Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements held in Rome, 5-17 October 1981, (WHO, 1985) suggested that energy requirements be based on estimates of energy expenditures rather than energy intakes and that the total energy expenditure of an individual be expressed as multiples of the measured or predicted basal metabolic rate (BMR). This ratio of the total energy expended to the BMR has been termed the physical activity level (PAL). Since BMR is based on body weight, the actual value obtained from this PAL ratio will reflect both the body weight and the level of physical activity of the individual. The minimum level of energy expenditure compatible with health, termed maintenance requirement, may be taken as corresponding to a PAL of 1.4 for the sake of simplicity. Thus, this value of 1.4 x BMR may be used as a cut-off point for assessing the prevalence of CED. Furthermore, studies have confirmed that both total energy expenditure and BMR are highly reproducible and stable (Shetty & Soares, 1988).
http://www.fao.org/docrep/T1970E/t1970e02.htm#P79_19554