title: Daily physical activity as determined by age, body mass and energy balance.
abstract: OBJECTIVE
  Insight into the determinants of physical activity, including age, body mass and energy balance, facilitates the design of intervention studies with body mass and energy balance as determinants of health and optimal performance.
  
  
  METHODS
  An analysis of physical activity energy expenditure in relation to age and body mass and in relation to energy balance, where activity energy expenditure is derived from daily energy expenditure as measured with doubly labelled water and body movement is measured with accelerometers, was conducted in healthy subjects under daily living conditions over intervals of one or more weeks.
  
  
  RESULTS
  Activity energy expenditure as a fraction of daily energy expenditure is highest in adults at the reproductive age. Then, activity energy expenditure is a function of fat-free mass. Excess body mass as fat does not affect daily activity energy expenditure, but body movement decreases with increasing fatness. Overweight and obesity possibly affect daily physical activity energy expenditure through endurance. Physical activity is affected by energy availability; a negative energy balance induces a reduction of activity expenditure.
  
  
  CONCLUSIONS
  Optimal performance and health require prevention of excess body fat and maintenance of energy balance, where energy balance determines physical activity rather than physical activity affecting energy balance.
authors.affiliation: Department of Human Biology, Maastricht University Medical Centre, PO Box 616, 6200 MD, Maastricht, The Netherlands, k.westerterp@maastrichtuniversity.nl.
authors.forename: Klaas R
authors.initials: KR
authors.lastname: Westerterp
mesh_terms: D000273:Adipose Tissue / Q000254:growth & development / Q000378:metabolism; D000367:Age Factors; D001835:Body Weight; D004734:Energy Metabolism; D006801:Humans; D009043:Motor Activity / Q000502:physiology*