Fatty tissue, known medically as adipose tissue, is a type of connective tissue made up of specialized cells that store energy in the form of fat. It sits beneath your skin, surrounds your internal organs, and plays an active role in metabolism, hormone production, and temperature regulation. Far from being inert padding, fatty tissue functions as one of the largest endocrine organs in your body.
What Fatty Tissue Is Made Of
The primary cells in fatty tissue are called adipocytes, or fat cells. White fat cells have a simple structure: each one contains a single large droplet of stored fat, with minimal internal machinery. But fat cells aren’t the only residents. Fatty tissue also contains nerve cells, blood vessels, immune cells, and a supporting cast of structural cells collectively known as the stromal vascular fraction. This mix of cell types is what allows fatty tissue to do far more than just store calories.
Three Types of Fat Cells
Your body contains three distinct types of fat, each with different jobs.
White fat is by far the most abundant. It stores energy as triglycerides (a type of fat molecule), cushions your organs, and releases hormones that influence appetite and metabolism. When people talk about body fat, they’re almost always talking about white fat.
Brown fat generates heat. Brown fat cells look different under a microscope: instead of one big fat droplet, they contain many smaller droplets and are packed with mitochondria, the structures inside cells that produce energy. Those mitochondria burn fat to produce warmth, which is why brown fat tends to cluster around the body’s core, along the spine, and near vital organs. Babies have relatively large amounts of brown fat. Adults retain some, though far less.
Beige fat is a newer discovery. These cells live within white fat deposits but can switch on heat-producing functions similar to brown fat, particularly in response to cold exposure or exercise. Both brown and beige fat play a role in regulating blood sugar through mechanisms that go beyond simple calorie burning.
Where Your Body Stores Fat
Fatty tissue is distributed across two main zones, and where your fat lives matters more for health than how much you carry overall.
Subcutaneous fat sits just beneath the skin, most noticeably in the upper arms, thighs, buttocks, and belly. In healthy individuals, roughly 80% of total body fat is subcutaneous. This layer protects muscles and bones from impact, provides insulation against cold, stores energy for later use, and anchors your skin to the deeper tissues underneath. It’s the fat you can pinch.
Visceral fat is stored deeper inside the body, packed between and around internal organs like the liver and intestines. A small amount of visceral fat cushions those organs, but excess visceral fat is strongly linked to higher risks of type 2 diabetes, heart disease, fatty liver disease, stroke, and certain cancers. Visceral fat drains its blood supply directly into the liver through the portal vein, which means fatty acids released from this depot hit the liver first and can drive metabolic problems in ways that subcutaneous fat does not.
There’s also fat in less obvious places: bone marrow contains fatty tissue, and specialized fat pads cushion the soles of your feet and surround your eyeballs.
How Fat Stores and Releases Energy
When you eat more calories than you need, your body converts the excess into triglycerides and packs them into fat cells for storage. This isn’t just about saving energy for later. Storing fat safely inside adipocytes also protects other cells from being exposed to high levels of circulating fatty acids, which can damage tissues when they build up in places like the liver, heart, or muscles.
When you fast or exercise, a process called lipolysis kicks in. Hormonal signals activate a series of enzymes inside fat cells that break triglycerides down into fatty acids and glycerol, which are then released into the bloodstream and used as fuel. This system is tightly regulated. During periods of high energy demand, the rate of fatty acid release from fat cells can increase dramatically, up to 100-fold in response to the right hormonal signals.
Fatty Tissue as a Hormone Factory
One of the most important discoveries about fatty tissue in recent decades is that it actively secretes hormones. Two of the most well-studied are leptin and adiponectin.
Leptin acts as a satiety signal. As your fat stores grow, fat cells release more leptin, which tells your brain to reduce appetite and ramp up metabolism. In theory, this creates a feedback loop that prevents excessive weight gain. In practice, people with large amounts of body fat can develop leptin resistance, where the brain stops responding effectively to the signal.
Adiponectin works differently. It improves insulin sensitivity, helping your cells absorb glucose from the blood more efficiently, and it reduces inflammation. Paradoxically, adiponectin levels tend to drop as body fat increases, which partly explains why excess weight raises the risk of insulin resistance and type 2 diabetes.
When Fat Tissue Becomes Inflamed
Fatty tissue isn’t just a passive victim of weight gain. When fat cells expand beyond their healthy capacity, they begin to malfunction. Overstuffed adipocytes release inflammatory signaling molecules, and immune cells from the bone marrow infiltrate the tissue, producing additional inflammatory compounds. This creates a state of chronic, low-grade inflammation that can persist for years.
That inflammation ripples outward. It contributes to insulin resistance, raises blood pressure, disrupts cholesterol balance, and increases the risk of cardiovascular disease. The ratio of visceral fat to subcutaneous fat matters here: visceral fat is more metabolically active and more prone to this inflammatory cascade than subcutaneous fat.
How Fatty Tissue Changes With Age
The distribution and behavior of body fat shifts significantly over a lifetime. With a sedentary lifestyle and consistent calorie surplus, adults tend to accumulate more visceral fat while losing muscle mass and strength. Individual fat cells also grow larger, a process called adipocyte hypertrophy, particularly in subcutaneous fat. Enlarged fat cells release more free fatty acids into the bloodstream, which drives insulin resistance and other metabolic problems.
Different fat depots also behave differently as you age. Upper body subcutaneous fat is more efficient at storing dietary fat than lower body fat, and it also releases fatty acids at a higher rate. Visceral fat storage efficiency varies widely between individuals, and people who already have large visceral fat stores tend to be less efficient at adding more to that depot, which can redirect fat into the liver and other organs where it causes harm.
In extreme old age, declining hormone levels, reduced physical activity, and direct effects of aging combine to accelerate muscle loss and worsen fatty tissue dysfunction. The result is a body composition that shifts progressively toward higher fat and lower muscle, even if overall weight stays stable.
Subcutaneous Fat as Thermal Insulation
One of the oldest evolutionary roles of fatty tissue is temperature regulation. Subcutaneous fat is a poor conductor of heat, which makes it an effective insulator. Marine mammals take this to an extreme with blubber, a thick subcutaneous fat layer that prevents heat loss in frigid water. In humans, the same principle operates on a smaller scale: your subcutaneous fat helps maintain core body temperature in cold environments.
Brown fat contributes to thermoregulation from the opposite direction. Rather than trapping heat, it generates heat by burning stored fat directly. This is particularly important in newborns, who have limited ability to shiver, and it plays a smaller but measurable role in adults exposed to cold temperatures.