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Chapter 7
blood pressure and pubertal Tanner stage (13). Subjects were eligible when they met the following inclusion criteria: 1) age of 11-18 years; 2) obesity according to the definition of Cole et al. (14). Cole et al provide international cut off points for body mass index for overweight and obesity by sex in childhood (2-18 years), based on international data and linked to the widely accepted adult cut off points of a body mass index of 25 and 30 kg/m2. Exclusion criteria were: not Dutch-speaking, obesity as a result of a known syndrome or organic cause (hypothyroidism), developmentally delayed , physical limitations (e.g. wheelchair dependent) and diagnosed type 2 diabetes mellitus. Subjects were measured between November 2006 and August 2008.
The medical ethical committee of VU University Medical Center approved the study protocol. Adolescents, as well as their parents, gave written informed consent.
Anthropometrics
All measurements were performed using the same protocol. After an overnight fast, the subjects attended the outpatient clinic. Height was measured with an accuracy of 0.1 cm with an electronic stadiometer (KERN250D, De Grood Metaaltechniek, Nijmegen, the Netherlands). Body weight (WT) was measured (in underwear) within 0.1 kg with a calibrated electronic flat scale (SECA861, Schinkel, Nieuwegein, the Netherlands). Weight and height were used to calculate BMI (weight in kilograms divided by the square of height in meters). For the body mass index standard deviation score (BMIsds) reference data of Dutch children collected in 1997, were used (www.growthanalyser.org; version 3.5) and the sds, or z-score, calculated. The BMIsds indicates how many standard deviations a measurement is above or below the mean of the normal distribution.
Dual Energy X-ray Absorptiometry
After measuring height and weight, a full body scan was performed by Dual- Energy X-ray Absorptiometry (DXA; Hologic QDR4500-Delphi, software 12.3.3. S/N 45665, Tromp Medical, Castricum, the Netherlands). DXA is based on the measurement of the attenuation of a collimated x-ray beam from a source with 2 energies passing through the body. Subjects (in the fasting state) were scanned for 10 minutes in underwear while lying in the supine position. The DXA method assessed, total body weight (WTdxa), fat mass (FM) and fat-free mass (FFM) defined as lean tissue mass including bone mineral content.
Bioelectrical impedance analyzes (BIA)
On the same morning as the DXA and also in the fasting state a BIA measurement was performed. Shoes and socks were removed, and the subjects were in a supine position. The BIA measurements were carried out on the non- dominant side of the subject, using a Hydra ECF/ICF Bio-Impedance Spectrum
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