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Chapter 2
Randomisation procedure
All participants will be randomly assigned to the intervention or control group, using SPSS for a random selection of a sample. Randomisation was stratified by age group (12-14 years, 15-18 years) and gender. Randomisation took place after the baseline measurements.
Sample size calculation
The sample size calculation is based on a difference in BMIsds of 0.29 (10%) between the intervention and control group after 18 months. The standard deviation for this BMIsds is considered to be 0.5. Based on these data, in an intention-to-treat analysis, 43 subjects per group are needed to detect this difference in mean BMIsds with an alpha of .05 and a power of .80. A sample of 108 persons (n=54 per group) is required taking into account a dropout rate of 25% (5,11).
Measurements
Primary outcome measures of the randomised trial was BMIsds. Secondary outcome measures were glucose tolerance, dietary behaviour, physical activity, sedentary behaviour and self-esteem. Data were collected at baseline and after 6 and 18 months intervention. All measurements were performed after an overnight fast, except for the Dual Energy X-ray Absorptiometry (DXA).
Body composition
- Height, weight and waist circumference
Height was measured and recorded with an accuracy of 1 mm with an electronic stadiometer (KERN 250D). Body weight was measured (in underwear) and recorded within 0.1 kg with a calibrated electronic flat scale (SECA 861). Weight and height were used to calculate BMI (weight in kilograms divided by the square of height in meters). The degree of overweight was quantified using Cole's least mean square method, which normalized the BMI’s skewed distribution and expressed BMI as BMIsds (15). Reference data for Dutch children were used (www.growthanalyser.org; versie 3.5). The BMIsds indicates how many standard deviations a measurement is above or below the median of the distribution.
- Bio-electrical impedance spectroscopy
After an overnight fast, bio-electrical impedance spectroscopy (BIS) measurements were performed on the non-dominant side of the subject, using a Hydra ECF/ICF Bio-Impedance Spectrum Analyzer, model 4200 (Xitron Technologies, San Diego, CA, USA). Shoes and socks were removed and the subjects were in a supine position. Two current electrodes (tetra-polar electrodes (3M Red Dot AG/AgCl)) were placed on the dorsal surfaces of the hand and foot on the distal portion of the second metacarpal and metatarsal,
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