HOW IS PONDERAL OVERLOAD AND DISTRIBUTION OF BODY FAT DETERMINED?
Direct Methods:
The methods that stand out are imaging techniques (computerized axial tomography, magnetic resonance and dual absorpsiometric double photon x-ray), electric methods (impedanciometric) and physical techniques (densitometric, dilution techniques and capturing of liposoluble inert gases). They all permit a direct form of obtaining the quantity of body fat in an individual and, in some cases, obtain information about the distribution of this fat.
Computerized Axial Tomography and Nuclear Magnetic Resonance
Computerized Axial Tomography (CAT) is based on the images obtained from the attenuation produced by the different tissues of the body by means of successive x-rays. It permits the total reconstruction of the images in shorts, at approximately every 10mm. The area of total abdominal fat and visceral fat is calculated through a computerized pen, which allows for the calculation of the quantity of fat stored in each of the body's compartments. It is considered to be part of the evaluation process of all patients with abdominal obesity; however it does not make available comparative references and has the inconvenience of high cost and the use of ionised radiation. From the moment the images are done through CAT there are established equations that can calculate the total fat volume and abdominal fat, when combined with the anthropometric parameters discussed above.
An alternative to the CAT, with the objective of limiting ionised radiation, a method has been developed to estimate the value of body fat and its distribution through Nuclear Magnetic Resonance (NMR). NMR allows for the estimation of total body fat, subcutaneous and the different intra-abdominal compartments (retroperitoneal and intraperitoneal) through the use of predictive mathematical formulas. With the most modern techniques the time of exploration to obtain abdominal images is minimized. However, like with the CAT, its clinical use is uncommon and, only available in specialized studies of investigation.
Dual Absorptiometric X-Rays and Dual Absorptiometric Double Photon (DADP)
Dual Absorptiometric X-rays (DEXA) uses two sources of x-rays with different energies. These types of x-rays are seen differently in different tissues of the body. Through them, and after putting the results through a computer program, the fat mass and the muscle mass are estimated and are differentiate the latter into osseous mass and non equisetic muscle mass. This same technique is capable of determining the grade of osseous mineralisation. The use of mathematical formulas that combine the results of the DEXA and the anthropometric parameters allows for the establishment of the quantity of visceral fat. The DEXA is easy to have done, comfortable for the patient, the person who completes it can vary, and the quantity of radiation applied is minimal. The drawbacks of the DEXA are that the instrument used is expensive and in comparing the results, there have been discrepancies with other more precise techniques.
The Dual Absorptiometric Double Photon (DAP) uses a very similar basis to that of the DEXA, the difference being that the source of photon emission is the Gadolinio-153, which emits two energies at different levels. Initially the DAP was developed for the evaluation of osseous mineral levels, although it can also be used to find composition. Its drawbacks are its high price and, for now, is not capable of distinguishing between fat mass and muscle mass.
Electric Methods (Impedanciometry)
This technique is based on the fact that the different components of the body have a distinct resistance to alternate currents. To do the test, electrodes are emitted from a weak alternate current along with residual receptors of the same current (once they are in the body). After a series of equations that take into account the length of the body and the results of the measurements of the currents, it is capable of obtaining the percentage of the different components in a direct and indirect form (fat mass, muscle mass and total body water). However, a drawback is that the formula considers the body a perfect cylinder and it is known that resistance of currents varies depending on the tissue, arms as much as legs contribute to total resistance. For this reason, errors in the estimation of total body water and muscle mass are between 2-3.5 L. In addition, in situations in which the there is an alteration in the hydric or water equilibrium (for example oedemas or dehydration) errors are made in these calculations. The most advanced impedanciometry techniques are capable of estimating the percentage of fat mass and muscle mass in different anatomic areas (abdomen, arms, legs, etc.).
Certain precautions are needed to assure the safety of this test, including the posture of the patient (it is recommended that they are seated), avoidance of meals shortly before, etc. More than anything, because it is a fast and simple technique that is not excessively expensive and does not cause problems, it has become very popular.
Densitometry
The density of the body derives from the distinct densities of each of its components (fat, muscle, viscera and water) and the different proportions found in each one. Thus, the density of fat mass is 0.9 g/ml and of muscle mass is 1.1 g/ml. Therefore, when there is a greater proportion of fat in the body, the total density of the body is less.
To do this test the individual is submerged in a tank of water without clothing and the amount of displaced water is measured, after doing some adjustments to eliminate air in the abdomen and lungs. It is a low-cost technique that has no risk to the patient, but the drawbacks are that it does not take into account the fat mass, that can have more or less density depending on the mineralisation of the osseous of each subject (causing possible errors of up to 3% in the calculation of body fat). There also needs to be adequate space, special infrastructure and precise collaboration with the patient, making it impractical for children. It estimates the total quantity of fat in the individual, but is not capable of differentiating between the different compartments of fat.
Dilution Techniques
These are complex techniques using trace substances (like deuterium, bromine, tritium oxide or ethanol) that are administered to the patient. Concentration in the blood, saliva and urine is determined before administering the test. With this data, through mathematical equations, the total volume of water can be extrapolated. As it is assumed that water makes up 73% of the fat mass, subtracting the total weight of the muscle mass of the patient (muscle mass = the quantity of water obtained by dilution/0.73) shows the quantity of total fat. This test can be done in 2-3 hours, but the estimated possibility of error is approximately 3% and does not differentiate between the different fat mass compartments.
Capturing of liposoluble inert gases
This is one of the most costly and difficult methods used to determine fat mass. Different inert gases (like krypton, xenon or cyclopropane) that are fat, but not water-soluble are used. This gas is inhaled over various hours so that it reaches a balance in the tissues and is then exhaled once this balance is obtained. The proportion of gas retained in the body reflects the quantity of body fat.