Adipose tissue: anatomy and metabolic functions

Adipose tissue is more than just an energy reserve in the form of triglycerides. It is a flexible and heterogeneous organ that maintains energy balance, influences insulin sensitivity, body temperature, and immune responses, and secretes signaling molecules that affect the brain, liver, muscles, and blood vessels. [1]

Classically, white adipose tissue, brown adipose tissue, and beige adipose tissue are distinguished. White adipose tissue specializes in storing energy and releasing fatty acids when needed, while brown and beige adipose tissue are capable of expending energy on heat production, including through uncoupling protein 1 (UCP1), which "uncouples" substrate oxidation and adenosine triphosphate synthesis, converting some of the energy into heat. [2]

Beige adipose tissue is interesting because it can appear within some white adipose tissue depots in response to certain stimuli, including cold and sympathetic activation. However, modern reviews emphasize the complexity of thermogenesis: both UCP1-dependent mechanisms and additional pathways are discussed, but the physiological significance of these different mechanisms may differ in humans and experimental models. [3]

Anatomically, adipose tissue is present in almost all areas of the body, but its depots differ in origin, blood supply, innervation, the composition of adipocyte precursors, and the profile of secreted molecules. These interdepot differences help explain why visceral fat is more often associated with metabolic complications, while gluteofemoral subcutaneous fat is often associated with a more favorable metabolic profile. [4]

From a practical standpoint, "healthy" adipose tissue is tissue that can safely store excess energy without significant inflammation, fibrosis, or lipid leakage into organs. "Unhealthy" adipose tissue is a depot where expansion occurs primarily due to abnormally large adipocytes, impaired blood supply, increased fibrosis, and an influx of inflammatory cells, which increases the likelihood of insulin resistance. [5]

Table 1. Main types of adipose tissue and key functions

Type of adipose tissue	What predominates in cells	Main function	Typical localizations
White adipose tissue	1 large lipid droplet in an adipocyte	Energy storage, fatty acid buffering, endocrine signaling	Subcutaneous depots, visceral depots
Brown adipose tissue	Many mitochondria, thermogenesis	Heat production, energy consumption	Cervical-supraclavicular zone, paravertebral (variable)
Beige adipose tissue	"Thermogenic" cells inside white depots	Adaptive thermogenesis in response to stimuli	More common in some subcutaneous depots

[6]

Topography and "depot": subcutaneous, visceral, perivascular and ectopic fat

Subcutaneous adipose tissue is located beneath the skin and is generally considered the "safest" depot for storing excess energy. Current data emphasize that subcutaneous fat, under certain conditions, functions as a "metabolic buffer," reducing fatty acid overload in the liver and muscles and thereby protecting systemic insulin sensitivity. [7]

Subcutaneous adipose tissue is heterogeneous even within a single region. In the abdominal wall, there is a more superficial adipose tissue above the fascial layer and a deeper adipose tissue below the fascial layer, often referred to as Scarpa's fascia. Research suggests that superficial and deep subcutaneous adipose tissue may be differentially associated with metabolic risks, including hepatic steatosis and metabolic syndrome. [8]

Visceral adipose tissue is located within the abdominal cavity around the organs and in the mesentery. Visceral fat has been shown in many studies to be more strongly associated with an unfavorable cardiometabolic profile than subcutaneous fat, including a stronger association with insulin resistance and cardiovascular disease risk factors. [9]

Perivascular adipose tissue surrounds blood vessels and can influence vascular tone and inflammation through local signaling molecules. Under physiological conditions, it can support vascular function, but in obesity and chronic inflammation, its secretory profile can shift toward vasoconstriction and endothelial dysfunction, which is associated with increased cardiovascular risk. [10]

Ectopic fat is the accumulation of lipids in organs and tissues not designed for long-term fat storage, such as the liver, pancreas, and skeletal muscle. Reviews highlight that ectopic fat deposition may play a causal role in metabolic syndrome and carbohydrate metabolism disorders, and that reduction in ectopic fat is often accompanied by improved insulin sensitivity. [11]

Table 2. Adipose tissue depots and typical metabolic associations

Depot	Where is it located?	What reflects more often	Typical risk association
Subcutaneous adipose tissue	Under the skin	"Safe" energy reserve	Often neutral or protective with good plasticity
Deep subcutaneous adipose tissue	Under the fascial layer	The more "metabolic active" part of the subcutaneous depot	May be more strongly associated with metabolic disorders than superficial
Visceral adipose tissue	Inside the abdominal cavity	Central obesity	Often strongly associated with insulin resistance and cardiac risk
Perivascular adipose tissue	Around the vessels	Local regulation of blood vessels	In case of dysfunction, it supports inflammation and endothelial dysfunction.
Ectopic fat	Liver, pancreas, muscles	"Overflow" of lipids from the depot	Associated with impaired glucose and lipid metabolism

[12]

Microanatomy: What cells does adipose tissue consist of and how is it regulated?

Adipocytes are the main cells of adipose tissue, but they constitute only a fraction of its biology. The stromal-vascular fraction plays a crucial role: adipocyte precursors, endothelial cells, pericytes, fibroblasts, immune cells, and extracellular matrix components. It is this "surrounding layer" that determines how well the depot can expand and the extent of inflammation and fibrosis. [13]

Adipose tissue growth can occur in two ways: by increasing adipocyte size and by increasing adipocyte number. In "healthy" expansion, the ability to recruit new adipocytes from precursors is important to prevent individual cells from reaching abnormally large sizes. In "unhealthy" expansion, adipocyte hypertrophy predominates, increasing the risk of hypoxia, cellular stress, and inflammatory cell influx. [14]

Extracellular matrix and fibrosis are one of the key switches between "healthy" and "unhealthy" depots. With chronic overnutrition, the matrix can accumulate, reducing tissue elasticity and limiting its plasticity. Reviews link adipose tissue fibrosis to metabolic dysregulation and impaired insulin sensitivity. [15]

Adipose tissue immune cells are particularly important in obesity. Macrophages can become the predominant immune population in inflamed adipose tissue and are involved in adipocyte dysfunction, matrix remodeling, and fibrosis. Recent reviews emphasize that simply dividing macrophages into two types is insufficient, and the actual heterogeneity is much more complex. [16]

Innervation and vascularity ensure rapid metabolic responses. The sympathetic nervous system regulates lipolysis, beige adipocyte development, and thermogenesis, while sensory neurons may be involved in feedback between adipose tissue and the brain. This explains why hormones, stress, and environmental temperature can significantly alter the metabolic behavior of depots. [17]

Table 3. Cellular components of adipose tissue and their functions

Component	What does it do?	Why is it important for metabolism?
Adipocytes	Lipid storage and release, adipokine secretion	Fatty acid buffer, hormonal signaling
Adipocyte precursors	Formation of new adipocytes	Determine the plasticity of the depot
Endothelium and pericytes	Vascular network and oxygen delivery	Limit hypoxia and tissue stress
Immune cells	Inflammation and remodeling	Dysregulation increases insulin resistance.
Extracellular matrix	Mechanical fabric "frame"	In excess, it forms fibrosis and limits expansion.

[18]

Metabolic functions: energy storage, lipolysis, fatty acid buffering, and thermogenesis

The primary energetic role of adipose tissue is the accumulation of triglycerides during periods of energy excess and their mobilization during periods of energy deficiency. Under physiological conditions, this protects the body from the toxic accumulation of fatty acids in the liver, muscles, and pancreas, thereby reducing the likelihood of ectopic fat deposition. [19]

Food intake and insulin switch adipose tissue to lipogenesis mode: increased uptake of fatty acids and glucose and triglyceride synthesis. During starvation and stress, sympathetic activation via catecholamines switches the depot to lipolysis mode, releasing fatty acids and glycerol. This is why neural regulation is considered not secondary, but a central part of fat metabolism control. [20]

Metabolic risk increases when adipose tissue loses its ability to "safely expand." Excess energy then begins to "flow" into ectopic depots, and inflammation and fibrosis increase, impairing insulin signaling pathways. Reviews of ectopic fat highlight the association with liver and pancreatic dysfunction and impaired glucose control. [21]

Thermogenesis of brown and beige adipose tissue adds another layer of metabolic function: energy expenditure for heat. In humans, brown adipose tissue activity varies, depending on environmental temperature and neurohumoral stimuli, and scientific debate surrounds how best to assess the contribution of thermogenesis to overall energy expenditure and which mechanisms are most significant. [22]

Beyond classical thermogenesis via UCP1, additional pathways of heat production and mitochondrial cycling are discussed, particularly in the context of beige adipocytes. This does not negate the importance of UCP1, but it does indicate that the thermogenic biology of adipose tissue is richer than a simple "white versus brown" dichotomy. [23]

Table 4. Key metabolic processes in adipose tissue

Process	When activated	Main regulators	Potential contribution to risk
Lipogenesis	After eating	Insulin, substrate availability	Protective if the depot is plastic
Lipolysis	Fasting, stress, physical activity	Sympathetic activation, catecholamines	In chronic hyperactivation, it increases the flow of fatty acids
"Overflow" into ectopic fat	When the depot is overloaded	Limited plasticity of the subcutaneous depot	Increases the risk of insulin resistance
Thermogenesis of brown and beige tissue	Cold, stimulation of the sympathetic system	Uncoupling protein 1 (UCP1) and related pathways	Potentially reduces energy excess

[24]

Endocrine and Immune Functions: Adipokines, Batokines, and Inflammation

Adipose tissue secretes numerous signaling molecules that influence appetite, energy expenditure, insulin sensitivity, and inflammatory responses. The concept of adipose tissue as an endocrine organ has been supported by decades of research and remains central to understanding obesity and metabolic disease. [25]

Leptin is a key hormone in adipose tissue, reflecting "storage levels" and acting on brain structures associated with appetite and energy balance regulation. Recent reviews describe the complex organization of leptin signals in the hypothalamus and the mechanisms that may weaken this signal in obesity. [26]

Adiponectin is an adipokine commonly associated with improved insulin sensitivity and anti-inflammatory effects. Unlike leptin, adiponectin levels are often reduced in obesity, and its mechanisms are associated with increased fatty acid oxidation, improved glucose utilization, and effects on hepatic glucose production. [27]

Brown and beige adipose tissue also have a secretory function: they secrete "batokines" and other signaling molecules, including components of extracellular vesicles. This part of biology is actively developing because it links thermogenesis and endocrine effects into a single system of interorgan communication. [28]

The immune function of adipose tissue is particularly evident in obesity, when chronic low-grade inflammation develops. Reviews highlight the role of macrophages, T lymphocytes, and matrix remodeling in maintaining adipocyte dysfunction and the development of insulin resistance. [29]

Table 5. Examples of adipokines and their typical effects

signaling molecule	Main source	Typical action	What changes with obesity
Leptin	White tissue adipocytes	Energy reserve signal, appetite regulation	Usually increases, but the effect may decrease
Adiponectin	Adipocytes	Improved insulin sensitivity, anti-inflammatory effects	Often decreases
Proinflammatory cytokines	Immune cells and adipocytes	Supports inflammation and insulin resistance	Often increase
Batokins	Brown and beige fabric	Interorgan signals, connection with thermogenesis	Biology is being actively refined

[30]

Why it matters clinically: Fat distribution, 'metabolically healthy obesity' and the limitations of cosmetic approaches

Cardiometabolic risk is determined not only by total fat mass but also by where the fat is located and how "healthily" the fat depot functions. Visceral adipose tissue is, on average, more strongly associated with an unfavorable risk factor profile than subcutaneous fat, while gluteal-femoral subcutaneous fat is often associated with a more favorable metabolism. [31]

The concept of "metabolic healthy obesity" is partly explained by the fact that in some people, the subcutaneous fat depot remains flexible for a long time, accumulating excess energy without significant inflammation or ectopic fat. As this flexibility deteriorates and fibrosis and inflammation increase, the likelihood of developing insulin resistance increases. [32]

Ectopic fat deposition in the liver and pancreas is considered an important part of the mechanism of impaired glucose metabolism. Current reviews link it to impaired hepatic insulin sensitivity and impaired beta-cell function, making reduction of ectopic fat a target in metabolic improvement studies. [33]

Perivascular adipose tissue highlights that fat depots influence not only metabolism but also local vascular biology. In obesity, its "vascular" signals can contribute to vascular wall inflammation and endothelial dysfunction, adding a mechanism to cardiac risk beyond the classic lipids and blood pressure. [34]

Cosmetic interventions targeting subcutaneous fat can alter body contours, but are not equivalent to correcting visceral and ectopic fat. This is important for setting appropriate expectations: improving appearance does not guarantee a reduction in cardiometabolic risk if the key risk depot remains visceral or ectopic. [35]

Table 6. Risk map by type of fat tissue distribution

Distribution pattern	What is more common?	Typical metabolic consequences	What is more often useful to monitor?
Central visceral accumulation	Visceral tissue	Higher risk of insulin resistance	Waist circumference, signs of fatty liver disease
Mainly subcutaneous accumulation	Subcutaneous tissue	The risk depends on the plasticity of the depot	Dynamics of body weight and metabolic markers
Ectopic fat deposition	Liver, pancreas, muscles	Glucose metabolism disorder	Evaluation of steatosis according to indications
Severe fibrosis and inflammation of the depot	"Unhealthy" expansion	Increasing insulin resistance	Metabolic parameters and inflammatory markers as indicated

[36]