Evaluation of the formulae for calculating nutrient utilization rates from respiratory gas measurements in fed subjects.

A method is proposed for calculating nutrient utilization rates from measurements of respiratory gas exchange by considering the balance of the elements C, O, H, N and S, without the need to define the pathways of intermediary metabolism. This approach enables the sources of error in the calculation of nutrient utilization to be readily identified, and their effect on precision to be assessed in different nutritional and metabolic states. Equations have been derived for calculating fat and carbohydrate utilization in the fed state. These are different from the conventional equations, which were intended for use in the fasted state. They take account of variations in the compositions of fat, carbohydrate and protein in the nutrients utilized, whether from the body stores during fasting or from the diet during feeding. Examples of the use of these equations show that failure to account adequately for the composition of food or nutritional support regimens can cause avoidable errors in the calculated rates of fat and carbohydrate utilization.