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| Understanding Body Fat Analysis | |||||||||||||||||||||
Body CompositionThere is growing evidence that clearly links body composition with health risks and the development of certain diseases. New research indicates that fat loss, not weight loss, can extend human longevity. Adding further to the acceptance of this practice is the importance of body composition in athletic performance and its move from being a laboratory-only procedure to one used in ordinary medical practice and now health clubs or at home. By measuring body composition, a person's health status can be more accurately assessed and the effects of both dietary and physical activity programs better directed. Most people don't realize that there is only one "direct" method of measuring body composition that is close to 100% accurate, and that an autopsy - performed Post Mortem. All other current methods for measuring body composition rely on "indirect" measurement techniques and are called In Vivo methods - meaning they are performed on a living body. In Vivo methods give estimates of percentage of body fat, fat-free mass, muscle, bone density, hydration, or other body components. Each method uses one or more measurable body component (such as Skin fold thickness, resistance, etc.) to make educated predictions about the other components. These predictions are based on years of research that define statistical relationships between different body components. According to the National Institutes of Health, no trial data exist to indicate that one method of measuring body fat is better than any other for following overweight and obese patients during treatment. Good results depend upon accurately taken measurements and an adequate, scientifically derived database. Every measurement method has strengths as well as defined sources of error. Most research studies employ several methods used in combination. Body composition equipment manufacturers should have scientific studies available to support accuracy claims, but often companies fail to explain the problems encountered in day-to-day use outside the controlled environment of a research lab. Tanita feels it is very important for people to fully understand the benefits - and limitations - of body composition analysis. This information will enable people to make better decisions about which method is the best or most appropriate for their particular needs. Body Composition ModelsThe more traditional methods are based on a two-compartment model that simply divides the body into fat and fat-free mass. Hydrodensitometry (underwater weighing) is based on the two-compartment model. Newer, more sophisticated techniques, such as Dual Energy X-ray Absorptiometry (DEXA), measure the body as multiple compartments. This approach improves the accuracy of the calculation for determining the real density of fat free mass. Reference ModelsOften referred to as "gold standards", these are clinical techniques that have been validated through repeated scientific studies and against which other clinical and field method results are evaluated. The two main reference models today are Hydrodensitometry and DEXA. Prediction EquationsIn Vivo methods use equations to predict percentage of body fat, fat-free mass, muscle, hydration, etc. Using a form of statistics known as multiple regressional analysis, this allows an unmeasurable component, such as body fat, to be predicted from one or more measured variable, where studies have proved there is a correlation. For example, calipers use external Skin fold measurements (a method that estimates fat found just under the skin) to calculate total body fat. BIA measures the body's impedance (resistance) to an electrical signal to estimate total body fat. Equations can be population-specific (developed for specific types of people, including such categories as gender, age, ethnicity, fitness level, disease, etc.) or generalized to cover a wide range of people types. A given equation is validated according to how well the results match the results of the reference method. It is important to note that results of reference methods themselves do not agree 100 percent. Therefore, when comparing different methods or products, you should consider which reference method was used and the appropriateness of both the method and particular product for the body type being analyzed. Basic Terms
Determining ObesityHeight-Weight Tables - were originally developed by insurance companies to establish recommended weight ranges for men and women. The "desirable" weights were those associated with the lowest mortality among large population studies of insured people. Unfortunately, these studies do not accurately represent a cross-section of the entire American population. Body Mass Index (BMI) - is a simple calculation that determines height to weight ratio. This index correlates a person's physical stature with mortality ratios based on actuarial studies. According to the National Institutes of Health and World Health Organization, overweight is defined as a BMI or 25-29.9, and obesity as a BMI equal to or greater than 30. A person with a BMI of 30 is about 30 pounds overweight/Over fat A BMI or 18 or lower indicates that a person is underweight/l While BMI is widely accepted, it can be misleading. Current guidelines do not differentiate for gender, ethnicity or age, and do not distinguish obesity or leanness for individuals who are extremely muscular. It is, however, more precise than height/weight tables and allows comparisons of population groups. Studies have confirmed that obesity-related health risks start in the BMI range of 25-30. Waist Measurement - Waist size is an additional, in dependant risk factor and can be used in conjunction with any other method. It reflects growing evidence that excess visceral fat - surrounding the abdominal organs - on its own increases the chance of heart disease or diabetes. Research indicates that visceral fat (waist size) is more important in the disease process than subcutaneous fat which is just under the skin.("love handles", "pinchable inches"). Abdominal fat cells appear to produce certain compounds that may influence cholesterol and glucose metabolism. A waist size of 35 inches or more is deemed a risk for people who have a BMI over 25. Calipers (Anthropometry, or Skin fold Measurements)Using hand-held calipers that exert a standard pressure, the Skin fold 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 3500 equations have been validated). This approach 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. Skin fold 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 Skin fold site measurement is made to reduce the time involved. Such a test should be used only for a rough estimate of obesity. Generally speaking, Skin fold 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 the home are usually less accurate than those used by an accredited caliper technician. The more obese the subject, the more difficult to "pinch" the Skin fold correctly, requiring even more skill to obtain an accurate measurement. The Dunk Tank (Hydrodensitometry, or 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 take with a chair or cot mounted on underwater scales, to a chair and scale suspended 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 weigh 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 than 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. Bioelectrical Impedance Analysis (BIA)Body impedance is measured when a small, safe electrical signal is passed through the body, carried by water and fluids. Impedance is greatest in fat tissue, which contains only 10-20% water, while fat-free mass, which conations 70-75% water, allows the signal to pass much more easily. By using the impedance measurements along with a person's height, weight and body type (gender, age, fitness level), it is possible to calculate the percentage of body fat, fat-free mass, hydration level, and other body composition values. Conventional BIA normally uses underwater weighing as its method of reference. Using BIA to estimate a person's body fat assumes that the body is within normal hydration ranges. When a person is dehydrated, the amount of fat tissue can be overestimated. Factors that can affect hydration include not drinking enough fluids, drinking too much caffeine or alcohol, exercising or eating just before measuring, certain prescription drugs or diuretics, illness, or a woman's menstrual cycle. Measuring under consistent conditions (proper hydration and same time of day) will yield best results with this method. Because BIA can be affected by body hydration, many professionals may use this method as a means of tracking the hydration status of their patients. This is especially important for athletes who are training or performing, as well as for the chronically ill. In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands or bare feet. Electrolyte gel is applied first, and then a current of 50 kHz is introduced. BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility and non-invasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important in determining the quality of the results. To minimize variables caused by a person's hydration level, measurements should always be taken under constant and controlled conditions. For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body. Tanita BIATanita has developed a simplified version of BIA that uses leg-to-leg bioimpedance analysis. In this system, two footpad electrodes (pressure contact) are incorporated to the platform of a precision electronic scale. A person's measurements are taken while in a standing position with the electrodes in contact with bare feet. The body fat monitor/analyzer automatically measures weight and then impedance. Computer software (a microprocessor) imbedded in the product uses the measured impedance, the subject's gender, height, fitness level and in some cases age, (which have been pre-programmed), and the weight to determine body fat percentage based on equation formulas. Tanita's reference method is DEXA. Through multiple regression analysis, Tanita has derived standard formulas to determine body fat percentage. Tanita's equations are generalized for standard adults, athletes and children. The Tanita method has all the advantages of traditional BIA as well as greater ease of use, speed, and portability. Professional versions of the product can be found in hospitals, health clubs and research labs and include computer printouts of comprehensive data such as BMI, fat percent, fat weight, total body water, fat-free mass, and BMR. The concept has been adapted for use as an affordable home monitoring device. Now ordinary people along with fitness enthusiasts and patients with health risks can measure body fat as part of a regular healthy lifestyle. The same variables apply with regard to hydration levels, and measuring should be done under consistent conditions. Near-Infrared Interactance (NIR)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 fibre 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. Dual Emery X-ray Absorptiometry (DEXA)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 non-invasive 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. Other Methods for Measuring Body CompositionMagnetic 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 uses 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 estimates body fat. The equipment is very expensive and limited in availability. *excerpted from the pamphlet "Understanding Body Fat Analysis", produced by the Tanita Corporation of America, c1999 |
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