[Frontiers in Bioscience 3, d1011-1027, September 15, 1998]

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John O. Holloszy, Wendy M. Kohrt and Polly A. Hansen

Department of Medicine, Washington University School of Medicine, St. Louis, MO, 63110, USA

Received 9/2/98 Accepted 9/8/98


Exercise intensity is expressed both in absolute and relative terms. Relative exercise intensity is generally expressed as a percentage of an individual’s maximal oxygen uptake capacity (VO2max). VO2max varies over a wide range among individuals, depending on level of aerobic training, genetic makeup, age, health status and sex. As a consequence, the same relative exercise intensity is attained at very different absolute work rates in individuals who differ markedly in their VO2max. For example, a champion cross-country skier with a VO2max of 90 ml×kg-1×min-1 has to generate energy at 6-times as great a rate as an untrained 85 year old man with a VO2max of 15 ml×kg-1×min-1, when both are working at 75% of VO2max.. Assuming that the fuel mixture used by these individuals is the same, the cross-country skier’s muscle mitochondria would be oxidizing 6-times as much pyruvate and fatty acids as those of the 85 yr old’s during the exercise, and the rates of glycogenolysis and lipolysis would, therefore, have to be similarly higher in the athlete than in the 85 yr old. As illustrated by this extreme example, the absolute work rate determines the rate at which fuel is utilized by the muscles during exercise, and therefore, plays an extremely important role in the regulation of substrate metabolism during exercise.

While the absolute work rate determines the total quantity of fuel required by the muscles during exercise, the relative exercise intensity is a major factor in determining the fuel mixture, i.e. the proportions of carbohydrate and fat, oxidized by the working muscles (27,68). During exercise performed after an overnight fast, 70-90% of the energy required at low exercise intensities in the range of ~25-30% of VO2max is supplied by the oxidation of fat. As shown in figure 1, as relative exercise intensity is increased from ~40% to ~85% of VO2max, there is a decrease in the percentage of the total energy requirement derived from fat oxidation and a reciprocal increase in carbohydrate oxidation. In addition to the decline in the relative contribution of fat oxidation with increasing exercise intensity, there is a decrease in the absolute amount of fat that is oxidized at higher relative work rates (27,37). Both plasma glucose and muscle glycogen utilization increase as exercise intensity is raised, with plasma glucose providing ~10 to 15% of total energy at all work rates and muscle glycogen providing the bulk (60% or more) of the energy required for very strenuous exercise requiring more than ~80% of VO2max (27,68). At low exercise intensities (20-30% of VO2max) plasma fatty acids provide nearly all of the fat that is oxidized, while at moderate and heavy intensities (50-85% of VO2max) plasma fatty acids and muscle triglycerides provide roughly equal amounts of the fat that is oxidized (27,34,38).

Figure 1. Carbohydrate (left axis) and fat (right axis) oxidation during submaximal exercise in untrained (closed symbols) and endurance-trained (open symbols) individuals. Taken from references (27,28,33,34,85-87).

The regulatory mechanisms responsible for the progressive rise with increasing exercise intensity in the proportion of total energy provided by carbohydrate oxidation are still not fully understood. Factors that play important roles include a decrease in plasma fatty acid availability due to a reduction in the amount of fatty acids released from adipose tissue (27,69), increased activation of glycogenolysis (27), and an increased recruitment of fast twitch, i.e. Type II, muscle fibers. The decrease in fatty acid release is thought to be due, at least in part, to constriction of the vascular bed in adipose tissue as a result of increased b-adrenergic stimulation (69). Direct evidence that limited availability of plasma fatty acids plays a role in the decrease in fat oxidation during high intensity exercise is provided by the finding that fat oxidation increases and muscle glycogen utilization decreases when plasma fatty acids are raised by means of infusion of triglyceride emulsion and heparin even during very intense exercise requiring 85% of VO2max (50,52). That glycogenolysis-glycolysis has an inhibitory effect on fat oxidation has been demonstrated in a number of recent studies as discussed earlier; this effect appears to be mediated by an increase in malonyl CoA, which inhibits the enzyme responsible for transporting long chain fatty acids into the mitochondrial matrix, palmityl carnitine transferase I (59). With regard to the role of muscle fiber type, during low intensity exercise the work is performed by slow-twitch, Type I fibers which have a high capacity for fat oxidation and a low capacity for glycogenolysis-glycolysis; as the exercise intensity is increased, progressively more fast-twitch, Type II fibers, which have a high capacity for, and obtain much of their energy from, glycogenolysis-glycolysis are recruited to contract (70).