A person's total body fat percentage is the total weight of the person's fat divided by the person's weight. The resulting number reflects both essential fat and storage fat.
- Essential fat is that amount of fat necessary for maintenance of life and reproductive functions. The percentage for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Essential fat is 3–5% in men, and 8–12% in women.
- Storage fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. The minimum recommended total body fat percentage exceeds the essential fat percentage value reported above. A number of online tools are available for calculating estimated body fat percentage.
Arguably, body fat percentage is the superior gauge of an individual's fitness level, as it is the only body measurement which directly calculates the particular individual's body composition without regard to the individual's height or weight. The widely-used body mass index (BMI), on the other hand, simply makes blanket assumptions as to what every individual of a certain height should ideally weigh, regardless of the body composition which makes up that weight. The BMI gives particularly inaccurate information with regard to individuals with above-average lean muscle mass, classifying such individuals as "overweight" or "obese" despite the fact that their body fat percentage would indicate they are in excellent physical condition.
Recommendations
Different cultures value different body compositions differently at different times, and some are related to better health or improved athletic performance. levels of body fat are epidemiologically dependent on gender and age. Different authorities have developed different recommendations for ideal body fat percentages, such as these from the The American Council on Exercise:
Essential fat means that level below which health and normal physiological would be negatively affected. Controversy exists as to whether falling in a particular category of these body fat percentages is better for one's health than any other, but there seem to be enhancements in athletic performance as one nears the ideal body fat percentage range for one's particular sport. The leanest athletes typically compete at levels of about 5–8% for men or 10–15% for women. Bodybuilders will often compete at ranges even lower than these levels. Certified personal trainers will suggest to male bodybuilders that they should aim for a body fat percentage between 2–4% by contest time, however it is unclear that such levels are ever actually attained and the means to assay it are as noted below lacking in principle as 5% is generally considered a physiological minimum for human males.
Measurement techniques
A living person's exact body fat percentage generally cannot be determined, but there are several techniques which can be used to estimate it accurately.
Near-infrared interactance
A beam of infra-red light is transmitted into the biceps. The light is reflected from the underlying muscle and absorbed by the fat. The method is safe, noninvasive, rapid and easy to use.
Dual energy X-ray absorptiometry
Main article: Dual energy X-ray absorptiometryDual energy X-ray absorptiometry, or DXA (formerly DEXA), is a newer method for estimating body fat percentage, and is commonly cited as the current gold standard for body composition testing.
Two different types of X-ray scans the body, one that detects all tissues and another that doesn't detect fat. A computer can subtract the second picture from the first one, giving only fat detection. The mass of this can be estimated by the grade of exposure.
Expansions
There are several more complicated procedures that more accurately determine body fat percentage. Some, referred to as multicompartment models, can include DXA measurement of bone, plus independent measures of body water (using the dilution principle with isotopically labeled water) and body volume (either by water displacement or air plethysmography). Various other components may be independently measured, such as total body potassium.
In-vivo neutron activation can quantify all the elements of the body and use mathematical relations among the measured elements in the different components of the body (fat, water, protein, etc.) to develop simultaneous equations to estimate total body composition, including body fat.
Body average density measurement
Prior to the adoption of DXA, the most accurate method of estimating body fat percentage was to measure that person's average density (total mass divided by total volume) and apply a formula to convert that to body fat percentage.
Since fat tissue has a lower density than muscles and bones, it is possible to estimate the fat content. This estimate is distorted by the fact that muscles and bones have different densities: for a person with a more-than-average amount of bone mass, the estimate will be too low. However, this method gives highly reproducible results for individual persons (± 1%), unlike the methods discussed below, which can have an uncertainty up to ±10%. The body fat percentage is commonly calculated from one of two formulas:
- Brozek formula: BF = (4.57/ρ − 4.142) × 100
- Siri formula is: BF = (4.95/ρ − 4.50) × 100
Bioelectrical impedance analysis
Main article: Bioelectrical impedance analysisThe Bioelectrical impedance analysis (BIA) method is a more affordable but less accurate way to estimate body fat percentage. The general principle behind BIA: two conductors are attached to a person's body and a small electrical current is sent through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to electricity varies between adipose, muscular and skeletal tissue. Fat-free mass (muscles) is a good conductor as it contains a large amount of water (approximately 73%) and electrolytes, while fat is anhydrous and a poor conductor of electrical current. Factors that affect the accuracy and precision of this method include instrumentation, subject factors, technician skill, and the prediction equation formulated to estimate the Fat Free Mass. Criticism of this methodology is based on where the conductors are placed on the body; typically they are placed on the feet, with the current sent up one leg, across the abdomen and down the other leg. As technician error is minor, factors such as eating, drinking and exercising must be controlled since hydration level is an important source of error in determining the flow of the electrical current to estimate body fat. As men and women store fat differently around the abdomen and thigh region, the results can be less accurate as a measure of total body fat percentage. Another variable that can affect the amount of body fat this test measures is the amount of liquid an individual has consumed before the test. As electricity travels more easily through water, a person who has consumed a large amount of water before the test will measure as a lower body fat percentage. Less water will increase the percentage of body fat. Also reducing the reliability of this method is the variation between models of the BIA devices: for instance when comparing outputs from a Tanita scale to an Omron Body Logic handheld device the Tanita scale overestimated the percentage body fat in college-aged men by 40% and in college aged women by 55%.. Bioelectrical impedance analysis is available in a laboratory, or for home use in the form of body fat scales and hand held body fat analyzers.
Anthropometric methods
There exist various anthropometric methods for estimating body fat. The term anthropometric refers to measurements made of various parameters of the human body, such as circumferences of various body parts or thicknesses of skinfolds. Most of these methods are based on a statistical model. Some measurements are selected, and are applied to a population sample. For each individual in the sample, the method's measurements are recorded, and that individual's body density is also recorded, being determined by, for instance, under-water weighing, in combination with a multi-compartment body density model. From this data, a formula relating the body measurements to density is developed.
Because most anthropomorphic formulas such as the Durnin-Womersley skinfold method, the Jackson-Pollock skinfold method, and the US Navy circumference method, actually estimate body density, not body fat percentage, the body fat percentage is obtained by applying a second formula, such as the Siri or Brozek described in the above section on density. Consequently, the body fat percentage calculated from skin folds or other anthropometric methods carries the cumulative error from the application of two separate statistical models.
These methods are therefore inferior to a direct measurement of body density and the application of just one formula to estimate body fat percentage. One way to regard these methods is that they trade accuracy for convenience, since it is much more convenient to take a few body measurements than to submerge individuals in water tanks.
The chief problem with all statistically derived formulas is that in order to be widely applicable, they must be based on a broad sample of individuals. Yet, that breadth makes them inherently inaccurate. The ideal statistical estimation method
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