FIGURE 12-7 Illustration of the effects of relative exercise intensity, recent carbohydrate feeding, and training status on the relative use of carbohydrate (CHO) and lipid (black) energy sources as determined by indirect calorimetry. Untrained men (UT) and trained (T) male cyclists were studied after being recently fed (3–4 h after a 550-kcal meal [87% CHO, 11% protein, 2% fat]) or after an overnight (12-h) fast, during continuous cycling at graded relative exercise intensities over periods of 120 min (22% and 40% Vo₂max), 90 min (59% Vo₂max), and 45 min (75% Vo₂max). Exercise intensity expressed as a percentage of maximal oxygen consumption (Vo₂max), which averaged 39 and 58 mL of oxygen/min/kg body weight among the UT and T cyclists. p < 0.05 for #. Reprinted, with permission, from Bergman and Brooks (1999). Copyright 1999 by the American Physiological Society.