Anthropometric measures of body composition are based on the measurement of two compartments of the body, fat and fat-free mass. Fat-free mass consists of the water, protein, and minerals of the skeletal muscle, non-skeletal muscle, soft lean tissues, and the skeleton.
Fat is a variable component in the body. In women, fat comprises a higher percentage of total body weight (averaging 27%) than in men (averaging 15%).
Body fat is commonly assessed by skinfold thickness. This is an estimated measure of subcutaneous fat. There are various sites on the body where skinfold measurements are taken, and one or more are used to estimate body fat. Triceps skinfold is measured at the midpoint of the back of the left upper arm. Biceps skinfold measures the thickness of the vertical fold in the front upper left arm. Subscapular skinfold is taken below and lateral to the angle of the left shoulder blade. Suprailiac is measured at the midaxillary line superior to the iliac crest. Skinfold measures are taken with an instrument called a caliper that squeezes the skinfold with a pressure of 10 g/mm2 and an area of 20 to 40 g/mm2. There is no real consensus on what combination of skinfold measures best estimates body fat, though experts often recommend measuring skinfolds from the triceps, subscapular, suprailiac, and abdomen.
The waste-to-hip circumference ratio is a measure of the distribution of subcutaneous and intra-abdominal fat. The ratio tends to increase with age and excess weight.
Muscle is the major protein store in the body; a measure of muscle protein can be indicative of body protein reserves. Mid-upper-arm circumference is indicative of muscle mass and subcutaneous fat, and changes in arm circumference are relatively easy to detect. In less developed countries, where people tend to have little subcutaneous fat, the mid-upper-arm-circumference is a good measure of muscle mass and can be used in the diagnosis of protein-energy malnutrition. The measure is taken using a flexible tape, at the midpoint of the upper arm between the acromion process and the tip of the olecranon process.
The measure of the mid-upper-arm muscle circumference is calculated from the mid-upper-arm circumference and triceps skinfold thickness measurements. It is a measure of the circumference of the inner circle of muscle mass around the bone. The measure is used to assess protein-energy malnutrition.
Mid-upper-arm muscle area is a two-dimensional measure of muscle mass and provides more accurate information than the mid-upper-arm muscle circumference. It is also calculated from the mid-upper arm circumference and triceps skinfold thickness. The measure is based on the assumption that the mid-upper-arm cross- section is circular.
Hydrostatic weighing has been the traditional gold standard for assessing body fat. This method determines body volume, which can be used in turn to calculate total body density. The method is based on the principle that the volume of an object is equal to the volume of water it displaces when submerged in water. The volume of water displaced is equal to the difference in weight of the object in air and in water. Body volume is obtained by weighing an individual as normal, and then weighing the person under water. The calculation must take into account any residual air trapped in the lungs and intestines, and must consider temperature-related changes in the density of the water; the individual must expel as much air as possible while being weighed under water, to measure the residual lung volume. Once the total body volume has been measured, one can calculate a patient's percentage of body fat.
Total body water can be measured by isotope dilution. The size of fat-free mass can be estimated from the proportion of water in fat-free mass. A dose of isotope (deuterium or tritium) labeled water is administered; then the technician measures the concentration of the isotope in the body after equilibrium. Malnourished individuals have more water per unit of fat-free mass. It should be noted that the measurement of total body water tends to overestimate the size of the fat-free mass and underestimate the amount of body fat.
Dual-energy x-ray absorptiometry (DEXA) is a technique designed to measure bone density. The patient lies down on the DEXA device and an X-ray apparatus passes over the person, emitting X-rays through the body, measuring the composition of bone and soft tissue. The resultant data includes, specifically, measurements of bone mineral, fat mass, lean soft tissue mass, and percentage of body fat.
Bioelectrical impedance is a measurement based on the electrical conductivity of fat and fat-free mass. The impedance instrument measures the impedance of an electrical current passed through the body when electrodes are placed on the wrist and ankle. This allows for the calculation of body volume. Combining this measurement with body weight, a patient's density and percentage of body fat can be determined. Note that the technique is sensitive to the position of the electrodes on the body, hydration status, and consumption of certain food and drink.