Fat facts: An overview of adipose tissue and lipids

Abstract

The term fat evokes a multitude of ideas, images, and prejudices. It encompasses the different types of adipose tissue (AT) and cellular components as well as the myriad of lipid molecules. The AT and lipid molecules throughout the body carry out scores of vital functions ranging from thermal insulation to energy homeostasis to signal transduction. A fact that is not generally appreciated is that in addition to its roles in energy balance and thermoregulation, the AT is also an integral part of both the endocrine and immune systems. Fatty acids (FAs) are the primary building blocks of most lipids. They serve as fuel, structural components, and regulatory molecules (mediators). Most of the free FAs in the body are either obtained from the diet or released by the AT (lipolysis). However, most of the short-chain FAs such as propionate and butyrate are generated in the colon by the fermentation of dietary fiber by the gut microbiota. In addition to providing fuel for the colon enterocytes, these molecules act on specific G protein-coupled receptors in the gut cells stimulating the release of glucagon-like peptide-1 and peptide YY simultaneously improving insulin sensitivity and curbing appetite. The essential FAs linoleic and α-linolenic give rise to two distinct classes of omega FAs, n-6 and n-3, respectively, and hence to more complex lipid derivatives (eicosanoids) which are involved in virtually all aspects of cellular function including immunomodulation and inflammation. These include prostacyclins, thromboxanes, leukotrienes, and epoxyeicosatrienoic acids.