Programme Contents

The scaling of human basal metabolic rate in adult males

         Body size is the primary determinant of basal metabolic rate, with that relationship being an alinear function across mammalian species (mass^0.67: White and Seymour, 2003) and revealing disproportionate increases in metabolic rate with increments in body size. Therefore, inter-species (mass-specific) comparisons made using arithmetic normalisation are erroneous. To date, however, it remains to be established whether human-specific, basal metabolic rate also follows this same alinear trend. Accordingly, this investigation was designed to test the hypothesis that basal metabolism in adult males will scale as a power function of body mass, and with an exponent similar to that observed for other mammalian species.

  Oxygen consumption was measured in 30 males (22 y [±2.5], 78.14 kg [±11.98, range 59.19-108.5]) using open-circuit, indirect respirometry. Basal metabolic rate was then estimated using Weir’s non-protein respiratory quotient formula. Absolute metabolic rate (kJ.day^-1) and oxygen consumption (mL.min^-1) were used to assess the scaling relationship between basal metabolism and body size. Deep-body temperature was measured using an auditory-canal thermistor and used to standardise metabolic rate to a common temperature (36.2°C). Data were collected at 07: 00 h after a 12-h fast, and over a 60-min duration in a low-stimulus and normothermic environment (∼23°C; ∼50% relative humidity). To reduce inter-individual variability, participants were recruited who had a height-adjusted (170.18 cm) sum of skinfolds (seven sites) <80 mm, and metabolic data were standardised to a common deep-body temperature.

A group comparison, taking the ten largest (89.9 kg [±11.3]) and ten smallest (64.5 kg [±4.1]) individuals, revealed a significant difference between mass-specific (linearly normalized) basal metabolic rates (kJ.kg^-1: P<0.05). This verified that, even across this relatively small adult mass range, mass-specific basal metabolism rate is size dependent and alinear. It was therefore not surprising to find that the basal metabolic rate could be scaled as a power-function of body mass (mass^0.64 : r² = 0.73, P<0.05). Moreover, that power function was not dissimilar to that reported for a wide range of mammals, with the 95% confidence interval for the exponent ranging between mass^0.53 and mass^0.82. Normalising these basal data to a common deep-body temperature, however, did not significantly alter the strength of the association between body mass and basal metabolic rate (r² = 0.73, P<0.05).

To the best of our knowledge, these observations represent the first verification that human basal metabolic rate also scales as a power function of body mass in healthy, adult males. This may be partially explained on the basis of inter-individual variations in the sizes of the most metabolically active tissues. Regardless of the causal mechanism, this non-linear relationship has confirmed that the frequently applied arithmetic mass normalisation of human basal metabolism (kJ.kg^-1) lacks validity and can introduce spurious interpretations. As a continuation of this theme, we are extending this work to include steady-state, non-basal conditions involving postural changes, ambulation and load carriage.

White CR & Seymour RS. (2003). Proc Natl Acad Sci USA, 100: 4046-9.