An approach to quantifying abnormalities in energy expenditure and lean mass in metabolic disease

Background/objectives: The objective of this study was to develop approaches to expressing resting energy expenditure (REE) and lean body mass (LM) phenotypes of metabolic disorders in terms of Z-scores relative to their predicted healthy values. Subjects/methods: Body composition and REE were measured in 135 healthy participants. Prediction equations for LM and REE were obtained from linear regression and the range of normality by the standard deviation of residuals. Application is demonstrated in patients from three metabolic disorder groups (lipodystrophy, n=7; thyrotoxicosis, n=16; and resistance to thyroid hormone (RTH), n=46) in which altered REE and/or LM were characterised by departure from the predicted healthy values, expressed as a Z-score. Results: REE (kJ/min)=−0.010 × age (years)+0.016 × FM (kg)+0.054 × fat-free mass (kg)+1.736 (R2=0.732, RSD=0.36 kJ/min). LM (kg)=5.30 × bone mineral content (kg)+10.66 × height2 (m)+6.40 (male). LM (kg)=0.20 × fat (kg)+14.08 × height2 (m)−2.93 (female). (male R2=0.55, RSD=3.90 kg; female R2=0.59, RSD=3.85 kg). We found average Z-scores for REE and LM of 1.77 kJ/min and −0.17 kg in the RTH group, 5.82 kJ/min and −1.23 kg in the thyrotoxic group and 2.97 kJ/min and 4.20 kg in the LD group. Conclusion: This approach enables comparison of data from individuals with metabolic disorders with those of healthy individuals, describing their departure from the healthy mean by a Z-score.