The respiratory quotient (RQ) indicates the ratio between carbon dioxide production and oxygen consumption and reflects whole body substrate oxidation; the ratio of fat to carbohydrate oxidation. This study aims at measuring the 24-hour measurements reliability of the RQ. To assess this, between week and between day respiratory quotient variability were measured during highly controlled weight, activity and energy balance conditions. Energy balance is the regulation of the energy inflow (food intake) and energy outflow (energy expenditure). The same applies to weight and activity balance, in order to create homeostasis.

This study included data of 15 participants. Measurements of free-living energy expenditure, physical activity and body composition were determined, using an accelerometry bracelet. In order to measure the total energy expenditure and the respiratory quotient of the participants over a 24-hour measurements period, a respiration chamber was used. All meals during the stay in the whole room calorimeter were provided and all participants had to follow a certain diet prior to the study. The number of calories the participants consumed were determined by the accelerometry measurements.

The mean 24-hour, rest and sleep respiratory quotients, weight stability and energy balance were assessed. The periods of the RQ that were evaluated, had a high reliability. The 24-hour measurements RQ had the highest between-day and between-week reliabilities. Since no significant difference was found between the parameters of the two test periods, we can ascribe the findings to the protocol and not to the variation between participants. The protocol is thus reproducible.

In summary, the RQ is highly reliable parameter under sleeping, resting and active conditions. The tight diet pre-study and protocol during the study, contributed significantly in differentiating this study from other studies who did not have such high coefficient of variation.

Elvis A. Carnero, Christopher P. Bock, Karen D. Corbin, Rosa Krajmalnik-Brown, Steven R. Smith