Intensity of Physical Activity

Oxidation of lipid provides most of the energy (~ 60 percent) for non-contracting skeletal muscle and overall for the body at rest in people who have not eaten for 10 to 12 hours (i.e., postabsorptive conditions) (Brooks, 1997). Glucose released from the liver into the circulation provides the remainder of the energy for the body overall, particularly the brain, kidneys,

PHYSICAL ACTIVITY

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Exercise Intensity (Vo2max)

Fed Subjects

UT T

Exercise Intensity (Vo2max)

Fasted Subjects

UT T

Fasted Subjects

UT T

UT T

UT T

Exercise Intensity (Vo2max)

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 Soci-

UT T

UT T

UT T

Exercise Intensity (Vo2max)

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 Soci-

920 DIETARY REFERENCE INTAKES

and blood. During mild exercise, the use of lipid increases, but if the level of effort increases, carbohydrate energy sources are used to a relatively greater extent (Figure 12-7). Peak rate of lipid oxidation is achieved at approximately 45 percent of Vo2max. For exercises intensities greater than 50 percent of Vo2max, the oxidation of free fatty acids declines in muscle, both as a percentage of total energy as well as on an absolute basis. In other words, there is crossover from prevalence of lipid oxidation at rest and during mild exercise to predominance of carbohydrate energy sources during moderate and greater efforts. The main carbohydrate energy source is muscle glycogen, and this is supplemented to some extent by glucose and lactate—glucose mobilized from the liver and lactate produced by muscle glycogen breakdown. If exercise persists beyond 60 to 90 minutes, lipid use will rise as carbohydrate fuel sources become depleted. In this case, the intensity of exercise must drop because of the depletion of muscle glycogen, decreasing levels of blood glucose, and other fatiguing consequences of the effort (Graham and Adamo, 1999).

Dietary carbohydrate is relatively rapidly assimilated compared to fat and protein, thus raising blood glucose and insulin levels. The increments in blood glucose and insulin in response to carbohydrate intake are less in trained than in untrained individuals (Dela et al., 1991; King et al., 1987). Still, carbohydrate feeding stimulates carbohydrate oxidation, raising the respiratory exchange ratio (RER = R = Vco2/Vo2) in all individuals. Hence, as shown in Figure 12-7 for fed individuals, crossover to predominant carbohydrate oxidation occurs already during mild (22% Vo2max) exercise, even in trained individuals, if they have recently consumed carbohydrates.

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