Blood Spotlight Blood spotlight on leukocytes and obesity

The obesity epidemic, which started in the industrialized world, has progressed to become a worldwide pandemic in which the number of obese individuals worldwide now exceeds those who are malnourished. Unlike other pandemics, however, obesity does not cause disease directly but instead functions as a potent independent risk factor for a number of chronic pathologies, including type 2 diabetes, coronary artery disease, neurodegenerative disorders, and cancer, wreaking immense harm in both health and economic terms. Despite such infamy, our understanding of obesity remains inadequate. The most overt effect of obesity is an increase in fat mass, which results from chronic imbalance between energy intake and energy expenditure. In rodents and humans, these excess calories are primarily stored in visceral (also referred to as gonadal in rodents) and subcutaneous white adipose tissues (WATs), the preferred sites for long-term nutrient storage in mammals. This observation initially led many to viewWAT simply as a depot for excess nutrient storage; however, research over the past 2 decades has revealed that WAT is not an inert organ but rather a dynamic endocrine tissue that secretes adipokines (such as leptin, adiponectin, and resistin) to coordinate changes in nutrient intake, utilization, and storage. Importantly, this active coordination of organism-wide metabolism is mediated not only by canonical neuroendocrine control but also by leukocytes that traffic to and reside in metabolic tissues. In this review, we review the salient contributions of individual leukocyte lineages to the function and control of adipose tissue and mammalian energy metabolism as a whole.