ABSTRACT
Adipose tissue is composed of at least three different types of depot (i.e., brown, beige, and white), which can differ with the stage of development and anatomical location in individuals and between species [1]. In addition, the distribution and composition of adipose tissue changes throughout the life span of individuals (Figure 22.1). Generally, the most abundant fat is white adipose tissue, which comprises ~95% of fat mass in children and adults, with obesity being accompanied with an increased number and size of white adipocytes [2]. It not only serves as an energy reserve in the form of lipid but also can act as an important endocrine organ with a number of roles, of which the best documented is appetite regulation via the release of leptin [3]. Brown adipose tissue (BAT) is only ever present in comparatively small quantities and, even in the newborn, when it is most abundant, it comprises just up to ~4% of total body weight [4]. However, as a consequence of its unique location(s), protein composition (including a substantial mitochondrial component), and high rate of blood supply, it is capable of using/dissipating exceptional amounts of energy and releasing heat when maximally stimulated [5]. This capacity is rarely, if ever, reached again in later life [6].
