I. Introduction

Adipose tissue plays a central role in the interplay between nutrition, energy balance, and human health. There are 2 types of adipose tissue, white and brown. White adipose tissue (WAT) stores energy, whereas brown adipose tissue (BAT) dissipates it. Overnutrition and/or physical inactivity result in an excess of WAT, the hallmark of obesity. In contrast, BAT is thermogenic, a property conferred by the presence of a unique protein, uncoupling protein 1 (UCP1). Located in the inner mitochondrial membrane, UCP1 uncouples mitochondrial respiration, releasing energy as heat. This unique property protects animals from hypothermia (1).
The traditional belief that BAT exists only in infants but not in adults has resulted in a paucity of research in humans. However, the discovery of fat with high metabolic activity in adults by functional imaging using positron emission tomography (PET) brought about a resurgence in research interest on BAT identity, abundance, prevalence, regulation, and significance in humans (2ñ9).
This review will cover 1) the characteristics and ontogeny of BAT, 2) its prevalence and regulation, 3) metabolic relevance, 4) the potential roles of BAT in health and diseases, and 5) the avenues for therapeutic targeting of BAT in obesity. These questions will be discussed on the background of known biology of BAT in rodents.