The respiratory quotient (RQ) is a useful indicator of whole-body substrate oxidation. However, there is a lack data about its reliability. This data is valuable for study design considerations and interpretation of metabolic research. To address this gap, we assessed between-week and between-day RQ variability during highly controlled weight, activity and energy balance (EB) conditions.
Fifteen men and women were included in this analysis age=2.5±8.0 yrs., BMI=25.7±1.8 kg/m2). Energy expenditure and RQ over 24 hours (24h) were measured in whole room respiratory chambers under a tightly scripted activity protocol. All meals were provided. A run-in diet was consumed prior to calorimetry assessments and energy intake was accurately calculated. Twenty-five periods of single calorimetry assessments followed by 6-day blocks were used to calculate between-week (single day versus 6-day average 2 weeks later) and between-day (daily comparisons within 6-day blocks) reliability. 24h, resting and sleeping RQ were evaluated (24RQ, RQrest and RQsleep). Reliability was calculated using the square root of measurement error variance in absolute (RQ units, RQu) and relative units. Weight stability was reported as relative units and EB as kcal/day.
The means for RQ variables were: 24RQ=0.869±0.021, RQrest=0.875±0.023 and RQsleep=0.847±0.021. Weight stability and EB were maintained between very narrow limits during assessment periods (Weight=0.50±0.27% and EB= -5±63 kcal/day). All RQ periods evaluated had high reliability. 24RQ had the highest between-day (24RQ=0.009 RQu and 1.1%; RQrest=0.021 RQu and 2.4%; RQsleep=0.012 RQu and 1.4%) and between-week (24RQ=0.016 RQu and 1.8%; RQrest=0.023 RQu and 2.6%; RQrest=0.016 RQu and 1.9%) reliabilities.
Our data indicate that RQ can be assessed with high reliability under sleeping, resting and active conditions. These results reveal key design elements for maximizing RQ reliability and can be used to interpret results of metabolic studies.