Adipose Tissue (Body Fat): Types, Functions, Distribution and Health

What is adipose tissue?

Adipose tissue is a connective tissue found throughout the body. Clinical sources describe it as an active endocrine organ that communicates with other organ systems through hormones, not just a passive energy store [3]. Its main cell is the adipocyte, or fat cell. In white adipocytes, a single large lipid droplet takes up about 90% of the cell's volume [1]. The tissue also contains a non-fat cell population, called the stromal vascular fraction. This includes preadipocytes, mesenchymal stromal (stem) cells, pericytes, endothelial cells and immune cells such as macrophages [9].

Types of adipose tissue: white, brown and beige fat

Researchers generally recognise three types of adipose tissue. They differ in structure and in what they do [2].

Brown fat is mostly present in infancy and decreases with age [3]. For a long time it was thought to have little role in adults. In 2009, three independent studies in the New England Journal of Medicine showed that metabolically active brown fat is present in adults [5].

• One study (Cypess and colleagues) reviewed 3,640 PET-FDG scans done for other reasons. It found likely brown fat in 7.5% of women and 3.1% of men scanned at room temperature [5].
• Another study exposed volunteers to 16°C for two hours. After this cold exposure, 96% of scans showed brown fat [5].
• Across these studies, brown fat was found less often in older people and in people with a higher body-mass index (BMI) [5].

Where body fat is stored

Most fat sits in two main places: under the skin (subcutaneous fat) and around the internal organs (visceral fat). Smaller deposits are found in bone marrow, breast tissue, between muscles, around the heart, in the eye sockets and in the palms and soles [3]. Visceral fat lines the abdominal wall and wraps around many internal organs. One clinical source estimates that it normally makes up about 10% of total body fat [4].

Visceral fat vs subcutaneous fat

Where fat is stored matters for health, not only how much there is. Excess visceral fat has been linked to cardiovascular disease, diabetes, high cholesterol and atherosclerosis (hardening of the arteries) [4]. Visceral fat is hard to measure directly without imaging, so simple body measurements are often used as a guide [4]:

• Waist circumference: 35 inches (about 89 cm) or more in women, or 40 inches (about 102 cm) or more in men, suggests higher risk.
• Waist-to-hip ratio: above 0.85 in women or 0.90 in men suggests fat concentrated around the abdomen.
• Waist-to-height ratio: the waist should generally be no more than half of a person's height.

Functions of adipose tissue

Adipose tissue does several jobs [3]:

• Energy storage and release: fat is stored as triglycerides and released as fatty acids when the body needs energy.
• Insulation and heat production: fat under the skin reduces heat loss, and brown and beige fat generate heat.
• Protection: fat cushions organs and pressure points.
• Metabolic control: fat helps regulate hunger and fullness, blood sugar, cholesterol and insulin sensitivity.
• Immune activity and sex-hormone processing: fat contains immune cells and takes part in the metabolism of sex hormones.

Adipose tissue as an endocrine organ

Fat tissue releases signalling molecules. Protein hormones from fat are called adipokines; lipid-based signals are called lipokines. The best-known adipokine is leptin. Jeffrey Friedman's group reported it in December 1994 after identifying the mouse ob gene [6]. Leptin is made mainly in adipose tissue. It acts on neurons in the hypothalamus, a brain region that controls appetite, to promote a feeling of fullness [6]. Brown and beige fat also release signalling factors, including fibroblast growth factor 21 (FGF21) and neuregulin 4 (NRG4) [2].

How many fat cells do adults have, and are they replaced?

A 2008 study in Nature by Spalding and colleagues dated fat cells using carbon-14 from nuclear bomb tests, which had been built into the cells' DNA. It found that the total number of fat cells did not rise significantly between ages 20 and 70 . Fat cells were still being replaced over time. The estimated death rate was about 8.4% per year, meaning roughly half of all adipocytes are replaced about every 8.3 years . In other words, adults gain or lose fat mostly because existing fat cells grow or shrink, while the number of cells stays fairly constant.

Adipose tissue and disease

Too much fat: obesity

The World Health Organization (WHO) defines adult overweight as a BMI of 25 or more and obesity as a BMI of 30 or more. It notes that measurements such as waist circumference can help diagnose obesity [8]. In 2022, 43% of adults worldwide were overweight and 16% were living with obesity, about 890 million adults [8]. Clinical sources report that healthy body-fat percentages vary with age and sex, generally falling between 10% and 35% [3].

Too little fat: lipodystrophy

Lipodystrophies are rare disorders in which the body loses fat tissue, either across the whole body or in certain areas [7]. They are grouped into four main types: congenital generalized, familial partial, acquired generalized and acquired partial [7]. Congenital generalized lipodystrophy is estimated to affect between 1 in 200,000 and 1 in 12 million people [7]. Because too little fat is available to produce hormones such as leptin, patients can develop severe insulin resistance, diabetes, high triglycerides and fatty liver disease [7]. This shows that some healthy fat tissue is needed for normal metabolism. Metreleptin, a synthetic form of leptin, is the only FDA-approved replacement therapy for certain patients with lipodystrophy [7]. The FDA approved leptin treatment for lipodystrophy in 2014 [6].

Adipose tissue as a source of cells

Fat tissue is plentiful and easy to reach, so it is widely used as a source of adult mesenchymal stromal cells. It generally yields more usable cells, more easily, than bone marrow [9]. The usual way to collect it is liposuction. Each liposuction procedure produces anywhere from 100 ml to more than 3 litres of lipoaspirate (the fat removed by suction) . The cells are then separated in one of two ways:

• Enzymatic digestion: the standard method uses the enzyme collagenase to break down the tissue and release the stromal vascular fraction (SVF). Stem cells make up only about 2–10% of the SVF . Because of this enzyme step, US regulators treat SVF as more than a "minimally manipulated" product, which means stricter oversight .
• Mechanical methods: these are faster and cheaper. One 2024 study reported about 59 million cells, with 91% viability, from 20 ml of processed lipoaspirate [9]. Results from single studies like this vary with method and patient.

The US Food and Drug Administration (FDA) lists stromal vascular fraction (fat-derived cells) among regenerative products being marketed without approval. It has received reports of harms including blindness, tumor formation and infections linked to unapproved products [10]. Fat grafting (moving a patient's own fat to another part of the body), SVF and adipose-derived stem cells are covered on separate pages of this site.

Key points

• Adipose tissue is an active tissue that makes hormones, not just stored fat.
• White fat stores energy. Brown and beige fat burn energy to produce heat.
• Where fat is stored matters: visceral fat is more closely tied to cardiometabolic risk than fat under the skin.
• Adults keep a fairly stable number of fat cells, and these cells are gradually replaced over time.
• Having too much fat (obesity) and too little fat (lipodystrophy) can both disrupt metabolism.