance exercise in young adult men and women who were matched for fatness, aerobic fitness, and age (26).…”

“…Limitations in using intramuscular TG content as a measure of intramuscular TG oxidation during exercise and differences in matching men and women on body composition may be responsible for the discrepancy between studies. The mechanism(s) responsible for the differences between the sexes in lipolysis of adipose tissue TG observed in our subjects during exercise is not entirely understood but is partly related to differences in lipolytic sensitivity to ␤-adrenergic stimulation and ␣-adrenergic receptor activity [see (24,25)]. Most of the increase in lipolytic activity that occurs during exercise is mediated through catecholamine stimulation of adipose tissue ␤-adrenergic receptors; although ␣-adrenergic receptor activity was also shown to inhibit lipolysis during exercise, thereby counteracting the ␤-adrenergic effect in men, it is not involved in the regulation of lipolysis during exercise in women.…”

Lipid fuels for exercise

“…By doing this the whole complex process of transporting fuels in the blood from other tissues is avoided. Skeletal muscle stores a total of about 8 MJ of energy as glycogen and a further 15 MJ energy as the more energy dense triacyglycerol [2]. Being on the spot such fuels are rapidly available.…”

Editorial

“…Continuing with the theme of fat metabolism, Mittendorfer & Klein (2003) review the factors that control the use of fat and carbohydrate reserves during endurance exercise. They note that while the intensity of exercise determines the increased requirement for energy-yielding metabolism, it is the individual's maximum aerobic capacity that, together with the intensity of the exercise, determines the relative amounts of plasma glucose and fatty acids, muscle glycogen and intramuscular triacylglycerol that will be used.…”