ACUERDO DEL CONSEJO GENERAL DEL INS- INS-TITUTO ELECTORAL DE QUERETARO,

In recent decades a dozen or more techniques have been developed which use forms of body imaging, digital technologies, and electromagnetic systems. It is not appropriate to consider them all here, but it is useful briefly to review their main applications, their advantages and disadvantages. In most instances they have been excluded from this research because of the practical difficulties associated with measuring a large number of individuals in laboratory conditions, because of the time and complexity required in using

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some of the instruments, because many such devices and techniques are available only in hospitals and research institutes, because some would be intrusive and intimidating for children, and because of very high costs.

2.6.6.1 Bioelectrical impedance analysis

BIA is a method by which body fat is estimated by passing a very small, safe current through the body – usually from one hand to the other, or by hand to foot. This process identifies differences in impedance resulting from the fact that fat and lean tissues have different degrees of electrical conductivity (Dehghan and Merchant, 2008). Impedance is low in lean tissue, where cellular fluids and electrolytes are primarily contained, but resistance if higher in fat. Thus, impedance is proportional to total body water. As with other methods of measuring fat, adjustments must be made to the raw data in order to take account of the age and height of the subjects as well as other characteristics and local factors. Published BIA equations are population specific.

Since the 1980s many electronic and digital BIA devices have been produced, and they have steadily been refined and improved so that now they offer a range of effective techniques for use by clinicians and researchers. Most are held in the hand or floor based, like bathroom scales. They offer a number of advantages; they are generally inexpensive, quite reliable (when correctly adjusted), and safe. They are also particularly useful for studies such as this: they can be used quickly in large epidemiological surveys, they are safe for children, and they are unobtrusive. As detailed elsewhere, these are the reasons one model was selected for use here (chapter 3).

42 2.6.6.2 Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) is an imaging technique used to obtain pictures of the anatomy by means of strong magnetic fields and radio waves; it is different from the use of X-rays, considered much safer and, in many cases, more useful as a diagnostic tool (Wells and Fewtrell, 2006). In regard to its use for estimating body fat, it measures the volume of adipose tissue rather than the mass since mass density can differ between people and within individuals. Although MRI provide high imaging quality, it is difficult to compare results with those obtained using other techniques (Duren et al., 2008). A major advantage of using MRI imaging over other techniques is its capability for estimation of regional body composition, and it is presently the only precise and viable approach for the estimation of intra-abdominal adipose tissue. Despite its usefulness and advantages, MRI would be quite inappropriate for a study such as described here: it is relatively costly, too slow for use with large population surveys, available only in major hospitals, very noisy, and unsuitable for researching young children.

2.6.6.3 Dual X-ray absorptiometry

DXA uses a very small dose of radiation to produce images of the inside of the body. Initially it was a procedure used for estimating bone density and bone mass, but over the past two decades it has been applied more broadly for surveying body composition; indeed, it is regarded as very effective for estimating body fat (Kakinami et al., 2014). It functions on the principle that X-rays passing through various body tissues decrease at different rates and therefore tissues of different densities can be distinguished. Moreover, by using X-rays at two different energy levels, better tissue-differentiation is possible compared with single energy systems. DXA has many uses as a diagnostic tool, but it would be quite inappropriate for this study: it is expensive, available only in major facilities, and any

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unnecessary use of X-rays (even in small doses) for researching young children would be unacceptable.

2.6.6.4 Hydro-densitometry

Hydrostatic weighing is a method for determining body composition by means of immersion in water. It is based on Archimedes Principle where the density of the body can be calculated using weight and volume (Density = Mass /Volume), and it is regarded as an accurate approach for estimating body composition (Duren et al., 2008, Demerath et al., 2002). Hydro-densitometry entails weighing the participant prior to immersion in water, during immersion, and by measuring the volume of water displaced. A high proportion of body fat makes an individual more buoyant because fat is less dense; lean mass is more dense than water and makes the person sink, thus individuals with high proportions of fat experience a greater weight difference when submerged. While this is regarded as an accurate approach it has some serious disadvantages which make it unsuitable for this project: it is a slow procedure, entails unclothed individuals being fully submerged, and requires access to immersion and measuring equipment which is not portable. Moreover, it is inappropriate for medium or large population-surveys.

2.6.6.5 Air-displacement plethysmography (ADP)

ADP is a method which is akin to hydro-densitometry insofar as it determines body composition by way of displacement – in this case displacement of air rather than water (Wells and Fewtrell, 2006, Demerath et al., 2002). In ADP the volume of the body can be measured indirectly by calculating the volume of air a person displaces when inside an enclosed chamber (plethysmograph). That is, the volume of the human body is measured from the displacement of the volume of air equivalent to their body volume. Body volume is simply measured by calculating the difference between volume of air remaining inside the

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chamber when the individual is inside from the volume of air in the chamber when it is empty. Since lean tissue is denser than fat, it is feasible to determine the lean-to-fat ratio from an individual’s density, and in order to calculate density it is essential to identify both weight and volume.

This approach is regarded a reliable research methodology, but in a population survey it would entail a considerable number of mathematical calculations. It has the advantages of being quick, comfortable, automated, non-invasive, and safe. However these tend to be outweighed by the disadvantages that it takes time, is costly, and non-transportable – most commercially-produced air-chamber ‘pods’ being located in research institutions.

2.6.6.6 Other methods of body measurement

There are several other techniques for ascertaining body composition, but most are unsuitable for a medium-sized population survey such as this: in general they are expensive or laboratory-based and located in major hospitals or research facilities. Furthermore, some entail the use of radio-active materials and some are invasive - thus being unsuitable for use with young children. They cannot all be described here, except to note that they include such research techniques as computerized axial tomography, measurement of total body water by isotope dilution, spectroscopy total-body potassium counting, in vivo neutron activation analysis, and total body electrical conductivity.