Adipose tissue is a specialized connective tissue composed of lipid-rich cells called adipocytes. It constitutes around 20-25% of body weight in healthy individuals and is far more than a passive storage depot for fat. This dynamic tissue is metabolically active, playing a part in the body’s energy management and communication between different organ systems.
Types of Adipose Tissue
Not all fat in the body is identical; it is classified into distinct types based on structure and function. The most common form is white adipose tissue (WAT), which appears throughout the body. White adipocytes are characterized by a large, single lipid droplet that pushes the cell’s other components to the periphery. The primary purpose of this tissue is to serve as the body’s main energy reservoir.
A second type is brown adipose tissue (BAT), which has a different cellular structure and a distinct role. Brown adipocytes contain numerous smaller lipid droplets and a much higher concentration of mitochondria. These iron-rich mitochondria give the tissue its brown color and are responsible for generating heat, a process known as thermogenesis. BAT is most abundant in newborns, helping them regulate body temperature, and its proportion decreases with age.
A third category, known as beige or “brite” adipocytes, represents a transitional form of fat cell. These cells are found within WAT depots and can develop some characteristics of brown fat cells. Under certain conditions, such as exposure to cold, beige adipocytes can increase their number of mitochondria and begin to burn energy to produce heat.
Primary Functions of Adipose Tissue
Beyond the specialized function of brown fat, adipose tissue performs several foundational roles for the body. The most recognized function is energy storage. White adipose tissue efficiently stores excess calories from the diet in the form of triglycerides. When the body requires energy between meals or during physical exertion, hormones trigger the breakdown of these triglycerides, releasing fatty acids into the bloodstream for other tissues to use as fuel.
Another function of adipose tissue is providing thermal insulation. A layer of fat located just beneath the skin, known as subcutaneous fat, helps to protect the body from cold temperatures by reducing heat loss to the environment.
Adipose tissue also serves a protective role by cushioning organs and joints. Deposits of fat surround organs like the heart and kidneys, absorbing shock and protecting them from physical injury.
Adipose Tissue as an Endocrine Organ
In recent decades, scientific understanding has shifted to recognize adipose tissue as an endocrine organ. It produces and secretes a variety of bioactive molecules, including hormones known as adipokines. These hormones travel through the bloodstream to communicate with other organs like the brain, liver, and muscles, influencing a wide range of bodily processes.
One of the most studied of these hormones is leptin, often referred to as a satiety hormone. Leptin’s primary role is to signal the brain about the status of the body’s energy reserves. When fat stores are sufficient, leptin levels rise, which suppresses appetite and increases energy expenditure, helping to maintain energy balance.
Another hormone produced by fat cells is adiponectin. Adiponectin is involved in the regulation of glucose levels and the breakdown of fatty acids. It helps to improve the body’s sensitivity to insulin, the hormone that manages blood sugar. Higher levels of adiponectin are associated with a lower risk of developing insulin resistance.
Adipose tissue is also a significant site for the production of estrogen, a primary female sex hormone. While the ovaries are the main source of estrogen in premenopausal women, fat tissue becomes a producer of this hormone after menopause.
Location and Health Implications
The location of adipose tissue in the body has implications for an individual’s health. Fat is stored in two main compartments. Subcutaneous adipose tissue (SAT) is the fat found directly beneath the skin, which is the type you can pinch.
The second primary location is visceral adipose tissue (VAT), which is stored deeper within the abdominal cavity, surrounding organs such as the liver, pancreas, and intestines. Unlike subcutaneous fat, visceral fat is not visible from the outside. Its placement around organs makes it more metabolically active and influential on organ function.
An excessive accumulation of visceral fat is more detrimental to health than an abundance of subcutaneous fat. This is because VAT releases specific types of inflammatory cytokines and has a different hormonal profile. Its proximity to the portal vein means that fatty acids and inflammatory signals released from visceral fat travel directly to the liver, impacting blood lipid levels and insulin sensitivity. This is linked to an increased risk for a cluster of conditions known as metabolic syndrome, which includes type 2 diabetes, high blood pressure, and cardiovascular disease.