Objectives - The energy requirements of infants are determined by body size, growth rate, and physical activity. Little is known of the determinants of energy expended on activity. The relation between free-living energy expenditure and behaviour was investigated in infants aged 9 and 12 months.
Methods - Total energy expenditure (TEE) was estimated by the doubly labelled water method and fat free mass was estimated from the 18O dilution space. Behaviour was assessed by two 24 hour activity diaries. Subjects-Thirty four normal healthy infants.
Results - TEE was negatively related to the time spent feeding and negatively related to the time spent upset. Body size, represented by fat free mass, accounted for only 19% of the variation in TEE, whereas the combination of fat free mass and two behavioral variables explained 46% of the variation in TEE.
Conclusions- Behaviour contributed significantly to TEE. The energy requirements of individual subjects in this age group cannot be predicted with accuracy from body size alone.