Validation of Bioelectric Impedance Analysis (BIA) for estimation of

The high prevalence of paediatric overweight and obesity have led to several initiatives focussed on the need for better screening tools for childhood obesity as well as more effective strategies for accurate assessment of obesity and better management of its physiological, clinical and social consequences. Cardiovascular diseases (CVD) remain the commonest cause of death and disabilities in the UK (Scarborough, 2010) with

34 obesity as its major risk factor both in children and adults (Whincup et al, 2002). Ethnic differences in cardiovascular disease risk have been reported with increased risk and deaths observed in people of ethnic minority living in the UK (Nish, 2003). These risk factors which include high plasma triglycerides levels, increased insulin resistance and higher blood pressure levels, all of which are directly or indirectly linked to overweight and obesity, have been found even in children, contributing to an increased risk of suffering from CVD in later life (Whincup et al, 2002). Reports also show that the prevalence of type 2 diabetes, predisposed by overweight and obesity is now appearing in children and youth, which is higher in South Asian, Middle Eastern and black

children all from ethnic minority background (Haines et al, 2007; Ehtisham et al, 2005).

To date, body mass index (BMI) has been the most common way to rank body adiposity in the assessment of overweight and obesity. Although BMI correlates with adiposity, it does not adequately describe ethnic variability in terms of overweight and obesity. For example, for a given BMI, people of South Asian origin have been found to have higher body fat as well as insulin resistance compared with white Europeans (Dudeja et al. 2001, Deurenberg et al. 2002, Ehtisham et al. 2005). Even in children and adolescents, it has been shown that for the same age and sex a child can have a twofold increase of fat mass for the same BMI (Wells, 2000). Unlike Caucasian children, children of black descent have lower average fat mass at similar BMI levels compared to Asian children (Deurenberg et al, 1998).

Furthermore, BMI does not provide information on relative proportions of fat and lean mass in an individual. However, for the same weight and/or BMI, an increase of fat mass as well as its location in the body and a decrease of lean mass especially skeletal

35 muscle mass have been linked to high risk of developing cardiovascular diseases and type 2 diabetes (Barker 2005). Body weight and BMI do not reflect either body

composition or fat distribution and as such the use of BMI tables and charts despite their easy accessibility and simplicity as a main measure of overweight and obesity is not entirely acceptable.

For clinical purposes body composition measuring methods and tools must be simple, reliable, quick to administer and applicable to a wide variety of subjects. Skinfold- thickness and body circumference anthropometry have been used to ascertain body fatness for some populations and to predict body density by entering the data into multiple regression equations as in some National Health and Nutrition surveys

(Kushner et al, 1990). Unfortunately, the accuracy of skinfold-thickness anthropometry is limited in estimating body composition due to multiple technical errors, population specificity and biological variations (Lohman, 1981).

Bioelectrical impedance analysis (BIA) is one of the methods available for the estimation of body composition in ambulatory clinical populations. BIA is relatively cheap, simple, non-invasive, easy to use, rapid, portable, reliable and widely used for estimating of body composition (Houtkooper et al, 1996; Eisenkolbl et al, 2001). Its accuracy has been evaluated by studies which have demonstrated very good correlations between the BIA, total body water (TBW), fat-free mass (FFM, using

hydrodensitometry) and total body potassium in lean and obese adults as well as children (Schoeller et al, 1989; Houtkooper et al, 1989). BIA assumes the body as a series of cylinder of length equivalent to its height (HT). It works by passing a low level of electric current through the body to measure the impedance of conducting tissues.

36 Impedance factor (which is calculated as HT2/Z, where Z is the impedance produced by the BIA), is proportional to TBW as well as lean mass (Kushner, 1992; Schoeller, 2000).

Numerous studies have demonstrated that age, gender, ethnicity and extreme levels of fatness influence BIA estimates of body composition measurements (Bray et al. 2002, Lohman et al. 2000). BIA has been validated predominantly in white Europeans with minimal information on the accuracy of using these BIA prediction equations in other ethnic groups such as Asian and black populations, especially in children and

adolescents: hence the need to validate the BIA for specific ethnic groups to derive ethnic-specific equations for calculating body composition parameters accurately.

Several validation studies have been carried out in adults and children to assess the BIA system (Jebb et al, 2000; Jartti et al, 2000; Tyrrell et al, 2001; Sung et al, 2001; Goss et al, 2003; Parker et al, 2003; Pietrobelli et al, 2004). Previous studies have been limited by factors such as a wide age range, sample size, lack of criterion method for

comparison, lack of information on validity of BIA compared with DXA in young children and doubts over its accuracy (Lukaski and Siders, 2003; Tyrrell et al, 2001).

The goal of this study is to develop whole group and gender specific BIA equations to estimate body composition of black children and adolescents. DXA was used as the criterion method in this study. Although it is of limited availability, relatively expensive and time consuming, DXA is more suitable for use in children because the radiation exposure is minimal and it has no discomfort. It is non-invasive and has excellent reproducibility of measurements (Svendsen et al, 1993; Svendsen et al, 1990). In

37 addition, studies have confirmed that body composition measurements by DXA are accurate and precise (Lohman and Chen, 2005). Magnetic resonance imaging and computed tomography are also considered useful criterion methods which provide visible images of adipose tissues and have been directly validated against cadaver body composition analyses (Ross and Janssen, 2005). They can also distinguish between intra-abdominal and subcutaneous adipose tissue. In the past, a major limitation of validation of BIA in children and adolescents studies has been the use of criterion methods with invalid assumptions of chemical constancy leading to inflation of the error in the criterion (Lohman, 1992; Going et al, 2006). Using DXA reduces error due to criterion method since it is based on a three-compartment model of body composition requiring fewer assumptions than methods based on two-compartments. Furthermore the adoption of DXA as a criterion method is justified by the fact that it has been successfully validated against multi-compartment models (Lohman and Chen, 2005) and chemical analysis of animal models (Pintauro et al, 1996).