While weight can certainly be a factor in one's health, it's important to remember even thin people need to be aware of the benefits of diet and exercise.
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Body composition analysis is a physical test that measures the proportion of the various components of a person's body. The human body is comprised of water, protein, fat, and minerals — but for most purposes, it is the level of fat compared to lean mass that is of interest. In general, most tests measure the ratio of fat to lean tissue. Body fat, or adipose tissue, has chemical and physical properties that allow for a number of analytical methods, each with its own advantages and limitations. The most common forms of body composition analysis are the body mass index (BMI,) skin fold caliper testing, bioelectrical impedance, and hydrostatic weighing.
The easiest, cheapest, and most common form of body composition analysis is the BMI. Used by fitness centers, doctors, and insurance companies, the BMI attempts to give a picture of body composition by mathematically comparing height to weight, using one of the following formulas:
Using these formulas, a normal BMI measurement is considered to be between 19 and 21, with a higher number indicating overweight and a lower number indicating underweight. The obvious limitation of the BMI as a measurement is that it fails to take body composition into account, and so it is usually seen as a good estimate for the general population, but can be inaccurate for those with a very athletic build.
Skin fold caliper testing has been used for body composition analysis for years, and is generally considered to be an accurate measure of body fat. The test is based on the assumption that roughly half of the body's fat is immediately under the skin. Measurements are taken at strategic locations, including the biceps, triceps, the mid- and lower back, and the calf, and the results are then applied to an equation that is used to estimate the total body fat percentage. While the skin fold test is an accurate one, it requires a certain degree of expertise and precision by the person making the measurements. Additionally, for many people, the process of being measured can be an emotionally uncomfortable one, in which case less invasive methods might be preferable.
Bioelectrical impedance is based on the principle that lean tissue is more electrically conductive than fat. A mild, painless electrical current is passed through the body, usually from the wrist to the foot. This test is often the body composition analysis method of choice in health clubs because the test can be performed fully clothed, and requires minimal skill on the part of the examiner. Equipment required for bioelectrical impedance testing can be expensive compared to skin calipers, so this testing may be found mainly at large facilities.
Hydrostatic weighing is often referred to as the gold standard for this analysis. Generally believed to be the most accurate means of determining body composition, hydrostatic weighing is based on the idea that fat is less dense than water. The process compares a person's body weight on dry land to their underwater weight, and uses the results to calculate the proportion of fat to lean tissue. While accurate, hydrostatic weighing is generally only carried out in laboratory settings, as it requires equipment and skills that would be impractical to replicate in a family medicine or fitness setting.
No matter which method is used, normal body fat measurements are generally considered to be 18-30% for women and 15-25% for men. If fat levels fall any lower, there is a lessened ability to cushion organs and store certain vitamins, and an increased risk of osteoporosis. Any higher, and there is a risk of cardiovascular disease. Where a person optimally falls within the normal range is dependent on many variables, including genetics, activity level, and life stage.