Adipose is a type of connective tissue found in animals and humans composed primarily of cells that store lipids known as adipocytes. In the healthy adult state, about 20% of the body is adipose. The core function of adipose is to store energy, but in different locations, additional functions arise. Parietal or subcutaneous (Latin for "under the skin") fat is one general location. Visceral fat is internal fat that exists within the body cavities (abdomen and pelvis, chest cavity) and surrounding internal organs. This divides adipose into two types based on where it is located anatomically. Adipocytes are also commonly referred to as "fat cells".  These cells are further categorized based on their cellular and functional characteristics.  White adipocytes are primarily involved in energy storage in the form of lipids, so the cells have very little cytoplasm as compared to lipids. Brown adipocytes are primarily involved in thermogenesis and so they have a rich vascular network, are more densely innervated, and the cells have the machinery necessary to generate heat by "burning fat". Specifically, Brown Adipose Tissue (BAT) adipocytes have large and plentiful mitochondria within each fat cell. This, plus the increased vascular tissue is what makes them look "brown" under the microscope. The mechanism by which brown adipose produces heat is very interesting. Their mitochondria express a specialized protein called uncoupling protein-1 (UCP-1). UCP-1 is a guanosine diphosphate binding protein that prevents protons from binding to phosphate in ATP. So, the mitochondrial electron transport chain burns fat and produces protons and instead of creating ATP, the protons have nowhere to go except into the mitochondrial matrix and the gradient energy is dissipated as heat. I call this the brown adipose proton fireball machine. It is technically called “non-shivering-thermogenesis” or NST, and in humans, the process is highly sensitive to thyroid hormone. 

Adipose was previously not considered an organ and was referred to as a “tissue”. All body structures including organs are composed of “tissue”. Tissue is basically a network of cells and structures that are organized in such a way as to maintain the livelihood of the tissue. Tissue therefore has cells, and where the cells reside is called the extracellular matrix. A good analogy is a house - the people are the cells, and the house is the matrix. Today, most would agree that Adipose is an organ as it has many functions including energy storage, heat production, insulation, metabolism, and hormone production.  Adipose tissue is composed of fat cells (adipocytes), “baby” fat cells (pre-adipocytes), fibroblasts, and stem cells. Blood must supply nutrients and oxygen to the tissue so vascular cells are of course present including endothelial cells and large vascular cell smooth muscle cells. Lymphatic cells also pass through adipose and mast cells and macrophages maintain a presence. The adipose is sensate and so unmyelinated nerves course through Adipose as well. The cells, vessels and nerves exist within a structure called the extracellular matrix composed structurally of strands or fibers of collagen, primarily type III. 

But, circling back to function, let’s discuss NST more deeply. NST can be divided into cold-induced-thermogenesis (CIT) or diet-induced-thermogenesis (DIT). DIT is a fascinating concept. Depending on whether you eat, how much you eat, and what you eat, DIT can be triggered to burn calories. This is the “missing link” that can significantly explain why some people are “blessed” with the ability to eat as much as they want and seemingly never gain weight, while others suffer the consequences of food intake and gain weight.  I prefer to call DIT the “post-prandial furnace” because the body basically burns the food you eat. This furnace (or DIT), has been studied extensively, and although BAT is known to be important in human temperature regulation only to the age of 2 years, the concepts of NST and DIT are being studied bigly, as they relate to weight regulation.
 

So in summary, fat itself is not bad. Adipose has many important functions and can be very useful. Fat is metabolically useful, it is cosmetically useful, it has comfort functions. Try sitting down with no buttocks fat. A face without fat is sunken and wrinkled. Hips without fat are boney. So we like fat, we want fat, we just don't want too much fat.

What we want is

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 ADIPAUCITY!