## Macronutrient Balance Energy Balance and Storage

Since macronutrients (carbohydrate, fat, protein, and alcohol) are the sources of energy, it is logical to consider energy balance and macronutrient balance together as the opposite side of the same coin.

There is a direct relationship between energy balance and macronutrient balance, and the sum of individual substrate balance (expressed as energy) must be equivalent to the overall energy balance. Thus:

carbohydrate balance exogenous carbohydrate — carbohydrate oxidation protein balance exogenous protein — protein oxidation lipid balance = exogenous lipid — lipid oxidation

It follows that A substrate balance = AE balance. Fat balance is closely related to energy balance (Figure 6).

Indirect calorimetry also allows computation of the nutrient oxidation rates in the whole body. An index of protein oxidation is obtained from the total

150 n

la al

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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 (2000 kcal day-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).

Input

Intake

Input

Intake

Figure 7 Macronutrient (substrate) stores versus macronutrient intake.

## Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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