Physiology of Finfish and Shellfish (2+1)

4.4.2. Metabolic Rate

Metabolic rate refers to the energy metabolism per unit time. The measure that can be used to determine the metabolic rate is the amount of oxygen used in oxidation processes, provided information is available about which substances have been oxidized (and there is no anaerobic metabolism).

The determination of oxygen consumption is technically easy and is so commonly used for estimation of metabolic rates that the two terms are often used interchangeably.

The reason, oxygen can be used as a practical measure of metabolic rate is that the amount of heat produced for each liter of oxygen used in metabolism remains nearly constant, irrespective of whether fat, carbohydrate, or protein is oxidized (Table 6.1, column c). The highest figure (5.0 kcal per liter O₂ for carbohydrate metabolism) and the lowest (4.5 kcal per liter O₂ for protein) differ by only 10%, and it has become customary to use an average value of 4.8 kcal per liter O₂ as a measure of the metabolic rate. The largest error resulting from the use of this mean figure would be 6%, but if the metabolic fuel is a mixture of the common food.

Heat produced and oxygen consumed in the metabolism of common food stuffs. Cd the values for protein depend on whether the metabolic end product is urea or uric acid. The ratio between carbon dioxide formed and oxygenused is known as the respiratory quotient (RQ). (Data based on Lusk 1931; King 1957).

Food	a	b	c	d
	Kcal g-1	Liter O2 g-1	Kcal per liter O2	Co2 formed O2 used=RQ
Carbohydrate	4.2	0.84	5.0	1.00
Fat	9.4	2.0	4.7	0.71
Protein (urea)	4.3	0.96	4.5	0.81
Protein (uric acid)	4.25	0.97	4.4	0.74

Stuffs (carbohydrate, fat, and protein), the error is usually insignificant.

The energy value given for protein, 4.3 kcal g^-1, differs from the combustion value determined for proteins in the chemical laboratory, which is usually given as 5.3 kcal g^-1. The reason is that protein is not completely oxidized in the body. Mammals excrete the nitrogen from amino acids as urea, CO (NH₂)₂, an organic compound, whose combustion value accounts for the difference. This difference must be considered, if the energy balance of an animal is calculated from combustion values determined by employing a bomb calorimeter.

The amount of oxygen needed to oxidize 1 g fat is more than twice the amount needed for oxidation of carbohydrate or protein (Table 6.1, column b). In combination with the high energy yield, oxygen is approximately the same for fat as for carbohydrate and protein.

The last column in Table 6.1 shows the ratio between the carbon dioxide formed in metabolism and the oxygen used. This ratio, known as the respiratory quotient (RQ) or respiratory exchange ratio (R), is an important concept in metabolic physiology.

The RQ gives information about the fuel used in metabolism. Usually the RQ is between 0.7 and 1.0. An RQ near 0.7 suggests primarily fat metabolism; an RQ near 1.0 suggests primarily carbohydrate metabolism. To determine the amount of protein metabolized from the nitrogen excretion (urea in mammals, uric acid in birds and reptiles), and from this knowledge, we can calculate tha,t fraction of the oxygen consumption and carbon dioxide production, that is attributable to protein metabolism. The remainder of the gas exchange is attributable to fat and carbohydrate metabolism and the amount of each can then be calculated. From determinations of oxygen consumption, carbon dioxide elimination, and nitrogen excretion, it is thus possible to calculate the separate amounts of protein, fat and carbohydrate metabolized in a given period of time.