Body composition evaluation and bioimpedance analysis

By Dr. Davide Cacciola

Writing a training program is certainly not an easy task if you think about the fact that each person is unique and different from the others.

In fact, everyone responds differently to physical exercise because there are many factors that can influence ability and reaction to training stimuli, from the subjective response to training sessions and resilience, to lifestyle.

In light of these considerations, each training program should include an initial assessment of body composition, such as to provide detailed information on the level of physical fitness and nutritional status of the person who is to be trained.

In the case of weight loss, if we think of the body as a simplified model composed of lean mass and fat mass, it is good to be sure that the weight loss occurs in the fat part of our body and not in the lean. This simple example shows how important body composition analysis is.

For this purpose Bioimpedenziometria (BIA) is undoubtedly one of the most reliable methods and certainly the least invasive for assessing body composition, since it is based on a "tricompartmental" model.

The three-compartment model to which it refers is composed of:

  • Fat mass;
  • Cell mass;
  • Extracellular mass.

BIA is based on the principle that biological tissues behave as conductors, semiconductors or insulators. The intra and extracellular electrolytic solutions of lean tissues are excellent conductors, while bone and fat are insulators, and are not crossed by currents.

The body responds like an electrical circuit whenever electric currents flow through it. When a current is infused into the body, it flows through it more easily if it contains many body fluids, whereas when it meets the cell mass it encounters more resistance. The cells also function as capacitors for which they produce capacitance. We will thus have that a low frequency current applied to a fabric passes mainly through the extracellular fluids because, at low frequencies, the impedance of cell membranes is very high (therefore low frequency measurements provide information on extracellular water). With higher frequencies the current flows through all the fluids, extra and intracellular (higher frequencies provide information on intracellular water).

As anticipated, the fatty tissue is a bad conductor, it follows that the body impedance depends almost completely on the lean mass.

The test execution protocol requires the subject to lie down on his back. At this point the technician will place four electrodes, two on the hand and two on the foot and, activating the machine, will measure the resistance and reactance of his body.

Resistance (Rz) represents the ability of all biological structures to oppose the passage of electric current.

Grease-free fabrics, good conductors, are thus a way of low resistance, therefore ideal for the passage of current. The adipose tissues, bad conductors, instead represent a very resistive electrical pathway.

From this it can be deduced that a very fat subject with little total water represents a body with a high resistance compared to a muscular and thin subject.

Reactance (Xc), also known as capacitive resistance, is the force that opposes the passage of an electric current due to a capacitance, ie a capacitor. By definition, the condenser consists of two or more conductive plates separated from them by a layer of non-conductive or insulating material used to store electrical charges. In the human body the cell mass behaves like a condenser consisting of a membrane of non-conductive lipid material interposed between two layers of conductive protein molecules. Biologically the cell membrane functions as a selective permeable barrier that separates the extracellular fluids from the intracellular ones, protects the inner part of the cell allowing however the passage of some substances towards which it behaves as permeable material. It maintains the osmotic pressure and favors the establishment of an ionic concentration gradient between the intra and extracellular compartments. Reactance is therefore an indirect measure of intact cell membranes and is representative of cell mass. Therefore the determination of the reactance is fundamental for the determination of fat-free tissues.

Through a software supplied, from these two values ​​we obtain important parameters that I am going to describe below:

Phase angle (PA) : expresses the relationship between Reactance and Resistance, expresses the intra and extracellular proportions in the human body. The phase angle has shown to have a strong prognostic value in various chronic diseases.

Body water (TBW) and hydration: It is the largest part of the human body. If the subject is well hydrated, all other parameters are correct. In addition to determining the amount of water present in our body, the BIA determines its distribution inside and outside the cells: a correct hydration provides a distribution ranging from 38 to 45% in the extracellular spaces and from 55 to 62% in the intracellular space.

Lean mass (FFM): It is the resultant of the sum of the Cellular Mass (BCM) - the compartment that contains the tissue inside the cells, rich in potassium, which exchanges oxygen, which oxidizes glucose - with the Extracellular Mass (ECM) ), the part that includes the extra cellular tissues therefore the plasma, the interstitial fluids (the extracellular water), the trans-cellular water (cerebrospinal fluid, articular fluids), the tendons, the dermis, the collagen, the elastin and the skeleton.

Fat mass (FM): Expresses all the body fat that goes from the essential fat to the adipose tissue.

Sodium potassium exchange (Na / K): a very important value to verify the functionality of the cells.

Basal metabolism (BMR): means the minimum quantity of energy (heat) essential for the performance of vital functions, such as blood circulation, respiration, metabolic activity, thermoregulation. From this value the total metabolism can be derived through equations. As a result, training and nutritional programs can be developed that are much more precise and targeted.

Applications of bio-impedance analysis for training purposes

In summary, Bioimpedentiometry allows:

  • demonstrate that training and nutrition are actually losing fat, and not other more important tissues;
  • assess how much fat is in the body before starting a weight-loss program;
  • calculate the basal metabolism, the percentages of muscle and fat mass, in order to adapt training and nutrition;
  • exclude or assess the extent of any water retention states;
  • to verify if the total water in absolute value and in the intra and extracellular compartments remains stable, indicating a substantial water balance.

Above all, bio-impedance analysis makes it possible to demonstrate that it is not true that by exercising more than necessary more results can be obtained, that the weight trend is not constant and daily the water can vary greatly (resistance training for example brings significant changes of physiological parameters due to the noticeable sweating), that a weight loss is not synonymous with fat reduction (especially when it occurs in a short time), and that following an uncontrolled diet, the water and the protein mass vary first, that is the cell mass.

Therefore, any personal trainer should not prescribe training programs and food suggestions without knowing his student's body composition.


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