BMD is affected by the person’s size and tends to underestimate the concentration of minerals in bone in small individuals and overestimate it in larger individuals. BMD is a misnomer because the measurement is a two-dimensional, rather than a three-dimensional, unit that does not distinguish which of the two components of bone mass is being assessed, bone size (volume) or density. 26,27 DXA instruments measure the mass of mineral within a bone, that is, bone mineral content (BMC), and bone area (BA), then calculate the ratio of these terms as bone mineral density (BMD). Two potential errors in interpreting the DXA scans of children are the use of bone mineral density (BMD) or T-scores to report loss of bone mineral content. Because of their overall performance, DXA scans are accepted as a standard procedure in clinical practice. This technique is considered to be safe because the average radiation dose to the skin is only 1 to 3 mrem per scan. 24 In this method, the child is scanned with an x-ray source of two different energy levels, the difference in the absorption being proportional to the type of tissue scanned. 23ĭual-energy x-ray absorptiometry (DXA) can be used to estimate FFM, body fat, and bone mineral content of the whole body or regions such as the spine and hip. Ultrasonography, computerized axial tomography, and nuclear magnetic resonance imaging display a visual image of body fat and fat-free mass throughout the body, but the inaccuracy of some of these methods, the high level of radiation exposure, and the cost and maintenance of the equipment preclude their use in children. 22 The particular element being examined, such as nitrogen, can be identified by the characteristic energy of the electromagnetic radiation it emits and its decay rate. 21 Neutron activation analysis, a method that generates a known amount of radioactivity within a given mass from a defined dose of neutrons, characterizes the elemental composition of the body. Total body potassium counting, a method that measures the natural abundance of radioactive potassium ( 40K) in the body, assesses lean body mass based on the assumption that the potassium content of the FFM is proportional to tissue nitrogen content. 16,17 Total body water can be measured directly by isotope dilution using the stable isotopes of hydrogen ( 2H 2O) or oxygen (H 2 18O) 18 and indirectly by total body electrical conductance 19 or bioelectrical impedance analysis. Air displacement plethysmography is a density technique that measures body composition from body mass and volume within an air-filled chamber. Underwater weighing is a density technique based on the individual’s actual weight and the weight lost while under water. 14,15īody fat and lean body mass can be measured by densitometry or derived from total body water. Waist circumference 12 and waist-for-height 13 may serve as alternative indicators of adiposity because these measure correlate with truncal fat and obesity-related comorbidities. 10,11 Accurate estimates of body fat using anthropometry are difficult to obtain in morbidly overweight children or in children with generalized edema because of altered relationships between body tissue water and fat in these conditions. A method that uses multiple skinfold thickness measurements, such as biceps, triceps, subscapular, and suprailiac sites, may provide a better estimate of body fat and its regional distribution. 9 No single skinfold site is entirely representative of the combined subcutaneous and deep fat stores of the body. 9 The triceps skinfold thickness is most frequently selected as a measure of body fat because it can be compared with reference standards to determine the degree of overweight in children. The upper-arm muscle circumference (or area) serves as a measure of body protein stores because it can be compared with reference standards to determine the degree of muscle wasting in children. A metal tape measure and calibrated skinfold calipers are the only instruments needed. Claudia Conkin, in Pediatric Gastrointestinal and Liver Disease (Fourth Edition), 2011 Methods to Measure Body CompositionĪnthropometry may be used to characterize body composition in the clinical setting.
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