C H A P T E R 1 nomic conditions or a negative development because of an increasing incidence of diseases related to obesity. Obesity can be defined as an overload of fat mass in the body. The degree of over-‐ weight is determined by the Body Mass Index (BMI) calculated by weight (kg) di-‐ vided by the square height (m2). Adolphe Quetelet, a Belgian statistician, introduced the BMI. Therefore the BMI is also called the Quetelet index6. The World Health Organization defines a BMI between 18.5 kg/m2 and 25 kg/m2 as healthy while overweight and obesity are defined as a BMI > 25 and BMI > 30 respectively. One speaks of morbid obesity in case of a BMI > 40 kg/m2. With regard to children the value of the BMI is age dependent and thereby the cut-‐off point of abnormal weight may differ from adults. The Ponderal Index (PI) is comparable to the BMI, but the mass is normalized with the third power of body height rather than the second power. It is used in infants because their body form is more cubic instead of oblong. Another approach is to depict on a graph weight versus height whereby the two measurements are used independent of age. It makes an estimation of weight more reliable in the case of extreme tall or small children. To date, often-‐used method is the measurement of waist circumference as an estimate of the abdominal fat mass. The measurement of skinfolds is applied to measure subcutaneous fat but is less accurate in obese children because of the restricted possibility of verification. It must be taken into consideration that the BMI does not make a difference be-‐ tween fat mass and fat-‐free mass. The fat-‐free mass represents all tissues except fat. From the fat-‐free mass only the muscle mass changes in relation to fat mass during the development of obesity. This difference can be estimated by measuring Total Body Water (TBW), the amount of water in the body. TBW is measured by the Deu-‐ terium dilution method or doubly labeled water (DLW) method. DLW is a the stable isotope of hydrogen and according to the Maastricht protocol7 the fat-‐free mass can be calculated by 퐶퐶!×푉푉! = 퐶퐶!×푉푉! C1 = concentration of the label in the ingested fluid V1 = volume of the dose C2 = concentration of the tracer in the sample V2 = distribution volume in the body Due to the exchange of the tracer with non-‐aqueous substances in the body, V2 has to be divided by 1.04 to determine TBW. To interpret total body water in individuals who differed in height, the results are corrected for height as TBW/height2. 12
Proefschrift binnenwerk Manon Ernst_DEF.indd
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