title: Variability of measured resting metabolic rate.
abstract: BACKGROUND
  The necessity of a 12-h fast before resting metabolic rate (RMR) is measured is often a barrier to measuring RMR.
  
  
  OBJECTIVE
  We compared RMR measurements obtained in the morning and afternoon and across repeated days to elucidate the magnitude and sources of variability.
  
  
  METHODS
  Healthy men (n = 12) and women (n = 25) aged 21-67 y, with body mass indexes (in kg/m(2)) ranging from 17 to 34 and body fat ranging from 6% to 54%, completed 4 RMR measurements. RMR measurements were made in the morning (after a 12-h fast and 12 h postexercise) and in the afternoon (after a 4-h fast and 12 h postexercise) on 2 separate days with the ventilated-hood technique. Body composition was assessed by dual-energy X-ray absorptiometry.
  
  
  RESULTS
  Mean (+/- SE) afternoon RMR was significantly higher than morning RMR on both visit 1 (1593.5 +/- 35.6 compared with 1508.0 +/- 31.5 kcal/d; P = 0.001) and visit 2 (1602 +/- 29.3 compared with 1511.4 +/- 35.9 kcal/d; P = 0.001). The 2 morning measurements (r = 0.93) and the 2 afternoon measurements (r = 0.93) were highly correlated, and no significant differences between measurements were observed. The mean difference between the morning and afternoon measurements was 99.0 +/- 35.8 kcal/d (6%).
  
  
  CONCLUSIONS
  Repeated morning and evening measurements of RMR were stable and highly correlated. Day-to-day measurements of RMR were not significantly different. RMR measured in the afternoon after a 4-h fast and exercise was approximately 100 kcal/d higher than RMR measured in the morning.
authors.affiliation: Department of Clinical Sciences, University of Colorado Health Sciences Center, Denver, CO, USA. heather.haugen@uchsc.edu
authors.forename: Heather A
authors.initials: HA
authors.lastname: Haugen
authors.forename: Edward L
authors.initials: EL
authors.lastname: Melanson
authors.forename: Zung Vu
authors.initials: ZV
authors.lastname: Tran
authors.forename: Jay T
authors.initials: JT
authors.lastname: Kearney
authors.forename: James O
authors.initials: JO
authors.lastname: Hill
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D001481:Basal Metabolism / Q000502:physiology*; D001823:Body Composition / Q000502:physiology*; D015992:Body Mass Index; D002153:Calorimetry, Indirect; D002940:Circadian Rhythm / Q000502:physiology; D002149:Energy Intake / Q000502:physiology*; D005215:Fasting; D005260:Female; D006801:Humans; D008297:Male; D011939:Mental Recall; D008875:Middle Aged; D010101:Oxygen Consumption; D012044:Regression Analysis; D015203:Reproducibility of Results; D013997:Time Factors