When dietary calories consumed exceed energy expediture through basal metabolic rate plus physical activity, then the excess is stored as body fat. An overall estimate of adiposity can be made with body mass index (BMI) calculated as body weight in kilograms divided by height in meters squared. The ranges given for BMI in adults:
Underweight = <18.5 kg/m2
Normal weight = 18.5 - 24.9 kg/m2
Overweight = 25 to 29.9 kg/m2
Obese = >30 kg/m2
The degree of adiposity can be estimated in other ways. Measurement with estimation of total body fat is possible with several methodologies.
The imaging technique of dual energy x-ray absorptiometry (DEXA) was originally developed for measurement of bone mineral density. The energy absorbed by different tissues can be measured as differences in attenuation (brightness of the image obtained). However, DEXA can be used to analyze three tissue components; bone, fat, and lean tissue. DEXA tends to overestimate body fat in persons with high fat percentage and underestimate body fat in persons with low fat percentage.
Bioelectrical impedance analysis (BIA) can provide an estimate of body fat based upon the relationship between the volume of the human body, as a conductor of a weak electric current, the subject's height, and the impedance (resistance to current travel through tissues). Fat conducts the current less than lean body tissues that have a higher concentration of fluid with electrolytes. A higher percentage of body fat translates to higher impedance. Estimates of body fat may be modified by age, gender, hydration, temperature, and levels of physical activity.
Skin fold thickness measurement has been employed as a simple method for estimation of body fat. Estimates may take into account age, gender, nutritional status, genetic background, and levels of physical activity.
Hydrodensitometry involves obtaining measurement of body composition by having the subject immersed in water and measuring the subject's weight in air and weight while immersed. Calculations are based upon assumptions regarding fat free mass (tissues such as muscle and bone) and fat mass, which have different densities. The average density of fat is 0.92 grams per cubic centimeter and of lean tissues is 1.1 grams per cubic centimeter. Equations have been developed for conversion of body density to body fat percentage.