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Background
The prevalence of obesity in adolescents is high and increasing (1,2). Accurate assessment of fat mass (FM) and fat-free mass (FFM) in obese adolescents is necessary for establishing reachable goals for healthy weight loss and evaluation of treatment. One of the main objectives of obesity management is to reduce FM and to preserve FFM during weight loss. Especially in adolescents FFM changes will occur, and therefore weight change is less informative. Body composition (FFM and FM) can be assessed by several techniques such as underwater weighing, total body potassium, deuterium dilution and Dual Energy X-ray Absorptiometry (DXA). These methods are time-consuming, expensive; need trained operators and are hardly feasible in most dietetic settings (3,4). DXA is acknowledged as the standard (5) and most precise (6) method to assess body fat mass, although it can only be used in special settings and requires the use of a very low dose of radiation (7). Unlike other methods, DXA measures 3 components of body composition – bone mineral content, fat tissue mass, and lean tissue mass – as well as regional fat distribution.
In contrast to DXA, bioelectrical impedance analysis (BIA) is a commonly used, safe and simple, portable, non-invasive, inexpensive technique that needs minimal operator training, making it appropriate for use in daily clinical practice. The BIA method is based on the conduction of electrical current in the body and differences in electrical conductivity between the fat and water components of the body. The electrical resistance and reactance together with body weight and height can reliably estimate body composition. But, the results of the BIA is highly dependent on which FFM-BIA equation is used. In order to assess FFM with BIA, several FFM-BIA equations have been developed. Only a few FFM equations have been developed for obese adolescents (8-10). To the best of our knowledge no studies exist on validation of all available FFM equations in obese Caucasian adolescents. Because of their mean weight and BMI our study group was almost comparable with adults. This is the reason to include also BIA equations based on healthy and obese adults. As part of evidence-based practice, the aim of this study was to 1) examine the validity of published BIA- FFM equations, based on healthy and/or obese population (children and adults), for obese 11-18 year old adolescents using DXA as the reference method and 2) to develop a new FFM-BIA equation for obese adolescents.
Subjects and methods
Subjects
Adolescents were referred by their general practitioner or school doctor to the outpatient pediatric obesity clinic of the VU University Medical Center Amsterdam. At their first visit the paediatric-endocrinologist interviewed all adolescents concerning their medical history, weight development and ethnicity (11,12). The physical examination included height, weight, waist circumference,
Fat-free mass
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