Body fat percentage is the fraction of the total body mass that is adipose tissue, as opposed to lean body mass (muscle, bone, organ tissue, blood, and everything else). This index is often used as a means to monitor progress during a diet or as a measure of physical fitness for certain sports, such as body building. It is more accurate as a measure of excess body weight than body mass index (BMI) since it takes into account muscle mass and frame size. However, its popularity is less than BMI because most of the techniques used to measure body fat percentage require equipment and skills that are not readily available.
Total body fat percentage consists of essential fat and storage fat. Essential fat is that amount 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 2-5% in men, and 10-13% in women. Storage fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. Again, women have slightly more than men. The minimum recommended total body fat percentage exceeds the essential fat percentage value reported above.[1]
Recommendations
According to Thomas A. Owens, M.D. (Departments of Internal Medicine and Pediatrics, Duke University Medical Center, Durham, NC), body fat percentage is categorized as follows:[1] :
| Description | Women | Men |
|---|---|---|
| Recommended amount | 20-21% | 13-17% |
| Adults in United States, average | 22- 25% | 17-19% |
| Obese | 30%+ | 25%+ |
According to Health Check Systems[2], The American Council on Exercise[3] has categorized ranges of body fat percentages as follows:
| Description | Women | Men |
|---|---|---|
| Essential fat | 10-12% | 2-4% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Acceptable | 25–31% | 18–25% |
| Obese | 32%+ | 25%+ |
Measurement techniques
A person's exact body fat percentage generally cannot be determined, but there are several different ways we can estimate it, each with varying degrees of accuracy.
Dual energy X-ray absorptiometry
Dual energy X-ray absorptiometry, or DXA (formerly DEXA), is a good method for estimating body fat percentage.
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 plethsmography). Various other components may be independently measures, such as total body potassium.
In addition, the most refined method, 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. This is the most accurate method. You can also use many other methods to calculate body fat percentage/
Body Average Density Measurement (Hydrostatic Weighing)
Prior to the adoption of DXA, the most accurate method of estimating body fat percentage was to weigh a person underwater in order to obtain the average density (mass per unit volume). 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 tissue, 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 error up to ±10%.[5] 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
In these formulas, ρ is the body density in kg/L.
where ρw is the density of water [0.99780 kg/L at 22 °C (72 °F)]. For example, a person weighing 80 kg needs to hold a floater with a volume of 4.5 L and a mass of 0.5 kg has a density of 1.05 kg/L and hence a body fat percentage of 21%. Note that both the Brozek and Siri formulas are claimed to give systematically too high body fat percentages. [6]
Bioelectrical Impedance Analysis
The Bioelectrical impedance analysis (BIA) method is 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 charge 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. 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 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. Bioelectrical impedence analysis is available in a laboratory, or for home use in the form of body fat scales and hand held body fat analyzers.
Skinfold Measurements
A simpler procedure for estimating body fat is the skinfold test, whereby a pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation. It is of utmost importance to test in a precise location with a fixed pressure.
Girth Comparisons
There exist formulae for estimating body fat percentage from an individual's weight and girth measurements. For example, the U.S. Navy Circumference method compares abdomen or waist and hips measurements to neck measurement and height[7], and other sites claim to estimate one's body fat percentage by a conversion from the body mass index. Unfortunately, these measures are usually inaccurate as a way of determining body composition.
External links
- A Guide to Body Fat Percentage
- Gallagher D, Heymsfield S, Heo M, Jebb S, Murgatroyd P, Sakamoto Y (2000). "Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index.". Am J Clin Nutr 72 (3): 694-701. PMID 10966886.
- Body Composition
- Yuhasz Skinfold Test
- Fat Caliper Measurement Sites
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