One of the most precise and fast methods for assessing body composition
Bio-impedancemetry is a fast and precise method for assessing the body composition (CC) of human beings (1985 Lukaski).
Body composition
The analysis of body composition is used in various sectors, such as: medicine, anthropology, ergonomics, sports, auxology.
Recently, the specialists have channeled energy and resources into the deepening of the correlation between CC, health status and sports performance; it emerged that a body composition tending to be rich in adipose tissue (especially with abdominal distribution or even worse in intra-abdominal tissue), and poor in muscle mass, is correlated to a poor overall fitness (cardio-circulatory, respiratory, muscular, joint etc.), to a poor athletic-sporting ability and to a greater physical risk linked to inauspicious events such as hypertension, diabetes, obesity, dyslipidemia, metabolic syndrome, cardio-vascular complications, joint pathologies ... and DEATH DEATH.
Compartments
To deepen the knowledge of body composition it is necessary to have clear that the organism, from the compositional point of view, can be divided into compartments. There is no single classification and at least five can be described (modified later by Wang et al., 1992-1993-1995):
Basic model
- 2 compartments (fat mass / lean mass - FM / FFM)
Multi-compartmental models
- Atomic model - 4 compartments (carbon / hydrogen / oxygen / other elements)
- Molecular model - 4 compartments (water / fat / protein / minerals)
- Cell model - 4 compartments (cell mass / extracell solids / extracell fluids / fat).
- Functional model - 5 compartments (skeletal muscle / adipose tissue / bone / blood / other).
Modified in the years 1992-1993-1995 by Wang et al. In the following way:
Multi-compartmental models
- Elementary model - 5 compartments (carbon / hydrogen / oxygen / nitrogen / other elements)
- Molecular model - 5 compartments (water / fat / protein / minerals / glycogen )
- Cellular model - 5 compartments (cell mass / extracellular solids / extracell. Water / fat)
- Functional model - 4 compartments (skeletal muscle / adipose tissue / skeleton / visceral organs and residues ).
Body composition evaluation - analysis levels
The body structure must be considered as a growing organization of complexity; the various levels of analysis are: atoms, molecules, cells, tissues, organs, systems / apparatuses and finally organism (Body Whole - BW).
NB . Knowledge of the relationships between the different constituents at a certain level or between different levels is IMPORTANT for the INDIRECT estimate of a specific body compartment.
Whole body analysis - BW
The body can be considered as a single unit characterized by: DIMENSIONS, SHAPE, AREA AND SURFACE, DENSITY and OTHER EXTERNAL CHARACTERISTICS (weight, height, volume); in the BW analysis the atomic and cellular levels are of relative interest, therefore, the organization system is reduced mainly to the levels:
- Molecular - chemical
- Tissue - anatomical.
Methods: validity and accuracy
Validity is the degree to which an instrument or method actually measures what it says it measures; at the basis of validity lies the accuracy, or the precision of the measurement of a quantity whose real value is NOTO.
In the evaluation of the CC (therefore of the fat mass - FM) the validity levels are 3:
- 1st level - direct: dissection of corpses and extraction of fat with ether
- 2nd level - partially direct: measurement of "some" quantities by densitometry (DEXA) and subsequent quantitative relationship for the estimation of FM
- III ° level - indirect: detection of a measurement (such as a thickness or electrical resistance) and derivation of an equation regressed to the II level (in reality it would be better to define it doubly indirect).
Plicometry and bio-impedance are methods belonging to the third level of validity and therefore INDIRECT; they are HIGHLY "specific champions" since the relationship between fat and density depends on many variables such as: body hydration, body density, muscularity, compressibility and thickness of fat, fat distribution, amount of intra-abdominal fat.
Bioimpedentiometry - history
Bio-impedancemetry is based on the concept of bioelectric impedance, or the ratio between the amplitude of an alternating potential and the consequent amplitude of alternating current in a biological conductor .
The concept of bioelectrical impedance was deepened by Lukaski, in 1985:
Z = opposition of a biological conductor towards an alternating current
based on the studies:
- Impedance plethysmographic, concerning the electrical properties of cells, tissues and blood flow, carried out in 1959 by Nyboer, who concluded that the modifications of the conductive volume are associated with changes in the impedance of the conductor.
- Experimental on the invasive bipolar technique (subcutaneous electrodes hand-foot against lateral), Thomasset 1962.
- Further investigated by Hoffer (1969) who applied four skin electrodes
In the 1980s, monofrequency impedance (50KHz) was already in use for the CC evaluation, while in the following decade multi-frequency impedance meters were used to estimate the total body water compartmentalization (total body water - TBW): the XITRON, the first multi-frequency instrument for bioimpedance analysis.
Bioimpedentiometry - features and operation
Bioimpedance analysis is a method of evaluating indirect CC, a dependent sample but with numerous advantages and advantages; among these we recognize: speed of execution, ease of use, non-invasiveness, cheaper than DEXA (densitometry), conceivable both for the clinic and for field surveys (transportable).
The bioimpedentiometry measures the impedance offered by a body to the passage of an alternating electric current at low intensity (800µA) and fixed frequency; the lean tissues carry the fixed current more than the fat tissues since they contain a greater quantity of water and electrolytes. It follows that the conduction capacity is directly proportional to the quantity of water and electrolytes contained. Furthermore, TBW can be predicted by impedance (Z) since the electrolytes contained in the water are good conductors of electric current; if TBW is large, the current flows easily through the body with less resistance (R), which in itself appears inversely proportional to the lean mass (FFM). Logically, resistance is directly proportional (high) in individuals with greater amounts of adipose tissue because fat is a very bad conductor of current due to its low water content.
Bioimpedance analysis and body shapes
There is also a relationship between the opposition of a biological conductor towards an alternating current (Z) and the LENGTH and VOLUME of the conductor; the impedance (Z) to the current flow through the body is directly proportional to the length of the conductor (STATURE) and inversely proportional to the section, always taking into consideration that: impedance ( Z) = ƿ (resistivity) * [length (L) / section (A)] - where ƿ is equal to the specific RESISTIVITY of body tissues (constant).
Bioimpedance analysis and physical principles
- Biological tissues act as conductors or insulators and current flow follows a path of least resistance. The use of bio-impedancemetry to evaluate CC is based on different conductive and dielectric properties of biological tissues when the frequency referred to electric current varies; tissues that contain water and electrolytes such as cerebrospinal fluid, blood and muscles are good conductors, while fat, bone and air-filled spaces like the lungs are dielectric tissues. In the human body, the volume (V) of these tissues can be deduced from the measure of their resistance (R).
- Impedance is a function of resistance (R) and reactance (Xc): Z = R2 + Xc2
Impedance (Z) is the opposition dependent on the resistance of a conductor to the flow of an alternating electric current and can be broken down into two members: resistance (R) and reactance (Xc). Resistance (R) is the pure measure of opposition to the flow of electric current and is inverse to CONDUCTANCE. The reactance (Xc) is the opposition to the current flow caused by the body mass (MC) and is the reciprocal of the CAPACITY; in bioimpedance analysis, resistance (R) and impedance (Z) are interchangeable because the reactance (Xc) is very low (<4%). At 50Hz, resistance (R) is greater than reactance (Xc) so resistance (R) is the best predictor of impedance (Z).
The resistance index corresponds to: stature (S) 2 / resistance (R), while the best predictor of extra cellular water (ECW) is: stature ( H) 2 / reactance (Xc).
Resistance (R) between two points is defined by Ohm's law: resistance (R) = distance between two points (V) / current intensity (I).
As anticipated, for an isotropic cylindrical conductor, the resistance (R) is directly proportional to the length (L) and inversely proportional to its section (A), therefore, the specific resistivity ( ƿ ) of the trunk is 2 or 3 times higher than the resistivity ( ƿ ) of that of the extremities. Also the resistivity ( ƿ) of adults is greater than in children and the resistivity ( ƿ ) of the obese is greater than in normal weight.
Bioimpedentiometry - error factors
The "acceptable" error level for a CC analysis after bioimpedance analysis is <3.5kg for men and <2.5kg for women.
The level of accuracy and precision of the bioimpedance method is influenced above all by intra-instrumental variability (calibration) and inter-instrumental variability (different models).
In monofrequency impedance meters, the INTENSITY of the alternating current (800: 500 µA) can vary significantly even with the same 50KHz frequency, as well as the PREDICTION EQUATION (software diversity) and the type of CALIBRATION (internal or external).
Multi-frequency impedance meters have certainly higher prices than single frequency ones; they use a tri-frequency (5-50-100KHz) to measure resistance (R) and reactance (Xc), but are used above all in scientific research.
Ultimately, to obtain useful measures for the assessment of a person's CC, it is necessary to ALWAYS use the same instrument and ALWAYS CALIBRATE it before use. Better to use electrodes with a surface of 5cm 2 and arrange them in full body mode (distal / proximal).
It is also appropriate to specify that paraphysolophic conditions exist that can alter the detection of body composition. The first is the state of hydration; it has been observed that a state of solid and liquid fasting for at least 5 hours is able to modify the detection on the subject. Similarly, intense aerobic exercise can reduce resistance (R) due to imbalance between body electrolytes and total water; a ratio in favor of electrolytes with respect to water leads to greater conductivity. Body temperature also significantly affects bio-impedance detection; increasing it there is a reduction in resistance (R), therefore, with pyrexia or hyperthermia bio-impedance is NOT reliable. Finally, the skin on which the electrodes are applied increases its conductivity if cleaned with ethyl alcohol.
NB . Errors of 1 cm in the positioning of the electrodes in the body determines a modification of the detection of 2% of the total, as well as the environmental temperature <14 ° C compromises the estimate of the lean mass up to 2.2kg.
Benefits of bioimpedance with respect to plicometry
Both plicometry and bioimpedance geometry are indirect CC detection techniques and have the same degree of accuracy; however, sometimes it would be preferable to use a bioimpedance as it has some application advantages. Among these we mention:
- It does not require a high degree of manual skill and skill of the operator
- It is more comfortable
- It can be estimated for the evaluation of the obese and the bedridden
- It also evaluates the local CC
- Has the ability to evaluate ECW (extracellular water) and ICW (intracellular water)
In short: a good detection with bioimpedance analysis
To perform a correct bioimpedance measurement it is necessary:
- PROPOSE THE ELECTRODES PROPERLY (4 cm distance proximal red distal red)
- RECOGNIZE DEHYDRATION
- ASSESS THE IMPORTANCE OF THE PERFORMED PHYSICAL EXERCISE
- BUILD AN ENVIRONMENT OF A THERMICALLY SUITABLE DETECTION
- CLEAN THE CONDUCTING SURFACE
Furthermore, we remind you that in order to obtain reliable and repeatable data the subject must:
- BE FASTING FOR AT LEAST 4 HOURS
- BE ABSENT FROM THE PHYSICAL EXERCISE OF 12 HOURS
- HAVE THE EMPTY BLADDER
- BE ABOUT DRINKING FROM ALCOHOL FROM ATMENT 48 HOURS
- BE AVOIDING FROM DIURETICS FOR AT LEAST 7 DAYS
Wanting to be even more precise, let us remember that the pre-menstrual period in women determines a change in body balance and that the change in water and salt content in children requires the use of SPECIFIC predictive equations.
NB . According to some researchers the prediction accuracy with BIA can be improved by using:
- Eq. age-specific Lohman 1992
- Eq. race-specific Rising et al., 1991
- Eq. specific for level of adiposity Rye t al., 1988
- Eq. specific to the level of physical activity Houtkooper 1989
GENERALIZED EQUATIONS have been formulated which include AGE and SEX but it is also possible that OVERLAY THE FAT MASS IN INDIVIDUALS WITH LOW FATTY PERCENTAGE (the opposite of plicometry) AND SUBMIT THE FAT MASS IN HIGH PERCENTAGE PERSONS.
