Bioelectrical Impedance Analysis
A Technique for Direct Determination of Body Fat Percentage
Nov 8, 2008
Your total amount of body fat is made up of 2 basic kinds: essential fat and storage fat. Essential fat is necessary for the proper functioning of the immune and hormonal systems. However, it is not the minimum amount of fat that is recommended for good health. You also need storage fat which occurs in adipose tissue.
This extra fat has important roles such as cushioning internal organs, storing fat-soluble vitamins and controlling body temperature. It also stores energy to be used by the body when food isn’t available. However, if the amount of stored fat in your body is too high, it puts you at risk for a variety of diseases.
Body Fat Percentage
According to the American Council on Exercise (ACE), the amount of essential body fat required by women is 10-12%, compared with 2-4% for men. Body fat is necessary in reproduction, which is why women carry 3-6 times as much essential fat as men. In fact, women who have too little body fat (athletes and anorexics, for example) may actually stop menstruating.
ACE recommends that female athletes maintain 14-20% body fat and male athletes carry 6-13% fat. For non-athletes who are nonetheless fit, the desirable range is higher: 21-24% for women and 14-17% for men. Body fat percentages between 25 and 31 (for women) and 18 and 25 (for men) are considered “acceptable,” but are found in overweight individuals. People with higher amounts of body fat are considered obese.
An individual’s Body Mass Index (BMI) is a useful number that usually correlates well with the amount of body fat. However, it yields misleading information for people who are more muscular than average (e.g., bodybuilders). BMI calculations also depend on factors like gender, age, ethnicity and bone structure.
Several techniques are available that can be used to measure body fat directly. These include underwater weighing, skin-fold thickness measurements using calipers, dual-energy X-ray absorptiometry (DXA), computerized tomography and bioelectrical impedance analysis. The latter is a relatively new technique that has been shown to agree well with skin-fold measurements and DXA.
Bioimpedance Analysis
Bioelectrical impedance analysis, or bioimpedance, can be used to directly measure the weight and percentage of body fat in your body. It is a simple, rapid, non-invasive technique. A variety of systems are available that measure leg-to-leg or total body impedance.
The technique measures the resistance of the body to the flow of a safe, low-level (1 mA) electric current through fluids in fat and lean tissues. Lean tissue (muscle) contains abundant water and electrolytes, making it conductive; the electrical current passes freely through it. Fat tissue has less water, so the current encounters resistance and flows less easily.
Bioimpedance measures Total Body Water which is then used to calculate the relative amounts of skeletal muscle and fat. This is done by computer, using linear regressions that factor in height, weight, age, gender and ethnicity. Total Body Fat is calculated by difference. Systems vary, but some models output percent skeletal muscle mass, body fat, visceral (abdominal) fat, extracellular water and intracellular water.
Bioimpedance is very sensitive to the level of hydration in your body. The best time to measure your body fat is in the late afternoon or early evening when variations in hydration are lowest, if possible. If you plan to repeat the analysis over time (if you’re trying to lose or gain weight), the most important thing is to schedule the analysis for the same time of day each time.
Disclaimer: This information is for educational purposes only and should not be considered as medical advice. It does not replace the advice of the physician who cares for you. All medical advice and information should be considered as incomplete without a physical exam, which is not possible without a visit to your doctor.
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