Bioelectrical impedance analysis
The bioelectrical impedance analysis ( BIA ) is used to determine the body composition of people and other living beings. It has spread widely since the appearance of the first commercial measuring devices in the 1980s, because the devices required for this are simpler, cheaper and more portable than other, more precise methods of determination.
More than 2000 publications in scientific journals have dealt with the BIA.
The analyzed body compartments are in detail:
- Fat mass (FM), fat mass
- Body water (TBW), Total body water
- Muscle mass , muscle
- Fat-free mass (FFM), fat free mass
- Body cell mass (BCM), body cell mass
- Extracellular mass (ECM), Extracellular mass
- Lean mass (LBM) lean body mass
The internationally agreed, scientific standardizations for the BIA require a defined measurement positioning. The person being measured lies back and relaxed in a horizontal position and the limbs are slightly angled from the torso. The placement of the adhesive electrodes must be carefully observed. Instruction and training of the measuring staff is therefore essential.
principle
The resistance (impedance, Z) of the body is measured with the constant signal of an alternating current of 0.8 mA at a frequency of 50 kHz. An electromagnetic field is built up in the body via two external electrodes . The voltage drop and the phase shift of the signal voltage are measured using two additional electrodes inside this field ( four-wire measurement ). The positioning of the electrodes on this "inner measuring section" must be carefully observed for a valid and repeatable BIA measurement.
The partial resistances of the alternating current resistance ( impedance ) are real resistance R (resistance) , capacitive reactance Xc ( reactance ) and inductive reactance Xl ( inductance ). The inductive reactance has no significance for the BIA. The resistances depend on the length and volume of the body and the composition of tissues and organs of the body with different conductivity. As electrolytes, the intra- and extracellular body fluids primarily define the resistance R. The cell membranes show capacitor effects, which can be explained by the structure and charges of the double-layer membranes, and determine the Xc component.
The BIA uses a single measurement to measure two values that show different biological properties:
- R: the effective resistance R analyzes the body fluid status
- Xc: the sum of all capacitive reactances (due to the uniform membrane of the cells) Xc gives an indication of the quantity, the body cell mass BCM and the quality of the body cells.
With only slight fluctuations, the fat-free mass (FFM - for a definition see section body compartment models ) consists of 74% water. A direct physiological connection between the resistance and the conductive compartments can thus be established. Fat tissue is one of the insulators, so it conducts poorly or has a high resistance (R). Healthy cell systems with their intact cell membranes generate a high capacitive resistance Xc. As a phenomenon, high Xc values indicate an intact energetic state of the cells, i.e. a good nutritional state. Malnutrition and illnesses show characteristic deficits, which are typically reflected in the relationship between the two resistances. This value is called the phase angle (pA). The higher the Xc portion of the total resistance Z, the greater the phase angle. Healthy, well-nourished, sporty and well-muscled bodies are characterized by a large phase angle. Illnesses and malnutrition and malnutrition as well as physical inactivity reduce the phase angle.
Changes in the body fluids in the distribution spaces can also be measured. Physiological and thus dynamic processes can also be represented with special BIA formulas (multiple regression equations ), such as the fluctuations in the body fluid content over the course of a day, which can be measured very precisely by changing the conductivity. The formula sets have been developed from studies with comparatively analyzed reference methods.
Devices for private use for bioelectrical impedance analysis such as body fat scales , which carry out measurements on the lower extremities and the lower trunk area, as well as hand-hand measuring devices, which supply the upper extremities and the upper trunk area with a measurement signal, do not meet the minimum requirements for a BIA . Body weight has a dominant function in the formulas of these devices. An increase in weight is defined as an increase in fat, and a reduction in weight is represented as a loss of fat, even if the main muscle has been reduced. This is why these devices are not recommended from a medical perspective.
Requirements for a correct body analysis using BIA:
- The BIA measurement can only be carried out with high-quality measurement technology (phase-sensitive).
- The caregiver must be trained.
- The horizontal positioning and the exact electrode configuration must be observed.
- The software must provide a universal, statistically secured formula set.
- Pathological situations can only be assessed by professionals using special formulas.
Normal values
The international initiative of the working group “AG Wissenschaff” has published its new, scientifically proven normal value tables for fat content and muscle mass with the project Body Analysis Normal Values, BIAdata . For the light-skinned European population, still called Caucasian in the English-speaking world despite the outdated race theory (often also valid for the white , non-Hispanic population of the USA), there are now statistically reliable normal values for the entire age range from birth to the age of 100 for the first time :
Body compartment models
There are four different body compartment models that can be determined using bioelectrical impedance analysis:
- 1-compartment model: This model contains only one compartment: the total body mass. It can be determined as body weight easily and precisely with a bathroom scale, but without the possibility of analyzing and evaluating the composition of the body in more detail.
The BMI also falls into this category. An assessment of “overweight”, defined as too much fat tissue, is therefore not possible. Well-developed muscles are also defined as being overweight. In the case of poorly developed muscles, on the other hand, excess fat tissue is often not recognized as being overweight.
- 2-compartment model: This model divides the organism into body fat (FM) and fat-free mass (FFM) (often also referred to synonymously as lean body mass, LBM), whereby the LBM still contains residual fats, such as the intramuscular fat deposits. The FFM is the rest of the mass extracted fat-free using ether. With measurements in this model, one compartment is determined directly and the other is calculated as the difference to the body weight.
- 3-compartment model: This model extends the 2-compartment model by dividing the FFM into body cell mass (BCM) and extracellular mass (ECM). The body cell mass is defined according to Moore as the sum of oxygen consuming, glucose oxidizing, potassium-rich and work-performing cells.
- 3-compartment model with ICW and ECW: In addition to the 3-compartment model, an identification of the components of the body water is introduced. These components are the intracellular water (ICW), which is part of the body's cells (BCM), and the extracellular water (ECW), which is located outside the cells and is therefore part of the ECM.
See also
literature
- Jörg Tomczak: Body analysis: The bioelectrical impedance analysis BIA . In: FIT science magazine of the German Sport University Cologne . tape 1 . ALPHA Informationsgesellschaft mbH, 2003, p. 34-40 ( PDF ).
- Ursula G. Kyle et al. : Bioelectrical impedance analysis part I: review of principles and methods . In: Clinical Nutrition . tape 23 , no. 14 , 2004, p. 1226-1243 , doi : 10.1016 / j.clnu.2004.06.004 ( PDF ).
- Ursula G. Kyle et al. : Bioelectrical impedance analysis part II: utilization in clinical practice . In: Clinical Nutrition . tape 23 , 2004, pp. 1430–1450 , doi : 10.1016 / j.clnu.2004.09.012 ( PDF ).