Wednesday, May 26, 2010


After my body composition test yesterday, I decided to do some research to see how accurate my results were.  My doctor uses  DEXA (Dual Energy X-ray Absorptiometry), which is evidently very accurate compared to other testing methods.  Right click the title below to read more about the subject: 


Most people now understand that to have a healthier body means to have a leaner body. These people are ready for Body Composition Analysis (BCA.) This article will attempt to acquaint you with the most popular methods of BCA used in gyms, homes, and professional practices.

Hydrodensitometry Weighing (Underwater Weighing)
- This method measures whole body density by determining body volume. There is a variety of equipment available to do underwater weighing ranging in sophistication from the standard stainless steel tank with a chair or cot mounted on underwater scales, to a chair and scale The Dunk 
Tank - Underwater Weighing Modelsuspended from a diving board over a pool or hot tub.

This technique first requires weighing a person outside the tank, then immersing them totally in water and weighing them again. The densities of bone and muscles are higher than water, and fat is less dense than water. So a person with more bone and muscle will weight more in water than a person with less bone and muscle, meaning they have a higher body density and lower percentage of body fat. The volume of the body is calculated and the individual's body density is determined by using standard formulas. Then body fat percentage is calculated from body density using standard equations (either Siri or Brozek).

The underlying assumption with this method is that densities of fat mass and fat-free mass are constant. However, underwater weighing may not be the appropriate gold standard for everyone. For example, athletes tend to have denser bones and muscles than non-athletes, which may lead to an underestimation of body fat percentage. While the body fat of elderly patients suffering from osteoporosis may be overestimated. To date, specific equations have not been developed to accommodate these different population groups.

An important consideration in this method is the amount of air left in a person's lungs after breathing out. This residual lung volume can be estimated or measured, but it is established that a direct measure is desirable and it should be taken in the tank whenever possible. Another consideration is that the water in the tank must be completely still; there can be no wind or movement.

Although this method has long been considered the laboratory "gold standard", many people find it difficult, cumbersome, and uncomfortable, and others are afraid of total submersion or cannot expel all the air in their lungs. Clinical studies often require subjects to be measured three to five times and an average taken of the results.

Calipers - Skinfold Measurment ModelCalipers (Anthropometry- Skinfold Measurements) - Using hand-held calipers that exert a standard pressure, the skinfold thickness is measured at various body locations (3-7 test sites are common). Then a calculation is used to derive a body fat percentage based on the sum of the measurements. Different prediction equations are needed for children and specific ethnic groups (over 3,500 equations have been validated). This approach usually uses underwater weighing as a reference method. The caliper method is based upon the assumption that the thickness of the subcutaneous fat (found just under the skin) reflects a constant proportion of the total body fat (contained in the body cavities), and that the sites selected for measurement represent the average thickness of the subcutaneous fat.

Skinfold measurements are made by grasping the skin and underlying tissue, shaking it to exclude any muscle and pinching it between the jaws of the caliper. Duplicate readings are often made at each site to improve the accuracy and reproducibility of the measurements. Often to save time in large population studies, a single skinfold site measurement is made to reduce the time involved. Such a test should be used only for a rough estimate of obesity.

Generally speaking, skinfold measurements are easy to do, inexpensive and the method is portable. Overall, results, can be very subjective as precision ultimately depends on the skill of the technician and the site measured. The quality of the calipers is also a factor; they should be accurately calibrated and have a constant specified pressure. Inexpensive models sold for home use are usually less accurate than those used by an accredited caliper technician. The more obese the subject, the more difficult to "pinch" the skinfold correctly, requiring even more skill to obtain an accurate measurement.

Dual Energy X-ray Absorptiometry - DEXA ModelDEXA (Dual Energy X-ray Absorptiometry) - A relatively new technology that is very accurate and precise, DEXA is based on a three-compartment model that divides the body into total body mineral, fat-free soft (lean) mass, and fat tissue mass. This technique is based on the assumption that bone mineral content is directly proportional to the amount of photon energy absorbed by the bone being studied.

DEXA uses a whole body scanner that has two low dose x-rays at different sources that read bone and soft tissue mass simultaneously. The sources are mounted beneath a table with a detector overhead. The scanner passes across a person's reclining body with data collected at 0.5 cm intervals. A scan takes between 10-20 minutes. It is safe and noninvasive with little burden to the individual, although a person must lie still throughout the procedure.

DEXA is fast becoming the new "gold standard" because it provides a higher degree of precision in only one measurement and has the ability to show exactly where fat is distributed throughout the body. It is very reliable and its results extremely repeatable; in addition, the method is safe and presents little burden to the subject. Although this method is not as accurate in measuring the extremely obese and the cost of equipment is high, DEXA is quickly moving from the laboratory setting into clinical studies.

NIR (Near Infrared Interactance) - A fiber optic probe is connected to a digital analyzer that indirectly measures the tissue composition (fat and water) at various sites on the body. This method is based on studies that show optical densities are linearly related to subcutaneous and total body fat. The biceps is the most often used single site for estimating body fat using the NIR method. The NIR light penetrates the tissues and is reflected off the bone back to the detector. The NIR data is entered into a prediction equation with the person's height, weight, frame size, and level of activity to estimate the percent body fat.

This method has become popular outside of the laboratory because it is simple, fast, noninvasive, and the equipment is relatively inexpensive. However, the amount of pressure applied to the fiber optic probe during measurement may affect the values of optical densities, and skin color and hydration level may be potential sources of error. To date, studies conducted with this method have produced mixed results; a high degree of error has occurred with very lean and very obese people; and the validity of a single-site measurement at the biceps is questionable. Numerous sources report that more research is needed to substantiate the validity, accuracy and applicability of this method.

Magnetic Resonance Imaging (MRI) - An x-ray based method in which a magnetic field "excites" water and fat molecules in the body, producing a measurable signal. A person lies within the magnet as a computer scans the body. High-quality images show the amount of fat and where it is distributed. MRI takes about 30 minutes and is very safe as it uses no ionizing radiation, but use is limited due to the high cost of equipment and analysis.

Total Body Electrical Conductivity (TOBEC) - This method is based on lean tissue being a better conductor of electricity than fat. A person lies in a cylinder that generates a very weak electromagnetic field. The strength of the field depends on the electrolytes found in the person's body water. In about 10 seconds, TOBEC makes 10 conductivity readings that estimate lean body mass. Although very accurate, its use is limited due to the high cost of the equipment.

Computed Tomography (CT) -
CT produces cross-sectional scans of the body. An x-ray tube sends a beam of photons toward a detector. As the beam rotates around a person, data is collected, stored, and applied to complex algorithms to build images that determine body composition. CT is particularly useful in giving a ratio of intra-abdominal fat to extra-abdominal fat. It is noninvasive, but potential is limited by exposure to radiation and high equipment cost.

BOD POD (Air Displacement) -
Based on the same principle as underwater weighing, the BOD POD used computerized sensors to measure how much air is displaced while a person sits for 20 seconds in a capsule. It uses a calculation to determine body density, then estimated body fat. The equipment is very expensive and limited in availability.

BIA (Bioelectrical Impedance)
- The only method that is based on measuring something, not estimating anything, is Bio-Impedance measurement. Bio-Impedance is a means of measuring electrical signals as they pass through the fat, lean mass, and water in the body. Through laboratory research we know the actual impedance or conductivity of various tissues in the body, and we know that by measuring current between two electrodes and applying this information to complex proven scientific formulas accurate body composition can be determined. The fact that the measurement is based on a reading of lean mass and not an estimate of fat mass, lends to a much more comprehensive testing method and results.

Losing Weight Can Make You Fatter
"A" and "B" were the same height, weight and had the same percentage of body fat. Each lost 10 pounds over the same period of time. Although their weights are identical, "A" is now fatter. She simply cut calories, resulting in the loss of muscle tissue, meaning her percentage of body fat increased. By combining a low-carb diet with a program of exercises, "B" lost 10 lb. and her body fat dropped to 21%.

It's Important To Know Your Body Fat
It's not the amount of weight you have but the amount of body fat that's potentially dangerous to your health. Carrying too much body fat can increase your risk of developing serious health problems such as high blood pressure, high cholesterol, heart disease, diabetes, and cancer. Maintaining a healthy body fat percentage can reduce your risk and help prevent the onset of these conditions.

No comments:

Post a Comment