Body density is the proportion of body fat present in a human body, as compared to its overall mass. Also known as body fat composition, it is used as a tool for gauging how much fatty tissue, both essential and excess, exists in a body. Knowledge of body density is a critical piece of data in a nutrition and fitness plan.
It is impossible to determine the exact proportion of fat in a living body, but there are a number of methods that have been developed to estimate body density. The most common and simplest way is known as body mass index (BMI), which involves using body weight and height to generate a number that can be compared to a baseline chart. Individuals can use their BMI to determine whether or not their body density is at a healthy level.
BMI is not considered by health authorities to be a definitive measure of density, but rather a screening tool to identify individuals who may have excessive amounts of body fat for their size. While a reasonable tool for estimation for most body types, BMI loses accuracy among heavily muscled individuals, as muscle weighs more than fat and skews results. Bodybuilders, for instance, are often shown to be obese on a body mass index because of their high weight to height ratio, despite a relative lack of body fat.
Other, more scientific methods for estimating body density include skin fold measurements, bioelectrical impedance analysis (BIA), near-infrared interactance, dual energy X-ray absorptiometry (DXA), and underwater weighing. Skin fold, near-infrared interactance, and BIA techniques are less costly than the newer DXA method, but are less accurate and have more limitations.
Skin fold analysis is the oldest and least advanced method, involving the use of skin fold calipers to measure the amount of excess non-lean tissue present on the body. BIA leverages the differing densities of bone, muscle, and fat to provide a measure of density by running an electrical current between two conductors placed on various parts of the body, and gauging resistance. Near-infrared interactance involves shooting a near-infrared beam of light through the muscles of the upper arm. The light is absorbed by fatty tissue, but reflects back off muscle. Therefore the amount of light that exits the arm is a good indicator of how much fat is present.
DXA is currently considered a leading technique for estimating body density. Two different types of x-ray machines — one that detects all kinds of tissue, and one that only detects fat — scan a body and indicate to what degree fat is present. DXA is a relatively involved process compared to other methods. It is generally used in conjunction with a process known as underwater weighing, which uses principles of buoyancy and the known weight difference between equal amounts of muscle and fat to provide reliable results.