Physical Activity Assessment with the ActiGraph GT3X and Doubly Labeled Water

Abstract

Purpose: To compare the degree to which four accelerometer metrics—total activity counts per day (TAC/d), steps per day (steps/d), physical activity energy expenditure (PAEE, kcal/kg/day), and moderate- to vigorous-intensity physical activity (MVPA, min/d)— were correlated with PAEE measured by doubly-labeled water (DLW). Additionally, accelerometer metrics based on vertical axis counts and triaxial counts were compared. Methods: This analysis included 684 women and 611 men aged 43 – 83 years. Participants wore the Actigraph GT3X on the hip for seven days twice during the study and the average of the two measurements was used. Each participant also completed one DLW measurement, with a subset having a repeat. PAEE was estimated by subtracting resting metabolic rate and the thermic effect of food from total daily energy expenditure estimated by DLW. Partial Spearman correlations were used to estimate associations between PAEE and each accelerometer metric. Results: Correlations between the accelerometer metrics and DLW-determined PAEE were higher for triaxial counts than vertical axis counts. After adjusting for weight, age, accelerometer wear time, and fat free mass, the correlation between TAC/d based on triaxial counts and DLW-determined PAEE was 0.44 in women and 0.41 in men. Correlations for steps/d and accelerometer-estimated PAEE with DLW-determined PAEE were similar. After adjustment for within-person variation in DLW-determined PAEE, the correlations for TAC/d increased to 0.61 and 0.49, respectively. Correlations between MVPA and DLW-determined PAEE were lower, particularly for modified bouts of ≥10 minutes. Conclusion: Accelerometer measures that represent total activity volume, including TAC/d, steps/d, and PAEE, were more highly correlated with DLW-determined PAEE than MVPA using traditional thresholds and should be considered by researchers seeking to reduce accelerometer data to a single metric.