Adipose tissue

Adipose tissue, commonly known as body fat, is a specialized type of loose connective tissue. It is primarily composed of cells called adipocytes, which act as storage units for energy.

However, the tissue is more complex than just fat cells. It contains a group of various cells called the stromal vascular fraction (SVF). This fraction includes preadipocytes, which are cells that can become fat cells, and fibroblasts. It also contains vascular endothelial cells that form blood vessels, and immune cells like adipose tissue macrophages. While it was once thought to be hormonally inert, meaning it did not send signals, scientists now recognize it as a major endocrine organ. This means it produces essential hormones like leptin, estrogen, resistin, and cytokines.

The way adipose tissue manages energy is a continuous, regulated process. The tissue stores energy in the form of lipids, which are fats. To manage these fats, the body uses a process involving free fatty acids (FFAs). These FFAs are released from lipoproteins by an enzyme called lipoprotein lipase (LPL). Once inside the adipocyte, the FFAs are reassembled into triglycerides. This reassembly happens by attaching the FFAs to glycerol through a process called esterification.

The movement of these fats in and out of the tissue is a constant flux. This flux is controlled by the hormones insulin and leptin. When insulin levels are elevated, there is a net inward movement of FFAs into the cells. The FFAs can only leave the adipose tissue when insulin levels are low.

There are two distinct functional types of adipose tissue: white and brown. White adipose tissue (WAT) is the primary site for storing energy. It also helps protect the body from excess glucose by storing triglycerides produced by the liver. Brown adipose tissue (BAT) serves a different purpose by generating body heat.

These tissues are located in specific areas known as adipose depots. In humans, these depots include subcutaneous fat, which sits just beneath the skin in the hypodermis. It provides insulation against heat and cold. Another major type is visceral fat, which is located deep inside the abdominal cavity. This fat is packed between internal organs like the stomach, liver, and intestines.

History shows that our understanding of these tissues has evolved significantly. The Swiss naturalist Conrad Gessner first identified brown adipose tissue in 1551. For centuries, the scientific view was that fat was simply a passive storage depot. Modern research has changed this by revealing the active role of adipose tissue in the endocrine system. We now understand how it communicates with the brain to provide feedback regarding hunger and diet. For example, in mice, a lack of the hormone leptin can cause a mouse to feel constant hunger. This leads to excessive eating and eventually obesity.

The location of fat depots has major implications for human health. Subcutaneous fat, often found in the hips, thighs, and buttocks, is generally considered to pose a lower health risk. In contrast, an excess of visceral fat is linked to serious conditions. This includes type 2 diabetes, insulin resistance, and cardiovascular diseases like atherosclerosis.

Excess visceral fat is often called abdominal obesity or "belly fat." Men are often more likely to store fat in the abdomen due to sex hormone differences. For instance, estrogen helps women store fat in the hips and thighs. When estrogen levels decline during menopause, fat often migrates toward the waist.

Scientists use various methods to study and measure these fat stores. To estimate body density, researchers have used equations from Durnin and Wormersley based on skinfold measurements. While these are only estimates, they help model the relationship between subcutaneous fat and total body fat. For abdominal fat, new developments like the Body Volume Index (BVI) help measure abdominal volume specifically.

Another important area of study is marrow adipose tissue (MAT). This is a poorly understood depot located within the bone. It is interspersed with bone and hematopoietic cells. Interestingly, MAT increases during calorie restriction, which is different from how other fat depots behave.

Finally, it is important to distinguish between healthy fat storage and ectopic fat. Ectopic fat occurs when triglycerides are stored in tissues that are not meant to hold them. This includes the liver, skeletal muscle, heart, and pancreas. This accumulation can interfere with how these organs function and is associated with insulin resistance. While visceral fat is a specific depot in the abdomen, ectopic fat is the misplaced storage of fat within other organs. Managing total body fat through diet and exercise can help reduce both visceral and ectopic fat stores, improving overall organ function.