ANN 121: Applied Animal Nutrition-I (2+1)

Measurement of respiratory exchange

INDIRECT CALORIMETRY BY THE MEASUREMENT OF RESPIRATORY EXCHANGE

• The substances which are oxidised in the body, and whose energy is therefore converted into heat, fall mainly into the three nutrient classes of carbohydrates, fat and proteins.
• The overall reaction for the oxidation of a carbohydrate such as glucose is

• And for the oxidation of the typical fat, tripalmitin, is

 

• In an animal obtaining all its energy by the oxidation of glucose, the utilisation of 1 litre of oxygen would lead to production of 673/(6x22.4)=5.007 kcal of heat, for mixtures of carbohydrates an average value is 5.047 kcal per liter. Such values are known as thermal equivalents of oxygen, and are used in indirect calorimetry to estimate heat production from oxygen consumption.
• For an animal catabolising mixtures of fats alone, (of 4.715 kcal per liter calculated from equation (2) above).
• Animals do not normally obtain energy exclusively from either carbohydrate or fat. They oxidise a mixture of these (and of protein also), so that in order to apply the appropriate thermal equivalent when converting oxygen consumption to heat production it is necessary to know how much of the oxygen is used for each nutrient.
• The proportions are calculated from what is known as the respiratory quotient (RQ). This is the ratio between the volume of carbon dioxide produced by the animal and the volume of oxygen used. Since, under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of molecules, the RQ can be calculated from the molecules of carbon dioxide produced and oxygen used.
• From equation (1) the RQ for carbohydrate is calculated as 6 Co2/6 O2 = 1, and from equation (2) that of the fat, tripalmitin, as 51 CO2/72.5 O2=0.70. If the RQ of an animal is known, the proportions of fat and carbohydrate oxidised can then be determined from standard tables.
• For example, an RQ of 0.9 indicates the oxidation of a mixture of 67.5% carbohydrate and 32.5% fat, and the thermal equivalent of oxygen for such a mixture is 4.924 kcal/liter.
• The mixture oxidised generally includes protein.
• The quantity of protein catabolised can be estimated from the output of nitrogen in the urine, 0.16g of urinary N being excreted for each gram of protein.
• The heat of combustion of protein (i.e. the heat produced when it is completely oxidised) varies according to the amino acid proportions but averages 5.3 kcal per g.
• Protein, however, is incompletely oxidised in animals because the body cannot oxidise nitrogen, and the average amount of heat produced by the catabolism of 1 g. of protein is 4.3 kcal.
• For each gram of protein oxidised, 0.77 liters of carbondioxide is produced and 0.96 litres of oxygen used, giving an RQ of 0.8.