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-5 -4 -3 -2 -1 0 1 2 3 4 5 Energy balance (MJ day-1)

Figure 6 Relationship between energy balance and fat balance in lean and obese individuals. Note that at zero energy balance fat balance is zero. At an excess or deficit of 4.2MJday—1 (1000 kcal) the fat imbalance (about 100gday—1 = 900kcalday—1) accounts for more than 90% of the magnitude of energy balance. (Reproduced from Schrauwen P, Lichtenbelt WD, Saris WH, Westerterp KR (1998) Fat balance in obese subjects: role of glycogen stores. AM J Physiol. 274: E1027-33.)

amount of nitrogen excreted in the urine during the test period. One approach to calculate the nutrient oxidation rate is based on the oxygen consumption and CO2 production due to the oxidation rates of the three nutrients carbohydrate, fat, and protein Figure 6 respectively. In a subject oxidizing c grams permin of carbohydrate (as glucose), f grams per min of fat, and excreting n grams per min of urinary nitrogen, the following equations, can be used:

We can solve equations 4 and 5 for the unknown c and f this way:

Because 1 g urinary nitrogen arises from approximately 6.25 g protein, the protein oxidation rate (p in grams per min) is given by the equation p = 6.25n

Energy stores (constituted mainly of fat stores) are big as a proportion of the food intake (2000kcalday—1, mixed diet in a 60-kg nonobese woman with 25% body fat). The total energy stored is about 90 times total daily energy intake: typically fat stores are 175 times daily fat intake, protein 133 times daily protein intake, and carbohydrate only 1.3 times daily carbohydrate intake (Figure 7).

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Intake

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Figure 7 Macronutrient (substrate) stores versus macronutrient intake.
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