ORGAN-TISSUE MASS MEASUREMENT ALLOWS MODELING OF REE AND METABOLICALLY ACTIVE TISSUE MASS

Investigators have expressed interest in the associations between rest ing energy expenditure (REE) and body mass for over a century. Traditi onally, descriptive models using regression analysis are applied, link ing REE with metabolically active compartments such as body cell mass (BCM) and fat-free body mass (FFM). Recently developed whole body magn etic resonance imaging (MRI) and echocardiography methods now allow es timation of all major organs and tissue volumes in vivo. Because measu red values are available for REE, BCM, and FFM content of individual o rgans and tissues, it should now be possible to develop energy expendi ture-body composition estimation models based on MRI-measured organ-ti ssue volumes. Specifically, the present investigation tested the hypot hesis that in vivo estimation of whole body REE, BCM, and FFM is possi ble using MRI- and echocardiography-derived organ volumes combined wit h previously reported organ-tissue metabolic rates and chemical compos ition. Thirteen subjects (5 females, 8 males) had REE, BCM, and FFM me asured by indirect calorimetry, whole body K-40 counting, and dual-ene rgy X-ray absorptiometry, respectively. Models developed from estimate d and measured variables were highly correlated, with no significant d ifferences between those estimated and measured [e.g., calculated vs, measured REE: r = 0.92, P < 0.001; (mean +/- SD) 6,962 +/- 1,455 and 7 ,045 +/- 1,450 kJ/day, respectively (P = not significant)]. Strong ass ociations were observed between REE, individual or combined organ weig hts, BCM, and FFM that provide new insights into earlier observed meta bolic phenomena. The present approach, the first to establish an energ y expenditure-body composition link with a mechanistic model in vivo, has the potential to greatly expand our knowledge of energy expenditur e-body size relationships in humans.