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

Abstract

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 increased to 32 +/- 2 kJ min(-1) at 55 % W(max), but declined at 75 % W(max) (19 +/- 2 kJ min(-1)). This decline involved a decrease in the oxidation rate of both plasma FFA and triacylglycerol fat sources (sum of intramuscular plus lipoprotein-derived triacylglycerol), and was accompanied by increases in muscle pyruvate dehydrogenase complex activation and acetylation of the carnitine pool, resulting in a decline in muscle free carnitine concentration. 4. We conclude that the most likely mechanism for the reduction in fat oxidation during high-intensity exercise is a downregulation of carnitine palmitoyltransferase I, either by this marked decline in free carnitine availability or by a decrease in intracellular pH.

Figure 1. Palmitate tracer data

A, breath ¹³CO₂/¹²CO₂ ratio (mean + s.e.m..); B, plasma [U-¹³C]palmitate enrichment (mean + s.e.m..); C, plasma palmitate level (mean + s.e.m..). TTR, tracer/tracee ratio. * Significantly different from the mean resting value, † significantly different compared to the mean value observed during exercise at 40 % of the maximal workload (W_max), ‡ significantly different from the mean value observed during exercise at 55 % W_max (P < 0.05).

Figure 2. Glucose tracer data

A, plasma [6,6-²H₂]glucose enrichment (mean + s.e.m..); B, plasma glucose level (mean + s.e.m..). * Significantly different from the mean resting value, † significantly different from the mean value observed during exercise at 40 % W_max, ‡ significantly different from the mean value observed during exercise at 55 % W_max (P < 0.05).

Figure 3. Blood plasma metabolites

Plasma concentrations of free fatty acids (A), free glycerol (B) and lactate (C; means + s.e.m..) during the rest period (t = 0–60 min), and exercise at 40 %W_max (t = 60–90 min), 55 % W_max (t = 90–120 min) and 75 % W_max (t = 120–150 min). * Mean plasma level significantly different from the mean resting values, † mean plasma levels significantly different from the 40 % W_max values, ‡ mean plasma levels significantly different from the 55 % W_max values (P < 0.05).

Figure 4. Energy expenditure and fuel selection

Values are means. FFA, free fatty acid.