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Body Composition and Body Fat

The most common methods to measure body composition and body fat

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Updated May 30, 2014

Doctor checking woman's BMI
Peter Dazeley/Photographer's Choice/Getty Images

What Is Body Composition

Body composition is the term used to describe the different components that, when taken together, make up a person's body weight. The human body is composed of a variety of different tissue types including lean tissues (muscle, bone, and organs) that are metabolically active, and fat (adipose) tissue that is not.

Body Weight Measurements

Standard body weight scales provide a measure of total weight, but don't determine the lean-to-fat ratio of that weight. Standing on most scales can tell you only if you weigh more than the average person, but not if that weight is fat or muscle. Based only on scale weight, a 250-pound athlete with 8% body fat may be considered "overweight" by a typical weight chart. Such charts are not a good indication of ideal body weight for general health or for athletic performance.

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Assessing Body Composition and Percent Body Fat

There are many methods of assessing a person's body fat percent and lean mass. The most common methods include the following.

Underwater Weighing - Hydrostatic Weighing

One method of body composition analysis in which a person is weighed while submerged in a large tank of water is called underwater or hydrostatic weighing This method of determining body composition relies on Archimedes' Principle of displacement which states:

  1. The density of fat mass and fat-free mass are constant
  2. Lean tissue is more dense than water
  3. Fat tissue is less dense than water.
  4. Therefore person with more body fat will weigh less underwater and be more buoyant.
Underwater weighing has been considered the gold standard for body composition assessment, however new, more sophisticated methods may make underwater weighing obsolete in the future.

Skinfold Thickness Measurements

Because underwater weighting it is complicated and cumbersome and requires special equipment, most exercise physiologists use simple skinfold measurements to determine body fat percent. The American College of Sports Medicine says that when performed by a trained, skilled, tester, they are up to 98% accurate.

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Bioelectrical Impedance

Bioelectrical Impedance is another method of assessing body fat percentage. There are a variety of body composition and body fat analyzers and scales available for home use that provide more than just total weight measurements. These devices determine total weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While the readings can be affected by hydration levels, food intake, skin temperature, and other factors, if you follow the directions and take the reading under similar conditions, you will obtain the best results.

Compare prices: Body Composition and Body Fat Analyzers and Scales

BMI - Body Mass Index

BMI or Body Mass Index, is another method of estimating a person's body fat percentage based upon simple weight and height measurements. While the BMI calculation is an indirect measurement, it has been found to be a fairly reliable indicator of body fat measures in most people. Although some studies still question the accuracy of the BMI method of body fat measurement, especially for athletes.

Ideal Body Weight and Percent Body Fat

The ideal weight and fat-lean ratio varies considerably for men and women and by age, but the minimum percent of body fat considered safe for good health is 5 percent for males and 12% for females. The average adult body fat is closer to 15 to 18% for men and 22 to 25% for women.

Athletes tend to be at low end of this scale due to their increased lean weight (muscle mass). While low levels of body fat seem to be related to improved performance, body composition alone is not a great predictor of sports success. A linebacker needs to have enough body mass (lean and fat weight) to generate high forces and avoid injury. Body fat among elite athletes vary largely by sport. There is little evidence of any benefit when men drop under 8% and women drop under 14 percent body fat.

How Low Is Too Low?

While the average body fat percent in the United States and Europe is increasing, extremely low body fat percent is also a health problem. The female athlete triad highlights the problem. Women athletes who lose too much fat risk injury, decreased performance and health issues.

The female athlete triad refers specifically to three related health problems often found in women athletes:

This attempt to reduce body fat by extreme measures not only leads to decreased exercise performance, but can lead to severe health complications. Nutrient deficiencies and fluid/electrolyte imbalance from low food intake can lead to increased risk of fractures, illness, loss of reproductive function and serious medical conditions such as dehydration, and starvation. The medical complications of this triad involve almost every body function and include the cardiovascular, endocrine, reproductive, skeletal, gastrointestinal, renal, and central nervous systems.

Read the updated American College of Sports Medicine Position Stand on the Female Athlete Triad.

How Much Body Fat Is Too Much?

Just as too little body fat can cause physiological complications, too much body fat is also harmful. For men over 25% and women over 32% fat there is a dramatic correlation with illness and disease.

Isn't Body Composition Genetic?

Some aspects of your body composition are genetic (where you store fat), but most fat increase is related to lifestyle.

Can I Change My Body Composition?

Yes. To increase or decrease your percent of body fat you need to create the right balance between the calories in and calories out. The best way to do this is to decrease daily calories by about 500 and increase your exercise. Aerobic exercise along with strength training is ideal. If you are beginning a new exercise program, you are advised to first consult your physician.

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