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Body Impedance Assessment (BIA)

There are several ways to determine your body fat percent. 

A few of the most feasible approaches are listed below.

Body Impedance Assessment (BIA): Welcome

Metropolitan Life Insurance Height-Weight Tables were originally developed by this insurance company to establish recommended weight ranges for men and women. The “desirable” weights were those associated with the lowest death rates among large population studies of insured people. Unfortunately, these studies do not accurately represent a cross-section of the entire American population.

Android or “apple-shaped” obese people are more vulnerable to disease than those who are gynic or “pear-shaped.”

Waist Measurement is an additional, independent 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 promote inflammation and may influence cholesterol and glucose metabolism. In men, a waist size of >40″ and in women >35″ is an indication of increased health risk.

Underwater Weighing when done accurately is considered one of the best ways of determining body fat composition.  However, it is impractical and not always down with the precision necessary to achieve useful results.  More often, in research underwater weighing is used as the reference standard to confirm the accuracy of the technique in question.

Bioelectrical Impedance is measured when a very small electrical signal carried by water and fluids is passed through the body. Impedance is greatest in fat tissue, which contains only 10-20% water, while fat-free mass, which contains 70-75% water, allows the signal to pass much more easily. By using the impedance measurements along with a person’s height and 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. Using BIA to estimate 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. In Dr. Kaslow’s office, we use two devices to measure body composition – The EIS and the Quantum unit from RJL Systems, the pioneer of bioelectrical impedance analysis (BIA) and related sciences since 1979. Since that time RJL has delivered more than 12,000 instruments to customers throughout the world. In addition, more than 1,000 abstracts and peer review articles have been written about BIA using RJL Systems instruments. RJL instruments set the standard for body composition.

The RJL Systems Quantum series assesses three compartments of your body (fat, fat-free cells and tissue, and water) and composition with accuracy and repeatability, which is a significant step beyond all other analyzers.  Most equipment only reports body composition in two compartments: fat and fat-free mass.

RJL instruments and software assess body composition, or in other words, the different structures or parts of your body. The BIA can measure and track changes in the amounts of body fluids, fat, and lean body mass, which includes your muscles and organs. The BIA distinguishes where the water is located in your body – either intracellular or extracellular. Functionally the BIA assesses how much of your body is acting as functioning cells (called the body cell mass or “BCM” on the report), transport tissue (extracellular mass or “ECM” on the report) or storage cells (fat).

  • Functional Mass is where metabolic work is done and calories are burned.

  • Transport Mass is tissue that transports nutrients and oxygen to cells and removes waste products.  It has also been called mesenchyme, the tissue matrix, etc.

  • Storage Mass is the fat where energy is stored.

Preparing for a RJL Quantum BIA:

If you haven’t had a BIA done before, it is important to be prepared.  The Quantum BIA sends a very minute electrical current throughout your body. Although, the electrical current is so small that it is undetectable, if you have a pacemaker or automatic implantable cardiac defibrillator (AICD), you should not have a BIA done with an EIS or RJL Quantum unit.

  • All metal jewelry should be removed. Although you won’t feel anything, metal removal improves accuracy.

  • Avoid exercise or other activity that would make you sweat at least 8 hours before your test. This is important to get accurate body fluid results.

  • Avoid caffeine or alcohol in large quantities 12 hours before the test.

  • Go to the bathroom before the test to get rid of any waste products. If you have diarrhea, it is important to tell Dr. Kaslow, as diarrhea can affect your body fluid results.

  • Your height and weight will be measured. Your height will only be taken the first time, but your weight will be measured each time you come in for a BIA. Since an accurate weight is important, remember to empty out your pockets, remove your shoes and any other heavy clothing.

  • You will be asked to remove your right shoe and sock and lie down on a table.

  • Two electrodes will be placed on your right hand and two more on your right foot.  If you have a lot of hair in these areas, you may feel some discomfort when the pads are removed.

  • Once the leads are hooked up to the electrodes, the test only takes a few seconds.

  • The BIA data will be input a computer and the results printed for review.

Understanding the BIA Results

There are several measurements of importance that are reported in the BIA results.

Phase Angle: is a measurement of your body’s overall health. Phase angle is based on total body resistance and reactance and is independent of height, weight and body fat. Lower phase angles appear to be consistent with either cell death or a breakdown of the cell membrane.  Higher phase angles appear to be consistent large quantities of intact cell membranes and body cell mass. As you would expect the phase angle is increased with an increase in body mass, even though obesity itself is not associated with good health.  All living substances have a phase angle.  In fresh uncooked vegetables the phase angle can exceed 45 degrees. In cooked vegetables phase angle is zero because they are dead.

Phase Angle is a predictor of outcome and indicates the course of disease or increases as the result of optimal health based on good nutrition and consistent exercise.  Usually, a phase angle of 6 or greater is desired for men and 5 or greater is desired for women.

As we get older our phase angle will decrease and will be approximately 4 or less when we die. Fit adolescents may have a phase angle greater than 10.  This effect is a result of cell integrity due to age. Low phase angles are consistent with:

  • Malnutrition

  • Infection (HIV/AIDS, bacteremia)

  • Chronic disease (cirrhosis, renal disease, pulmonary tuberculosis)

  • Cancer (most types)

  • Abusive life style

  • Chronic Alcoholism

  • Old Age (80 – 100 years)


  1. Ott, H. Fisher, H. Polat, E. B. Helm, M. Frenz, W. F. Caspary B. Lembcke“Bioelectrical Impedance Analysis as a Predictor of Survival in Patient with HIV Infection” J. of Acquired Immune Deficiency Syndromes and Human Retrovirology 9:20-25 1995

  2. Liedtke “Principles of Bioelectrical Impedance”

Body Mass Index (BMI):  is a ratio between weight and height. It is a mathematical formula that correlates somewhat with body fat.  If your BMI is high, you may have an increased risk of developing certain diseases, including:

  1. Hypertension

  2. Cardiovascular Disease

  3. Dyslipidemia (elevated cholesterol, triglycerides etc.)

  4. Adult-Onset Diabetes (Type II – insulin resistance)

  5. Sleep Apnea

  6. Osteoarthritis

  7. Female Infertility

BMI is a better predictor of disease risk than body weight alone. However, there are certain people who should not use BMI as the basis for estimating body fat content: competitive athletes and body builders, whose BMI is high due to a relatively larger amount of muscle, and women who are pregnant or lactating. Nor is it intended for use in growing children or in frail and sedentary elderly individuals.   People with BMI between 19-22 have been shown to live the longest.

The energy storage compartment, Fat Mass.

Fat is the energy storage mass of the body and is the total lipid mass (triglycerides) with a density of .9 g/ml. Fat mass is equal to actual weight minus fat free mass

The functional compartment, Body Cell Mass.

BCM is the functional mass of the body where work is done. All oxygen consumption, carbon dioxide production, glucose oxidation, protein synthesis and other metabolic work takes place within the body cell mass. The body cell mass is, in effect, the total mass of all the cellular elements in the body, and therefore, represents the metabolically active component of the body. In the normally nourished individual, muscle tissue accounts for approximately 60% of the body cell mass, organ tissue for 20% of body cell mass, with the remaining 20% made up of red cells and tissue cells. It also contains the majority of the body’s potassium, (98 – 99%).

The support compartment, Extracellular Mass.

ECM is the support mass of the body and is metabolically inactive, consumes no oxygen, produces no carbon dioxide and performs no work. The extracellular mass consists of extracellular fluids and tissue, such as bone and cartilage, with its primary function that of support and transport. ECM is located outside of the cellular compartment or outside of the body cell mass. Lean body mass is the sum of body cell mass and extracellular mass.


Fat: this is reported both as a percentage of your total weight and your actual body weight in pounds that is fat.  Normal values are based on age and gender. Just as you don’t want too much fat, you also want to avoid too little.  Having too little fat can cause your body to stop producing important hormones. For men, this means testosterone.

Body Cell Mass (BCM): this is also reported as a percentage of your total weight and as the actual pounds that make up cells that are active like your muscles and organs. The body cell mass are the cells that create your metabolism and energy.   They are what keep you healthy. When the body cell mass goes up, you are usually putting on muscle weight. But, when it goes down, you are losing muscle mass.

ECM: stands for extracellular mass. This number is the amount of your body weight in pounds that is made up of your skeleton and other support structures, as well as ECW.  It is composed of tissue that is between cells.  When this number changes, it is showing that there was a change in the ECW. This number does not show changes in bone density.

FFM:  describes the combination of all cells and tissues that are not fat, in other words, the FatFree Mass.

Impedance Index: is another measurement of your general health. Normal values for both men and women are greater than 1273. If your impedance index is more than 1273, you are at a better level of health and fitness than if it is below 1273.

Parallel capacitance:

All living things are made of cells.  Cells are membrane bounded compartments filled with a concentrated solution of chemicals and salts.  Groups of cells perform specialized functions and are linked by an intricate communications system.  The cell membrane maintains an ion concentration gradient between the intracellular and extracellular spaces.  This gradient creates an electrical potential difference across the membrane which is essential to cell survival.  Electrical gradients are necessary to support movement of oxygen, carbon dioxide, and nutrients.  Therefore, the cell membrane has electrically insulating qualities or capacitance.

Electrical capacitance will increase or decrease depending on the health and the number of cells. Damage to the cell membrane, and its functions, is as lethal to the cell as direct damage to the nucleus itself.

The cell membrane functions as a permeable barrier separating the intracellular (cytoplasm) and extracellular components. The lipid membrane is transversed by proteins, which are soluble in water thus making pores through which water, ions and other chemicals can enter and exit the cell.

Parallel capacitance is somewhat like phase angle, whereas it is not affected by weight or body fat. It is a measure of cell membrane health in all living substances and can change dramatically depending on disease or good health.  A body builder, for example, would have a high parallel capacitance and low resistance, or more cell volume, because he is extremely muscular and fit. A malnourished AIDS patient would have a low parallel capacitance.

Basal Metabolic Rate (BMR) is how many calories are burned at rest during the average day. The metabolic rate is determined by how many cells are producing oxidative energy.  The more cells, the more energy, and the higher the basal metabolic rate.  Thyroid, other hormones, medications, etc. all can affect the basal metabolic rate.  A low basal metabolic rate means that any calories you consume above your unique basal metabolic rate are unnecessary to supporting you and will be converted into storage (fat).  A low body temperature would occur if your rate of calorie burn is too low.   If you consume a lot more calories than you need, then you may have an excessive appetite problem, which can be due to a neurotransmitter imbalance.

Metabolism occurs in two distinct and interdependent phases: 1.) catabolism, in which the body breaks down food into its component parts and harvests the energy stored in its atomic bonds, and 2.) anabolism, in which those component parts and energy are used to build new tissues and conduct basic life functions. Basal Metabolic Rate (BMR) is the amount of energy your body requires every day to perform its most basic function including:

  1. Breathing

  2. Digesting

  3. Heart beating

  4. Muscle activity

  5. Transportation of fluids and tissue

  6. Circulation of blood

This is the amount of energy you would require if you laid in bed all day without ever moving a single muscle. Since most of us do a bit more than that, a daily activity level must also be factored in. This ranges from everyday activities to working out strenuously.  BMR varies between the sexes.  Lean body mass is a major determinant.  Because women tend to have less lean muscle mass, their BMR is lower than that of otherwise comparable males.  BMR is at peak during infancy, then declines rapidly through childhood and adolescence.  It continues to fall slowly with increasing age and decline further with old age largely due to a loss of muscle mass.  However, this is not inevitable, because weight-bearing (resistance) exercise will prevent or reverse muscle loss among the elderly.

Fluid Assessment: is described in the section called Total Body Water Compartments, which tells about the fluid level and distribution in your body.  The fluid is actually primarily water.  Your total body water (TBW) is measured in liters (L) and is made up of:

Intracellular Water (ICW): the fluid inside all of your body’s cells.  The cells of your muscles and organs (liver, kidney, brain, etc) contain more water than fat cells.  The closer to ideal your ICW, the greater the number of cells that contribute to your metabolism.

Extracellular Water (ECW) is the fluid that circulates outside your cells and throughout your body. This includes your blood, lymphatic tissue, and the ECM discussed previously. It should be noted that oxygen delivery to the cells is markedly reduced when there is excess extracellular water (edema).

Total Body Water (TBW):  If you are dehydrated or losing a lot of fluid, your total body water may be low. If you are retaining fluid or have an infection, your total body water may be elevated compared to ideal.

Water Compartmentalization is among the most valuable features of the BIA since it tells how much water is in the cell and how much is outside of the cell.  The water content inside the cell (“intra-cellular water”) is termed the “ICW” and the water content outside of cells (“extra-cellular water’) is termed the “ECW.”  The total body water content is called “TBW.”  These three values implicate:

  1. The functional integrity of the cell membrane, which is responsible in part for the electro-osmotic gradient across the cell membrane.The content of the intracellular environment must be in balance with the surrounding tissue in order to function properly in terms of cell nutrition and detoxification.

  2. The electrolyte content inside the cell.In order to hold the water inside the cell, electrolytes must be in sufficient quantity to provide the osmotic pressure for water to stay inside the cell.  Potassium is one of the major intra-cellular electrolytes whereas sodium is one of the major extra-cellular electrolytes.

  3. The fatty acid content of the cell membrane.If we did not have a lipid layer around every cell in our body, all of the water would leak out.  The fat repels the water (like oil and vinegar) and helps keep water within the cell.

  4. The amount of mesenchyme or tissue that has built up with water.Usually this represents toxicity since normally there should be only a modest amount of tissue separating the cells from each other and from blood vessels, nerves, etc.  Often times this reflect the amount of ECM discussed above.

Body Impedance Assessment (BIA): Welcome
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