Encuestas a los jueces de paz de Arequipa

Table 2.2 gives details of predictive and simple measurements of body composition, which have the potential to be used in field based studies. Predictive measurements include measurements of skinfold thickness and bio- electric impedance analysis, which are used to predict body composition using equations, but in doing so make a number of assumptions (Wells and Fewtrell 2006).

2.2.2.1 Skinfold Measurements

Although skinfold measurements are simple to conduct and require little equipment, one of the disadvantages with this measurement stems from the difficulty in ensuring that only fat and not underlying muscle are measured by the callipers. This measurement is more difficult to conduct in obese individuals, and therefore has poorer accuracy and reproducibility in this group. A further difficulty is in the use of prediction equations for percent fat which may not be valid in populations other than in whom they were derived, with errors in prediction in individuals being +9% fat (Wells and Fewtrell 2006). Thus the prediction of percentage fat from skinfold measures is both inaccurate and not reproducible in obese children, and is unsuitable for longitudinal comparisons.

Table 2.1 - Strengths and Weaknesses of Main Methods to Measure Body Composition (Part I)

Method Description /Principle Strengths Weaknesses

(a) Multicomponent models Multicomponent models

(i.e. three and four component models)

e.g. Three component model divides body weight into: fat, water, fat-free tissue, by measuring body water

(hydrometry) and body volume (densitometry).

(1) Most accurate approach – gold standard to compare other methods (2) Assumptions are minimised as measurements are made on hydration, density etc

(1) Special equipment -limited to research settings. (2) Expensive (b) Two-component techniques/models Density-based methods:- (1) Hydrodensitometry (underwater weighing), (2) Air-displacement plethysmography (volume of air displaced in chamber) (Bodpod)

If the density of a body is known (i.e. weight per unit volume), the proportion of fat mass (FM) & fat free mass (FFM) can be estimated using equations.

(1) Acceptable two component technique (i.e. divides body into FM and FFM)

(1) Hydrodensitometry requires submersion - unsuitable in children (2) Effects of disease on lean mass reduces its accuracy

(3) Special equipment -limited to research settings.

Computerised tomography (CT) or Magnetic Resonance Imaging (MRI)

X-rays (CT) or magnetic field (MRI) generates cross-sectional high resolution internal images from which volume of adipose tissue is estimated

(1) Allows estimation of regional body composition i.e. intra- abdominal adipose tissue

(1) Radiation exposure (CT) (2) Expensive

(3) Special equipment -limited to research settings

Dual-energy x-ray absorptiometry (DXA)

Transverse scans using low- energy x-ray, with beams differentially absorbed by various tissues.

(1) Two component technique (2) Quick and acceptable to children (3) Assess regional fat distribution as well as overall fat mass.

(1) Radiation exposure (2) Problems with accuracy

(3) Special equipment -limited to research settings

Table 2.2 - Strengths and Weaknesses of Main Methods to Measure Body Composition (Part II)

Method Description /Principle Strengths Weaknesses

(c) Predictive Measurements

Skinfold Measures Subcutaneous fat is measured

by callipers, often from many sites (peripheral & trunk areas).

(1) Simple measures of regional fat if use raw figures

(2) Equations available to predict % body fat

(3) Cheap, simple & portable

(1) Need to partially undress – may put some children off

(2) Poor reproducibility in obesity (3) Poor accuracy, magnified by using prediction equations not derived from the population under study

Bio-electric impedance analysis (BIA) Electrical currents pass more easily through body fluids in muscle & blood, but encounter resistance from fat (as contains little water). i.e. conductivity is proportional to body water (predicts body fat)

(1) Can be used in field, as analysers as portable.

(2) Simple, quick, non-invasive measure

(3) Relatively inexpensive (4) Reference data available for some measurement methods

(1) Less accurate than sophisticated measures

(2) Prediction equations incorporate assumptions which make them useful only in the population from which the equations were derived.

(3) Affected by hydration status (d) Simple Measurements

Waist Circumference Assumption is that waist is

proportional to central fat.

(1) Simple, quick measure of central (abdominal) fat

(2) Reference data available

(1) Not as accurate as measure of visceral fat

Body Mass Index Weight (Kg) / height (m2) –

used as an index of relative weight for height

(1) Simple and quick

(2) Reference data available, taking into account age and sex

(expressed as SDS or z-score)

(1) Not a measure of body composition i.e. cannot distinguish between fat and lean mass.

(2) For a given BMI in children, there is a large variation in ‘fatness’

2.2.2.2 Bio-Electric Impedance Analysis

Bio-electric Impedance Analysis (BIA) involves the passing of an electric current through the body and measuring its impedance. There is no one standard method for the placement of electrodes with options including electrodes manually placed on the wrist and ankle; standing on the electrode with bare feet with a hand-grip in each hand; or just standing on the electrodes (foot-to-foot). The foot-to-foot (or leg-to-leg) method measures lower body impedance only. This is the simplest method and is now incorporated into commercially available weighing scales.

The advantages of bio-electric impedance analysis are that it is a portable, simple, relatively inexpensive and non-invasive measure, and therefore suited for use in field settings (Table 2.2). Reference data are becoming available for percentage fat in children using bio-electric impedance analysis using the Tanita BC-418 MA Segmental Body Composition Analyser (McCarthy 2006). However, without a study cross calibrating these results to other scales these reference curves can only be used if the Tanita BC-418 MA analyser is used.

The disadvantages of bio-electric impedance analysis are that it is not as accurate as multi- and two-compartmental techniques (Table 2.2). Many assumptions are incorporated into the measurements and prediction equations for fat free mass and total body water, with the simplest ‘foot-to-foot’ method relying most on these assumptions (Wells and Fewtrell 2006). The measure of impedance is first of all adjusted for height and then total body water is

equations are required for children and adults. Furthermore, it has been shown that equations for the relationship between bioelectrical data and total body water developed in lean children are not applicable to obese children (Wabitsch et al 1996). Thus reference equations have been developed for obese children (Wabitsch et al 1996), though these equations are specific to the method of measurement (e.g. electrode site).

Validity studies that have examined BIA in children include a study by Tyrrell et al (2001) which compared foot-to-foot bio-electric impedance analysis with dual- energy x-ray absorptiometry in 82 children aged 4 to 10 using Bland-Altman plots. They concluded that BIA was an accurate technique to estimate fat free mass and percent body fat. Wabitsch et al (1996) also assessed the validity of bio-electric impedance analysis to detect changes in body composition over a 40 day diet and exercise programme in 146 obese 5 to 18 year olds, comparing it with total body water measured by deuterium dilution. The authors concluded that the bio-electric impedance analysis equation developed in the obese children provides an accurate prediction of total body water, although prediction of changes in total body water with a small amount of weight loss over time is not possible by bio-electric impedance analysis. However, the intervention lasted only 40 days and is therefore unlikely to give a reliable assessment of the value of BIA to detect changes over time in children. Some caution must also be exercised with both of these validity studies because both studies used two- component models as the comparator, whereas only multi-component models are now considered sufficiently accurate to act as a reference (Wells and Fewtrell 2006).

In a 12 week weight-loss programme in adults which resulted in a significant mean loss of weight of 9.9 kg, Jebb et al (2007) examined the validity of leg-to- leg BIA to detect changes in body fat, compared with multi-compartment models as the gold standard. Leg-to-leg bio-impedance was shown to be superior to both tetrapolar bio-electrical impedance and to skinfold thickness (i.e. other ‘simple’ measures of body fat) at detecting both increases and decreases of body fat. Its performance was similar to air-displacement plethysmography, dual-energy x-ray absorptiometry and deuterium dilution. The authors concluded that leg-to-leg bio-electric impedance analysis is a useful method to measure body composition changes in weight management programmes. However, this study was in adults aged 24 to 65, and thus caution must be used in the direct transfer of this finding to treatment programmes in children.

Wells and Fewtrell (2006) indicate in their review that bio-electric impedance analysis has the potential to provide information on the direction (though not magnitude) of change in fat free mass over time in children. This is based on two assumptions:- first, that electrode placement is consistent and, second, that body build is relatively constant over short periods of time in children.

2.2.3 Simple Measurements of Body Composition