<P> The body fat percentage is a measure of fitness level, since it is the only body measurement which directly calculates a person's relative body composition without regard to height or weight . The widely used body mass index (BMI) provides a measure that allows the comparison of the adiposity of individuals of different heights and weights . While BMI largely increases as adiposity increases, due to differences in body composition, other indicators of body fat give more accurate results; for example, individuals with greater muscle mass or larger bones will have higher BMIs . As such, BMI is a useful indicator of overall fitness for a large group of people, but a poor tool for determining the health of an individual . </P> <P> Epidemiologically, the percentage of body fat in an individual varies according to sex and age . Various theoretical approaches exist on the relationships between body fat percentage, health, athletic capacity, etc . Different authorities have consequently developed different recommendations for ideal body fat percentages . </P> <P> This graph from the National Health and Nutrition Examination Survey in the United States charts the average body fat percentages of Americans from samples from 1999--2004: </P> <Table> <Tr> <Td> QuickStats: Mean Percentage Body Fat, by Age Group and Sex--- National Health and Nutrition Examination Survey, United States, 1999--2004 </Td> </Tr> </Table>

What body functions require fat or are affected by fat