The basic metabolic rate varies between individuals. One study of 150 adults representative of the population in Scotland reported basal metabolic rates from as low as  per day to as high as ; with a mean BMR of  per day. Statistically, the researchers calculated that 62% of this variation was explained by differences in fat free mass. Other factors explaining the variation included fat mass (7%), age (2%), and experimental error including within-subject difference (2%). The rest of the variation (27%) was unexplained. This remaining difference was not explained by sex nor by differing tissue size of highly energetic organs such as the brain.
A cross-sectional study of more than 1400 subjects in Europe and the US showed that once adjusted for differences in body composition (lean and fat mass) and age, BMR has fallen over the past 35 years. The decline was also observed in a meta-analysis of more than 150 studies dating back to the early 1920s, translating into a decline in total energy expenditure of about 6%.