It can be difficult to precisely measure the proportion of fat in an individual's body to determine if someone is obese or simply overweight. A quick way to estimate fat deposition is the waist and waist-to-hip measurement ratio. Waist measurements of above 37 in (94cm) in men and 34.6 in (88 cm) in women indicate increased risk of metabolic complications (e.g. diabetes, hypertension or heart disease).
The most widely used and accepted indicator of obesity is the body mass index (BMI). BMI uses body weight and height to evaluate weight. Using metric values, an individual's weight is divided by a square of their height to calculate BMI. It is then compared to a standard set of values. For instance, a BMI of 30 indicates obesity, while a BMI of 27 designates the person as overweight. This method is limited, however, because it does not take into account the percent of body weight that is accounted for by fat. So BMI, then, might indicate that a body-builder with a BMI of 31 as obese, even though he or she has more muscle mass than body fat. More accurate measures of percent body fat can be used to better assess obesity. These tests, such as hydrostatic weighing, skinfold measures, and bioelectric impedance help to distinguish between weight that is attributable to fat versus muscle. Hydrostatic weighing determines the body's density (weight/volume) for use as an estimate of body fat. Because lean tissue, like muscle or blood, is denser than fatty tissue, a person with more adipose is less dense than a lean person. Density of a patient is obtained by weighing the person, followed by submerging their entire body in a tank of water. The amount of water that is displaced from the tank is equal to the volume that their body occupies. From the density calculated, an estimate of body fat is obtained from standard values.
An accurate, but inconvenient method of estimating body fat uses a blood test. Here, a water-soluble compound that is detectable and measurable in solution is injected into the person. Because the substance is water soluble, it does not mix well with fat, nor highly fatty tissues such as adipose tissue. After allowing some time for the substance to distribute throughout the body, blood samples are taken. The concentration of the substance in blood then indicates how much it has been diluted in the body. This concentration, then, gives an estimate of lean body tissue. Using this, the amount of fatty tissue can be calculated. Another method is skinfold measurement. A simpler and less invasive way to determine percent body fat, this technique uses skinfold calipers. A caliper is an instrument consisting of a pair of movable curved arms that measures the thickness of skin folds at certain body regions. Thicker skinfolds have more fat content, and the amount of fat under skin is thus, correlated with total body fat. A more technologically oriented way of estimating percent body fat uses bioelectrical impedance. By transmitting a small electrical pulse through the body, the amount of body fat can be estimated. Because water conducts electricity more readily than fat, and as lean tissue contains more water than fat, the electrical surge can detect the amount of lean tissue, and thus fat tissue, present.
Science EncyclopediaScience & Philosophy: Nicotinamide adenine dinucleotide phosphate (NADP) to Ockham's razorObesity - The Widespread Weight Problem, Measuring Obesity, Causes Of Obesity, The Health Effects Of Obesity