Body composition analysis using radionuclides

Abstract

Body composition studies were first recorded around the time of the Renaissance, and advances by the mid-twentieth century facilitated growth in the study of physiology, metabolism and different diseases. They are divided into two main groups, i.e. radionuclide and non-radionuclide methods. Simple methods based on body mass index (BMI), skinfolds and circumference measurements along with bioelectrical impedance analysis (BIA) technique are useful tools in everyday clinical practice, while more sophisticated non-radionuclide methods include underwater weighing (UWW), air-displacement plethysmography (ADP) and dual energy X-ray absorptiometry (DXA). In radionuclide methods, total body water (TBW) and total body potassium (TBK) are the two main traditional methods which were considered the 'reference' standard for body fat measurement. Computed tomography (CT), magnetic resonance imaging (MRI), positron-emission tomography (PET) scan and neutron activation analysis (NAA) are new methods which provide a detailed description of different tissues. Imaging methods (CT and MRI), in particular, opened a new era in body composition research by offering the possibility of qualitative and quantitative measurements of various tissues such as bone, skeletal mass, hepatic tissue and adipose tissue. CT and MRI estimates of visceral and ectopic adipose tissue using a single slice at L4-L5 level or multiple slices are considered the reference methods for assessing regional fat distributions. PET scan using specific radiopharmaceuticals can visualize active metabolic tissues like brown adipose tissue, which is found primarily in infants and has been considered to be essentially nonexistent and of no physiological relevance in adults. Additionally, this method can follow and provide direct depot-specific measurements of substrates uptake (e.g. insulin - stimulated glucose uptake) in various tissues and organs during insulin resistant states. Therefore, functional imaging with PET scan provides a tool for a better understanding of the physiology of obesity, type 2 diabetes mellitus and related metabolic disorders. This chapter provides an overview of the present status, mainly of radionuclide methods in everyday clinical practice for patients with obesity and related metabolic diseases, as well as in human research.