OTHER EXPLORATIONS
The body is mainly made of fat body mass and lean body mass (or non-fat mass). Lean mass is divided into body cells and extracellular mass (body water and non-aqueous mass). Composition varies with age and the sex of the individual.
Likewise, fat body mass is divided into subcutaneous fat and into visceral fat due to the higher incidence of cardiovascular and metabolic diseases (diabetes, hypertension, etc.) associated with a predominance of the latter.
Anthropometric measures for an individual's assessment (BMI, perimeters, folds, etc.) are easy to use; however, many times these measurements provide imprecise information. On occasions, a more thorough evaluation of the different body compartments is required. Technology has developed a series of techniques that permit a very accurate estimation of the water content in the body, fat body mass and lean body mass, as well as extracellular and intracellular volumes.
Computerised Axial Tomography
Computerised axial tomography (CAT) is based on images obtained from the attenuation produced by the different body tissues when they are radiated by successive X ray beams. CAT permits the reconstruction of images in cuts of approximately 10 mm. The area of total abdominal fat and visceral fat is calculated through this delimitation using a computerised pencil that allows quantification of the contents of each compartment. The main disadvantage is the high cost of this procedure and the use of ionising radiation.
Based on CAT images, equations have been established that allow us to calculate the total volume of fat and abdominal fat, when these parameters are combined with the aforementioned anthropometric parameters.
Nuclear Magnetic Resonance
As an alternative to CAT and to avoid ionising radiation, other methods have been evaluated to estimate the value of body fat and its distribution in the different compartments. One of these methods is nuclear magnetic resonance (NMR). NMR allows us to evaluate the amount of muscle and total abdominal fat, subcutaneous fat, and the different intra-abdominal compartments (retroperitoneal and intraperitoneal) by means of mathematical predictive formulas. As with CAT, the clinical use of NMR for nutritional purposes is low, and is only available for studies conducted within the scope of specialised research units.
Dual X-Ray Absorptiometry
Dual Energy X-ray Absorptiometry (DEXA) uses two sources of X rays with different energies. These X ray beams are attenuated in a different way by different body tissues. After the results obtained are interpreted by means of a computer programme, fat body and lean body mass are calculated, differentiating the latter from bone mass and non-skeletal lean mass. This same technique is capable of determining the degree of bone mineralisation. The utilisation of mathematical formulas which combine DEXA results with those of anthropometric measurements allow us to establish the amount of visceral fat. DEXA is easy to use, comfortable for the patient, does not vary depending on the observer performing the test, and the amount of radiation administered is small. As drawbacks for DEXA we have the expensive cost of the instruments used, and some discrepancy when results are compared against those from other very precise tests.
Dual Double Photon Absorptiometry (Dpa)
The fundamentals of dual double photon absorptiometry (DPA) are very similar to those used by DEXA, with the difference that the emission source is Gadolinium-153, which emits two different level energies. Initially, DPA was developed to assess mineral bone content although it can also be used as an adequate method to determine bone composition. Drawbacks include its high cost and that so far it has been unable to distinguish between fat and lean mass.
Impedancemetry
The technique is based on the fact that the different components of the body present a different resistance to the passage of an alternating current. The test involves the application of electrodes that emit a weak alternating current and receptors of the same residual current (once it goes through the body). Depending on a series of equations that take into account the length of the body and the result of the obtained current measurements, it is possible to obtain the percentage of the different components either direct or indirectly (fat mass, lean mass and total body water): Nonetheless, the drawbacks are that for the purpose of calculation, the body is considered as a perfect cylinder. It is known that the resistance to the current varies depending on the different tissues and both arms and legs contribute towards the total resistance. Because of this, the error margin when estimating the total amount of water and lean meat is about 2-3.5 L. Furthermore, in those situations in which there is an alteration of the water balance (for instance, oedemas or dehydration) errors in the calculation occur. Advanced impedancemetry techniques are able to estimate the percentage of fat mass, lean mass and water in the different anatomical areas (abdomen, arms, legs, etc. ).
Certain precautions such as the posture of the patient (a sitting down position is preferable), avoidance of recent food intake, etc, need to be observed to ensure the correct performance of this test. Despite this, this technique has become very popular because it is fast, simple and not excessively expensive.
Densitometry
The body's density results from the different density of each of its components (fat, muscle, viscera, lean mass, water) and also from the different proportion in which each of these components is found in the body. Thus, fat mass density is 0.9 g/ml, and lean mass density is 1.1 g/ml. Consequently, the greater the ratio if fat, the lower the total body's density.
To carry out these measurements, the individual is submerged in a water tank undressed. The displaced water is then measured, after doing some corrections to eliminate the abdominal and pulmonary air. It is a cost-effective technique with no risks. A drawback is that it does not take into account that fat mass may be of greater or lesser density depending on bone mineralisation of each subject (and thus an error margin of up to 3% in the calculation of the fat percentage), that it needs space and special infrastructures, and that it requires the patient's collaboration and therefore it cannot be used on children. It estimates the total amount of fat of the individual, but cannot differentiate among the different far compartments.
Dilution Techniques
These are complex techniques in which the individual is administered a certain amount of trace elements (such as deuterium, bromide, tritium oxide or ethanol). Concentration of these substances is then measured in serum, saliva or urine. The total water volume is then extrapolated with this datum, by means of mathematical equations. Since it is assumed that water makes up 73% of fat mass, substracting the lean mass from the patient's total weight (lean mass = amount of water obtained by dilution/0.73) yields the total amount of fat. The test can be performed in 2 to 3 hours, but the estimation error is approximately 3% and does not make a differentiation among the different fat mass comportments.
Uptake Of Fat-Soluble Inert Gases
This is one of the most costly and hard to use methods to determine fat mass. Different inert gases (such as krypton, xenon or cyclopropane) that are soluble in fat but not that much in water are used. The gas must be breathed in for several hours until a balance has been reached in the different tissues and exhaled once such balance has been accomplished. The proportion of gas retained by the body shows the amount of body fat.
Spectrophotometry
This technique aims to determine the thickness of the subcutaneous fat fold by emitting near infrared electromagnetic radiation aimed at the subcutaneous tissue of the arm and measures reflected and refracted energy. It is a simple non- aggressive method for the patient.
This test compensates the inaccuracy of measuring the subcutaneous fold with the lipocaliper, but is not very accurate to determine total body fat.
Total Body Potassium
Potassium is the major intracellular cation. The method consists in evaluating the amount of potassium present in the body (by measuring one potassium isotope, potassium 40, which emits a characteristic radiation and that is found in a constant percentage in total potassium). This test will thus allow us to measure the intracellular volume of water in the body. Assuming that 70% of fat-free mass is water, the body's cell mass can be calculated.
The results of this measurement may be altered by factors such as age, sex, or some conditions. Nonetheless, it is a very useful method, with the exception of obesity subjects with BMIs over 35 Kg/m2.