Body fat is primarily made of triglycerides, a type of molecule built from three fatty acid chains attached to a small glycerol backbone. In adults, about 85% of adipose tissue by weight is pure fat (lipid), with the remaining 15% consisting of water, proteins, blood vessels, nerve fibers, and various support cells. This composition isn’t fixed from birth: newborns carry adipose tissue that’s only about 40% lipid by weight, with the rest being water and structural proteins. The lipid percentage climbs steadily through childhood and reaches adult levels by adolescence.
The Chemistry Inside a Fat Cell
Each fat cell, or adipocyte, is essentially a storage container for triglycerides. A triglyceride molecule forms when your body links three fatty acid chains to a single glycerol molecule. The process starts when your liver or fat cells convert excess energy from food into fatty acid chains, typically 16 carbons long. These chains are then stitched onto a glycerol backbone in a stepwise process, first one chain, then a second, then a third, creating a compact, energy-dense molecule that sits inside the cell as a large oil droplet.
This stored fat packs roughly 8 calories per gram of tissue. That’s slightly less than the 9 calories per gram of pure dietary fat because adipose tissue also contains water and cellular infrastructure. Still, it’s more than twice the energy density of carbohydrates or protein, which is why fat is such an efficient fuel reserve. A person carrying 10 kilograms of body fat is storing around 80,000 calories of energy.
Fat Cells Aren’t Just Blobs of Oil
Adipose tissue is a living, active organ with a complex internal structure. The fat cells themselves are surrounded by a network of capillary blood vessels and nerve endings that regulate how fat is stored and released. Beyond the adipocytes, fat tissue contains a diverse mix of other cell types: stem cells capable of becoming new fat cells, immune cells, cells that line blood vessels, lymphatic cells, and smooth muscle cells that wrap around small arteries. This supporting cast of cells plays a major role in how your fat tissue communicates with the rest of your body, releasing hormones and immune signals that influence appetite, inflammation, and insulin sensitivity.
Three Types of Fat Cells
Not all fat cells look or function the same. White fat cells are the most abundant type in adults. Each one contains a single large lipid droplet that takes up almost the entire cell, pushing the nucleus and other structures to the edge. White fat cells have very few mitochondria (the energy-generating structures inside cells), because their main job is storage, not energy burning.
Brown fat cells are built for the opposite purpose: generating heat. They contain many small lipid droplets instead of one large one, and they’re packed with mitochondria. Those mitochondria contain a specialized protein that short-circuits the normal energy production process, converting stored fat directly into body heat instead of usable cellular energy. Babies have significant brown fat deposits to help maintain body temperature, and adults retain smaller amounts, mainly around the neck and upper back.
Beige fat cells are a hybrid. They normally look and behave like white fat cells, but when triggered by cold exposure or certain hormonal signals, they can switch on heat production at levels comparable to brown fat. This “browning” of white fat has generated significant interest because it represents a way the body can burn through stored energy more rapidly.
Where Fat Is Stored Changes What It Does
The fat sitting just beneath your skin (subcutaneous fat) and the fat packed around your internal organs (visceral fat) differ in important ways beyond location. Subcutaneous fat cells are generally larger and accumulate more lipid per cell, especially in people who are overweight. They’re also better at producing leptin, the hormone that signals fullness to your brain, and adiponectin, a hormone that helps your body respond to insulin.
Visceral fat is more metabolically active in ways that tend to cause problems. It produces higher levels of inflammatory molecules, particularly a signaling protein called IL-6 that drives systemic inflammation. Visceral fat cells also break down their stored fat more readily, releasing fatty acids into the portal vein that flows directly to the liver. This is one reason why carrying excess weight around the midsection is linked to a higher risk of metabolic disease compared to carrying the same amount of fat on the hips or thighs. Visceral fat cells also produce less adiponectin, which may partly explain the stronger connection between belly fat and insulin resistance.
How Fat Gets Stored and Released
Your body builds triglycerides for storage through a process called lipogenesis. When you eat more energy than you need, your liver converts the surplus into fatty acid chains using a series of enzymatic steps. These fatty acids travel through the bloodstream and are taken up by fat cells, where they’re assembled onto glycerol backbones and deposited into the cell’s lipid droplet. Your body can also convert excess carbohydrates and protein into fat through this same pathway, not just dietary fat.
The reverse process, lipolysis, happens when your body needs energy between meals or during exercise. Hormonal signals (primarily from adrenaline and related stress hormones) tell fat cells to break their stored triglycerides back into free fatty acids and glycerol. These components are released into the bloodstream and transported to muscles, the heart, and other tissues that burn them for fuel. The two processes exist in a constant tug of war: after a meal, storage dominates; during fasting or physical activity, release takes over. The balance between them, over weeks and months, determines whether your fat stores grow, shrink, or stay the same.
How Fat Tissue Changes With Age
The composition of adipose tissue shifts across a lifetime. In newborns, fat tissue is roughly 40% lipid and 60% water, nitrogen-containing proteins, and DNA. As a person ages, fat cells mature and accumulate more triglycerides, pushing the lipid fraction up to about 75% in adults. At the same time, the water content and density of supporting structures decline. This shift means that a kilogram of fat tissue in a child contains meaningfully less stored energy than a kilogram in an adult, because less of it is actual lipid. In older adults, fat also tends to redistribute, moving away from subcutaneous deposits under the skin and increasingly accumulating as visceral fat around the organs.