Validating measures of free-living physical activity in overweight and obese subjects using an accelerometer

Abstract

Background: Free-living physical activity can be assessed with an accelerometer to estimate energy expenditure but its validity in overweight and obese subjects remains unknown. Objective: Here, we validated published prediction equations derived in a lean population with the Tracmor D accelerometer (DirectLife, Philips Consumer Lifestyle) in a population of overweight and obese. We also explored possible improvements of new equations specifically developed in overweight and obese subjects. Design: Subjects were 11 men and 25 women (age: 41±7 years; body mass index: 31.0±2.5 kg m-2). Physical activity was monitored under free-living conditions with Tracmor D, whereas total energy expenditure was measured simultaneously with doubly-labeled water. Physical activity level (PAL) and activity energy expenditure (AEE) were calculated from total energy expenditure and sleeping metabolic rate. Results: The published prediction equation explained 47% of the variance of the measured PAL (P<0.001). PAL estimates were unbiased (errors (bias±95% confidence interval): -0.02±0.28). Measured and predicted AEE/body weight were highly correlated (r2 = 58%, P<0.001); however, the prediction model showed a significant bias of 8 kJ kg-1 per day or 17.4% of the average AEE/body weight. The new prediction equation of AEE/body weight developed in the obese group showed no bias. Conclusions: In conclusion, equations derived with the Tracmor D allow valid assessment of PAL and AEE/body weight in overweight and obese subjects. There is evidence that estimates of AEE/body weight could be affected by gender. Equations specifically developed in overweight and obese can improve the accuracy of predictions of AEE/body weight. © 2014 Macmillan Publishers Limited.