Innovations in Body Composition Using Dual X-ray Absorptiometry

Abstract

The body is composed of various tissues and the assessment of these tissues can be measured by various devices. Notably, the number of tissues or compartments measured vary by the device used to assess body composition. For example, a 2-compartment model separates the body tissues into fat mass and fat-free mass and can be measured by underwater weighing or air displacement plethysmography. While a 3-compartment model further separates the body into fat mass, lean mass, and bone mass. A 3-compartment model can be measured by dual X-ray absorptiometry (DXA). DXA allows for quick assessment of total as well as regional (arms, legs, trunk, etc.) body composition. Finally, a 4-compartment model separates the body into fat mass, lean mass, bone mass, and total body water. Due to more tissues or compartments being measured, the 4-compartment model is more accurate when compared to the 2- and 3-compartment models. However, a traditional 4-compartment model requires the use of multiple devices, ultimately making it costly and time consuming. Thus, using DXA to assess 3 of the 4 compartments in a 4-compartment model reduces the time of body composition assessment. In addition, DXA allows for exploration of the relationships between specific tissues throughout the body, specifically the relationship between muscle and bone. The purpose of this dissertation is two-fold: (1) to create total and regional DXA-derived volume and 4-compartment model, and (2) to explore the muscle-to-bone (MBR) and soft tissue-to-bone (SBR) ratio in children with obesity. This dissertation includes three manuscripts. The first manuscript examines the validity of the DXA for determining both total and regional volumes. These DXA-derived volumes were compared to a traditional method of measuring volume (e.g., water displacement and underwater weighing). The second manuscript explores DXA-derived total and regional volumes in a total and regional 4-compartment model for estimates of fat mass, fat-free mass, and percent fat. Finally, the third manuscript explores the MBR and SBR in children with obesity. In addition, this manuscript explores if MBR and SBR are associated with cardiometabolic risk factors. Collectively, this dissertation uses DXA in innovative ways to assess body composition in children and adults.