Accuracy and reliability of a portable indirect calorimeter compared to whole-body indirect calorimetry for measuring resting energy expenditure

Background/objectives: Resting energy expenditure (REE) is the largest component of total energy expenditure. As such, inaccurate estimation or measurement of REE may impact the development of individualized energy intake recommendations. The objective of this observational study was to assess the accuracy and reliability of REE measured with a portable indirect calorimeter with ventilated hood - Fitmate GS - compared to REE from whole-body indirect calorimetry (WBC).

Methods: REE was assessed using the Fitmate GS and compared to REE from WBC in male and female adults. Fitmate GS tests were repeated on a separate visit. Body fat percent, fat mass, and fat-free mass were assessed using dual X-ray absorptiometry in a subset of individuals with body mass index (BMI) ≥35 kg/m². Paired t-tests and Bland-Altman analyses were used to describe REE accuracy; intra-class correlation coefficient (ICC) examined test-retest reliability of repeated Fitmate GS assessments.

Results: Fitmate GS and WBC REE was assessed in 77 adults (age 32 ± 8 years; 63.6% females; 53.2% with BMI ≥ 30 kg/m²). The Fitmate GS underestimated REE (1680 ± 420 vs. 1916 ± 461 kcal/day, p < 0.001) and wide limits of agreement were observed (95% confidence intervals [CI]: -727, 246 kcal/day). Bias was similar across sex and BMI categories and did not correlate to any body composition parameter. Fitmate GS ICC was 0.80 (95% CI: 0.70-0.87), indicating 'good' reliability with repeated testing after median follow-up of 4 days (interquartile range 2-7 days).

Conclusion: The Fitmate GS did not accurately quantify REE but produced reliable REE measurements.