title: Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine.
abstract: BACKGROUND
  Basal metabolic rate (BMR) is the largest component of daily energy demand in Western societies. Previous studies indicated that BMR is highly variable, but the cause of this variation is disputed. All studies agree that variation in fat-free mass (FFM) plays a major role, but effects of fat mass (FM), age, sex, and the hormones leptin, triiodothyrionine (T3), and thyroxine (T4) remain uncertain.
  
  
  OBJECTIVE
  We partitioned the variance in BMR into within- and between-subject effects and explored the roles of FFM, FM, bone mineral content, sex, age, and circulating concentrations of plasma leptin, T3, and T4.
  
  
  METHODS
  This was a cross-sectional study of 150 white adults from northeast Scotland, United Kingdom.
  
  
  RESULTS
  Only 2% of the observed variability in BMR was attributable to within-subject effects, of which 0.5% was analytic error. Of the remaining variance, which reflected between-subject effects, 63% was explained by FFM, 6% by FM, and 2% by age. The effects of sex and bone mineral content were not significant (P > 0.05). Twenty-six percent of the variance remained unexplained. This variation was not associated with concentrations of circulating leptin or T3. T4 was not significant in women but explained 25% of the residual variance in men.
  
  
  CONCLUSIONS
  Our data confirm that both FFM and FM are significant contributors to BMR. When the effect of FM on BMR is removed, any association with leptin concentrations disappears, which suggests that previous links between circulating leptin concentrations and BMR occurred only because of inadequate control for the effects of FM.
authors.affiliation: Aberdeen Centre for Energy Regulation and Obesity, Division of Energy Balance and Obesity, Rowett Research Institute, Aberdeen, Scotland, United Kingdom. a.johnstone@rowett.ac.uk
authors.forename: Alexandra M
authors.initials: AM
authors.lastname: Johnstone
authors.forename: Sandra D
authors.initials: SD
authors.lastname: Murison
authors.forename: Jackie S
authors.initials: JS
authors.lastname: Duncan
authors.forename: Kellie A
authors.initials: KA
authors.lastname: Rance
authors.forename: John R
authors.initials: JR
authors.lastname: Speakman
mesh_terms: D015502:Absorptiometry, Photon; D000273:Adipose Tissue / Q000378:metabolism*; D000328:Adult; D000367:Age Factors; D001481:Basal Metabolism / Q000502:physiology*; D001823:Body Composition / Q000502:physiology*; D001824:Body Constitution / Q000502:physiology; D002153:Calorimetry, Indirect; D003430:Cross-Sectional Studies; D006801:Humans; D020738:Leptin / Q000097:blood; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism*; D010101:Oxygen Consumption; D012606:Scotland; D012737:Sex Factors; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood
chemical_list: D020738:Leptin; D014284:Triiodothyronine; D013974:Thyroxine