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Reference Guide to Body Composition

An In-Depth Look at Body Fat, BMI and More
  -- By Jen Mueller and Nicole Nichols, Fitness Experts
Body weight alone is not a clear indicator of health or fitness because it does not distinguish how many pounds are fat and how many are muscle. But body composition helps describe the amount (and distribution) of fat and lean muscle tissue in the body. The popularity of body composition (as a measure of progress) is growing as people realize its value in determining health risks and showing progress, even when the scale doesn’t.

There are several methods of measuring body composition—some are simply estimates based on formulas, while others are more accurate. Use this reference guide to find an option that best suits you.

Height & Weight Tables

What they are: In 1953 the Metropolitan Life Insurance Company developed the first height/weight table to calculate how overweight or underweight individuals were. The data were based on the "averages" of their client base for both men and women. In 1999, the tables were revised based on updated data.

What they measure: The function of a height and weight table is to help determine if a person’s weight is within an appropriate range for their height and frame size.

How they work: To use a Height/Weight Table, simply find the appropriate chart for your gender and the right column for your frame size and height. Frame size is an important, subjective factor in the development of the tables; small, medium and large frame sizes change the "ideal weight" recommendation.

Where to find them: Many life and health insurance companies use these charts, as well as doctors and other health care providers. You can find the 1999 Metropolitan Height/Weight Tables here to see how you compare to average. http://www.bcbst.com/MPManual/HW.htm

Accuracy: On an individual basis, height/weight tables can provide very little information about an individual's health risk. But they can be a good indicator of whether or not you are within an average weight range. If you outside of the range, conduct another measure (like the ones listed in this Guide) to confirm whether your weight poses problems for your health.

Limitations: Weight alone doesn’t say much. An individual can be "overweight" and not "over fat." A bodybuilder, for example, may be considered "overweight" by a typical height-weight chart, and a thin, weak person may be considered “average,” even if they’re not healthy. Therefore, these charts are not a good indication of a person's ideal body weight for optimal health, much less for athletic performance. Also, much of the data collected for the Life Insurance tables came from upper and middle-class Caucasians, and therefore may not reflect an appropriate weight for other races and socioeconomic groups.

Body Mass Index

What it is: Body Mass Index (BMI) is a quick and easy way to determine, in general terms, if one’s weight is appropriate for one’s height. It has recently been used to quantify an individual's obesity level.

What it measures: Like it says above, BMI helps determine if a person’s weight is within an appropriate range for their height and frame size. It does NOT measure one’s body fat level.

How it works: The equation for BMI is weight (in kilograms) divided by height squared (in meters). To convert pounds to kilograms, divide by 2.2. To convert inches to meters, multiply by .0254. A “healthy” BMI ranges from 18.5 to 24.9.

Where to find it: BMI is commonly used in doctor’s offices, in gyms, and in many weight loss programs. You can use our BMI calculator to find out where you stand.

Accuracy: Since only an individual's height and weight are used, BMI does not provide a differentiation of fat and lean muscle weight. For most adults, however, there is a clear correlation between higher BMI and negative health consequences.

Limitations: BMI is an average that is based on population studies. Because it does not differentiate between fat and nonfat weight, it may overestimate body fat in athletes and others who have a muscular build. In the same way, it may underestimate body fat in older persons and others who have lost muscle mass.

Girth Measurements

What they are: The use of girth and length measurements is a quick, easy and inexpensive method to estimate body composition or describe body proportions. These measures are based on the assumption that body fat tends to be distributed at various sites on the body, such as the waist, neck and thighs, so that is where measurements are often taken. (Muscle tissue, on the other hand, is usually located in places such as the biceps, forearm and calves, which tend to store very little fat.)

What they measure: Some girth measurements use the circumference of various sites on the body to estimate one’s true body fat percentage. Other girth measurements (such as the waist-to-hip ratio) estimate one’s health risk based on these measures.

How they work: Using a cloth tape, girth and length measurements are taken from specific points on the body, such as those described above. The waist-to-hip ratio is one of the most commonly used values to reflect the degree of abdominal obesity.

To calculate your waist-to-hip ratio, use a measuring tape to measure the circumference of your hips at the widest part of your buttocks. Then measure your waist at the smaller circumference of your natural waist, usually just above the belly button. To determine the ratio, divide your waist measurement by your hip measurement.

Where to find them: While it’s fairly easy to find calculators online that estimate your body fat based on girth measurements, it’s best to leave these measures to trained professionals who are adept to the proper measuring techniques and sites. You can easily perform the waist-to-hip ratio yourself (to estimate related health risks).

Accuracy: Girth measurements provide a high-level assessment of an individual. When conducted properly, their accuracy is typically within 5% of the value measured using underwater weighing (the “gold standard” for body composition analysis).

Limitations: The major disadvantage of this method is that the measurements provide little information about the fat and nonfat components of the body. For example, a body builder may have a thigh that has a larger circumference (yet less fat) than an obese individual.

Skinfold Tests (Calipers)

What they are: Skinfold tests require the use of specially-designed "calipers" to measure the thickness of several sites on the body. Calipers are devices that pinch your skin, pulling fat away from muscles and bones.

What they measure: Skin fold measurements are used to calculate a person’s body fat percentage.

How they work: Typically, the tester uses the calipers to “pinch” at least three different sites on your body, such as the abdomen, arm, and back, but many more sites can be used as well (including the chest and thigh). This test is based on the assumption that the amount of fat stored at these various sites is proportional to a person’s overall body fat. By measuring several sites, total body fat may be calculated.

The thickness of each pinch is plugged into a formula to determine a person’s body fat percentage. Currently, over 100 different equations are available to estimate body fat when using skinfold calipers.

Where to find them: Individuals can buy various types of calipers at specialty stores and online. However, to ensure accuracy, it’s recommended that only trained professionals (who have been trained in skinfold measurement and have had many opportunities to practice) conduct skinfold tests. Because calipers are readily available and fairly inexpensive, this body fat test is pretty common. It can be done quickly, and the interpretation is simple. Many local gyms, YMCA’s, and community centers offer this as a free service free or charge a minimal fee.

Accuracy: The American College of Sports Medicine states that skinfold measurements, when performed by a trained, skilled tester, are up to 98% accurate. Because of the consistency in results, the high success rate, and the low margin of error, this is generally accepted as the best field test, outside of clinical testing, such as hydrostatic weighing (see below).

Limitations: The estimation results obtained from skinfold measurements vary widely from technician to technician. The "art" of skinfold measurements requires the technician to properly identify a site measurement and pinch the skin gathering only the fat store and no other tissue. If the tester does not pinch exactly the right spot, or pull all the fat away from the muscle, the test will be inaccurate. When skinfold calipers cannot open wide enough to measure the total fat thickness, this technique tends to grossly underestimate body fat percentage in the obese population. In addition, some people can best be described as “hard to pinch,” meaning that it’s difficult to pull their skin and fat away from their muscle and bone. Test results will be less accurate for these individuals.

The wide variety of equations (number and location of sites tested) reflects the problem with the accuracy of this method. When getting retested, it’s wise to have the same professional who conducted your initial assessment measure you again for consistency.

Bioelectrical Impedance

What it is: Bioelectrical impedance is a weak electric signal that goes through the body to measure body composition. A small 500-800 micro-amp (50 kilohertz) signal measures the body's ability to conduct the current.

What it measures: These tests estimate a person’s body fat percentage.

How it works: A special scale or handheld device sends a weak electrical signal through into the body. This current flows through the body, finding varying resistance depending on the density of the muscle, the amount of body fat encountered, and the hydration of the tissue. The slower the signal (measured in ohms), the more fat is present, because fat interferes with the signal. The faster the signal moves, the more lean (muscle) tissue is present, because lean tissue is highly conductive due to its high water content.

Along with other variables that you input into the device (such as gender, age, weight and height), the machine will estimate your body fat percentage based on the speed of the traveling signal.

Where to find it: Bioelectrical impedance testing is pretty common these days. You can buy your own handheld body fat testing device online or at the store, as well as special scales that use bioelectrical impedance to calculate your body fat percentage in addition to your weight. Both of these devices are often used in commercial and health care setting as well. Gyms and universities offer these tests, but prices can range from free to $30 or more per test. Home machines (handheld and scales) can be purchased for anywhere from $50-$400.

Accuracy: Bioelectrical impedance can have a large margin of error, especially if the subject is extremely obese or extremely lean. In one study, female distance runners averaged 20 percent body fat using this method, but more reliable methods showed that they were actually closer to 10 percent. Dehydration can also skew the results; the signal slows down, and the subject appears to have more fat than they actually do. Compared to other body fat testing methods (skinfold calipers and hydrostatic weighing) this is the less accurate.

Limitations: It’s essential to follow a specific protocol for an accurate result. These guidelines are usually extensive (no eating or drinking within 4 hours prior to the test, no exercising 12 hours before the test, etc.). If they’re not followed, the results will be inaccurate. Inconsistency in hydration, body fluids and intestinal content can result in high degree of variation from day to day too. This makes bioelectrical impedance less suitable for repeated testing (like measuring small changes in body fat). The test also tends to overestimate percent body fat in very lean individuals and underestimate body fat in obese people.

In addition, handheld devices only measure fat levels in the upper body (the signal travels through one arm and out the other arm) and digital scales only measure fat levels in the lower body (the signal travels up one leg and down the other). The result you get from each of these models won’t take abdominal fat storage into account either, so you’re not getting a full picture of the fat level of your entire body.

Hydrostatic (Underwater) Weighing

What it is: Hydrostatic weighing immerses a person in a large tank of water to calculate their body composition.

What it measures: Hydrostatic weighing measures a person’s overall weight and their lean (muscle) mass and body fat mass.

How it works: Based on the assumption that the density and specific gravity of lean tissue is greater than that of fat tissue (i.e. that lean tissue will sink and fat tissue will float in water), hydrostatic weighing submerges a person under water. By comparing a test subject's weight out of the water with their mass measured under the water and out of the water, body composition may be calculated. A person with more bone and muscle will weigh more in water than a person with less bone and muscle, meaning they have a higher bone density and lower percentage of body fat.

Hydrostatic weighing uses a large tank of water, usually 1000 gallons that must be maintained at a constant temperature. Equipment to measure residual lung volume, and a scale connected to an "under-water chair" are also required. Test subjects are asked to exhale as much air as possible from their lunges before they are completely immersed for 10 to 15 seconds for an underwater weight measurement to be taken. This procedure is repeated 7 to 10 times, and may last 45 minutes to one hour.

Where to find it: This test is usually only available at research institutions and universities. Typical costs vary from $10-$75 (or more) due to the involved nature of the test.

Accuracy: Hydrostatic weighing is currently considered the "Gold Standard" of body composition analysis. It is the most accurate way to measure one’s body fat.

Limitations: The equipment required to perform hydrostatic measurements is bulky and its maintenance is intense. The procedure is also time-consuming and complicated, especially if the subject is unable to expel all of the air from their lungs. Fear of immersion in a tank of water, fear of infection from the water, and obesity are additional barriers to this technique.

Considerations Related to Body Composition

Although two people can have the weight or the same body fat percentage, that doesn't mean they face the same health risks. Where body fat is located can place a person at far greater risk for fat-related health conditions such as: cardiovascular disease, high blood pressure, stroke, diabetes and even certain types of cancers.

It is important, for health and well-being, to not only know your body fat percentage but to also pay attention to where that fat is located. Fat around the abdomen may present the greatest risk for health problems. In contrast, fat around the hips and thighs is most common in females and seems relatively harmless with respect to these health problems.

So which test is the best one for you?
If you are looking for an average estimate of your body fat percentage and how you compare with the population, BMI and girth measurements are easy to do at home. For more accuracy and personalized results, the skin fold test is best and can be affordable. Bioelectrical impedance can be inexpensive and easy to find, but it has many limitations. If your time and finances allow, hydrostatic weighing is the best option in terms of accuracy.