The effects of increasing exercise intensity on muscle fuel utilisation in humans

Abstract

Fat and carbohydrate are the principal substrates that fuel aerobic ATP synthesis in human skeletal muscle. The relative utilisation of fat and carbohydrate during exercise can vary enormously and depends strongly on exercise intensity. An early stable isotope tracer study (Romijn et al. 1993), which used assumptions to estimate indirectly the oxidation rate of plasma free fatty acids (FFAs), suggested that plasma FFAs provide the majority of the substrate oxidised by skeletal muscle during low-and moderate-intensity (25 and 65 % of maximal oxygen uptake capacity, V O 2 ,max) exercise. In addition, it was suggested that whole-body fat and plasma FFA oxidation rates declined during high-intensity exercise (85 % V O 2 ,max), as muscle glycogen became the main fuel source utilised (Romijn et al. 1993). The primary aim of the present study was, therefore, to quantify accurately the oxidation rates of plasma FFA, triacylglycerol (TG; sum of intramuscular and lipoprotein-derived TG), plasma glucose and muscle glycogen during steady-state exercise at three intensities (40, 55 and 75 % of maximal workload, W max) using contemporary stable isotope tracer methodology (i.e. using direct measurements of plasma FFA oxidation and validated assumptions for plasma glucose oxidation). Understanding the regulation of fuel selection in human skeletal muscle is important, especially as fuel use abnormalities are present in metabolic diseases like type 2 diabetes (Kelley & Simoneau, 1994; Martin et al. 1995) and obesity (Colberg et al. 1995). However, the mechanisms that regulate the relative contribution of carbohydrate and fat during exercise have not been fully elucidated, and remain open to discussion. Randle et al. (1963) proposed the 'glucose-FFA cycle' in an effort to explain the reduction in muscle carbohydrate oxidation rate in the presence of high plasma FFA levels in resting muscle. It was proposed that an increased availability of plasma FFAs could stimulate fat oxidation and decrease carbohydrate oxidation by suppressing pyruvate dehydrogenase complex (PDC) activation (via a rise in the mitochondrial acetyl-CoA/CoA ratio) and by decreasing glycolytic flux (via the inhibitory effect of high citrate concentrations on phosphofructokinase activity). According to this concept, 1. Contemporary stable isotope methodology was applied in combination with muscle biopsy sampling to accurately quantify substrate utilisation and study the regulation of muscle fuel selection during exercise. 2. Eight cyclists were studied at rest and during three consecutive 30 min stages of exercise at intensities of 40, 55 and 75 % maximal workload (W max). A continuous infusion of [U-13 C]palmitate and [6,6-2 H 2 ]glucose was administered to determine plasma free fatty acid (FFA) oxidation and estimate plasma glucose oxidation, respectively. Biopsy samples were collected before and after each exercise stage. 3. Muscle glycogen and plasma glucose oxidation rates increased with every increment in exercise intensity. Whole-body fat oxidation increase...

Physiological and molecular responses to an acute bout of reduced-exertion high-intensity interval training (REHIT)

“…The absolute level of glycogen utilisation with HIT/REHIT is similar to that observed following a 30-min bout of moderate (50% VO2max) aerobic exercise (Van Loon et al 2001). However, the speed at which the glycogenolysis occurs varies greatly between these two exercise modes.…”

Optimal exercise intensities for fat metabolism in handbike cycling and cycling

“…5 In contrast to these results, Astorino 6 showed the highest oxidation rate of fat at 75% VO 2 peak in women on a treadmill. VO 2 max of 65 and 75% seem to be too high to burn fat because Van Loon et al 7 found in a further study the highest fat oxidation rate in cycling in well-trained men to be at lower intensities. In their study, the highest rate of fat oxidation was at 55% W max , corresponding to 5771% VO 2 max.…”

No effect of menstrual cycle phase on glycerol or palmitate kinetics during 90 min of moderate exercise

“…To do this, an average molecular mass, typical of the plasma NEFA pool (284 g), was used. Assuming 60 -100% of NEFA R d was oxidized during exercise, on the basis of previous studies (13,25,45,49,54), the contribution of plasma NEFA to whole body fat oxidation was very similar between cycle phases (minimal estimate: 34 Ϯ 4% EF, 36 Ϯ 3% MF, and 35 Ϯ 2% ML; maximal estimate ϭ 57 Ϯ 7% EF, 60 Ϯ 5% MF, and 58 Ϯ 4% ML). Consequently, the potential contribution of other lipid sources (plasma TG and muscle TG) to total fat oxidation was similar between cycle phases, and this ranged from a minimum of ϳ42% to a maximum of ϳ65%.…”

“…These former variables can be assessed using tracer measures of systemic nonesterified free fatty acid (NEFA) turnover and systemic glycerol turnover. Using these techniques, it has been shown that, during moderate exercise, both circulating NEFA and triglyceride (TG)-derived free fatty acids (FFA) are utilized as lipid fuel sources (1,40,54). Although it has been assumed that the predominant source of the TG-derived FFA is intramyocelluar TG (1,45), a contribution from circulating (VLDL) TG cannot be ruled out (40).…”