|Conclusion: Overall accuracy of the various methods is 3 – 6%, however in actual use calculations of an individual’s body fat can vary by as much as 10% due to differences in machines, methods, equations, variables, and the choice of reference method. BIA works well in healthy subjects with stable water and electrolytes balance and the appropriate equation with regard to age, sex and race. BIA can be used to follow body composition in persons with BMI 16-34, but must be interpreted with caution. Home bathroom scales that measure body fat (Tanita, Taylor) are not accurate for body fat, but can be used to monitor changes.
Bioelectrical Impedance (BIA) is a low-voltage, painless, alternating-current applied to the body. The resistance (R) of the body is a consequence of its length (L) and cross-sectional area (A), and how easily current can pass through the body’s tissue which is a function of the volume of water present in the cylinder (body). We will treat the body as one large cylinder; the conductive length of this cylinder is the length of the body from one electrode to the next. In order to measure the entire body we simply measure from the ankle to the wrist. However, it is much more convenient to use the height of the individual as the length of the cylinder instead measuring from ankle to wrist.
Fat has very, very little water, while lean tissue has approximately 73% water. It is the lean tissue (actually the water content of lean tissue) in the body that allows the current to pass and that is what we measure with BIA. Since we are trying to approximate a cylinder with the body, which is not a perfect cylinder, all the equations have some coefficient to try and match the body to a perfect cylinder.
PROBLEM 1: While the equations treat the body as one large cylinder, it is really several different cylinders of different length. The leg, torso, and arm. As the cross section of a cylinder increases, the resistance decreases. As a result, the arm and leg contribute a great deal more to resistance than the torso. In fact, the arms and legs contribute from 90% of the resistance while the torso is only 10%, yet the torso represents as much as 50% of the whole body mass.
PROBLEM 2: The torso represents 50% of body weight but only 3-10% of the body’s impedance. This means that impedance is more closely related to changes of the muscle mass of the limbs; changes in the muscle mass of the torso is not adequately represented; even large changes in the fluid volume in the abdominal cavity have only minor influence on the measured impedance.
PROBLEM 3: The equations use the height of the individual instead of the entire length between the ankle and wrist. A person with different ratio of arm/leg to torso relative to the average person used to derive the equation will have a different result.
PROBLEM 4: Use assume a constant hydration of FFM at 73%. Anything that changes that hydration will change the measured resistance. Such factors as dehydration, exercise and diuretics will effect the results. Even a full bladder will change the overall percent of water.
The cell membrane acts as an insulator against the current. At 0 kHz (impossible to do) no current will penetrate the cell and all the water measured is outside of the cell or extracellular. At infinite kHz (also impossible) the membrane does not insulate the fluid inside the cell (intracellular) and the resistance measures the entire body’s fluid, extracellular and intracellular. At various frequencies different amounts of intracellular or extracellular fluid is measured and could be important in monitoring certain disease states that effect fluid dynamics in the body.
METHODS OF BIA
Single frequency BIA (SF-BIA). Most BIA instruments use a single frequency at 50kHz which predominately measures the water outside of the cell (extracellular) and about 25% of the water inside the cells (intracellular). Various manufacturers have used hand to foot while some instruments use other locations such as hand to hand or food to foot.
Multi-frequency BIA (MF-BIA). Similar to SF-BIA, but multiple frequencies are used and measured. Some researchers report that MF-BIA is better at predicting extracellular fluid while SF-BIA was better at predicting total body water.
Bioelectrical spectroscopy (BIS). In contrast to MF-BIA, BIS uses mathematical modeling and mixture equations to generate relationships between R and body fluid compartments. The published equations seem accurate and reliable in healthy individuals but are wildly disparate in various disease states.
Segmental-BIA. This is done by adding two additional electrodes on the wrist and ankle of the opposite side (four electrodes total) or placing the two electrodes in various places to measure just the leg or arm or torso. There are several problems associated with this approach including new standardized equations
EQUATIONS AND MEASUREMENTS
The reproducibility of BIA is fairly good on the same day with results that are about 1-2% different. If the same individual is measured on different days or weeks the reproducibility is 2-3.5%. Overall reproducibility or precision is 2.7 – 4%. Prediction errors were estimated to be 3-8% for total body water and 3.5-6% for fat-free mass.
To maximize accuracy of readings, certain parameters must be maintained.
- Food – consumption of food and beverage can decrease impedance by 4-15 ohms over a 2-4 hour period representing an error < 3%.
- Exercise will decrease R by ~3% and Xc by ~8% immediately after and returns to normal in 1 hour.
- Lying down for 60 minutes has resulted in a 3% increase in R. This can result in errors of predicting TBW of 1-1.5 liters due to gravity pulling the water down.
- NO interference with pacemakers or defibrillators have been noted.
BIA has been shown to be valid in persons with BMIs up to 34 and changes in body fat or FFM over time can be monitored. Changes of less than 1.5 – 2 kg (3-4.5 pounds) are limited due to the limitations in BIA precision. In morbid obesity, most equations are unable to predict static body composition and are not reproducible for individuals over time.
If you are going to have a test at your doctor's office or local health spa, here is the test preparation protocol used at Formula Medical Group