Researchers Identify Body Fat as Communicative Organ Affecting Multiple Body Systems

The average UK adult carries around 22 kilograms of body fat. If melted, this would equate to 88 blocks of lard, enough to fill two-thirds of a small suitcase. For centuries, body fat was regarded as an inert substance. Researchers now describe it as a living organ that communicates with other parts of the body and influences processes ranging from bone health to mood.

Fat is not uniform. It exists in white, brown, beige and pink forms, each with distinct functions, and contains immune cells, nerves and blood vessels. "You wouldn’t get pushback today if you claimed fat was an organ, in the same way your lungs or liver or spleen are organs," said Paul Cohen at The Rockefeller University in New York.

This recognition is reshaping scientific views of body fat and obesity. It is prompting questions about approaches to reducing fat and leading some researchers to examine ways to reprogram fat tissue to improve health.

Body fat, also known as adipose tissue, was long viewed primarily as a storage depot for excess calories, insulation against cold and protective padding. Its evolution helped early humans survive colder climates after leaving Africa. "I think the first thing that people fail to appreciate is what a valuable evolutionary step it was to be able to store fuel," said Randy Seeley, who researches energy balance and metabolism at the University of Michigan.

In mammals, fat has developed beyond simple storage. It integrates into the regulation of blood glucose, body temperature and other functions including bone health.

The discovery of the hormone leptin in the 1990s provided initial evidence of fat's active role. Secreted by fat cells, leptin acts on the brain to suppress appetite and increase energy expenditure. When fat stores decrease rapidly, leptin levels fall.

The brain responds by increasing hunger signals and reducing energy use to restore fat reserves. Fat cells release multiple hormones and signalling molecules known as adipokines. These can act locally or travel to distant tissues. Nerve fibres extend into adipose tissue, creating bidirectional electrical communication with the brain.

"The nerve supply in adipose tissue enables a bidirectional and fast communication route with the brain," said Kristy Townsend, a neuroscientist at The Ohio State University. Nerves relay information about fat's health status, such as injury or inflammation.

Immune cells within the tissue also participate, releasing molecules that support nerve survival and growth. "If you look at the tissue in between all the adipocytes, there’s pretty much every immune cell you can imagine – so fat is also an immune organ," said Townsend.

Fat communicates with the brain about energy reserves. It also appears linked to mood regulation. Obesity, particularly when metabolically unhealthy, has been associated with higher rates of depression and anxiety, potentially through inflammation pathways and altered leptin levels affecting brain reward circuits.

Fat plays a key role in fertility. Menstruation does not begin or stops without sufficient body fat, an adaptation that protects against pregnancy without adequate energy stores. "People forget that fat is metabolically really important. Without fat, we have issues with hormonal control, infection [and] immunity," said Louise Thomas, a professor of metabolic imaging at the University of Westminster in London.

More than 95 per cent of human fat stores consist of white fat, located both under the skin and around internal organs. Visceral fat surrounding organs is associated with increased risk of type 2 diabetes, high blood pressure, heart attacks and some cancers.

Research indicates visceral fat may also affect brain function and contribute to Alzheimer's disease. In obesity, fat cells can enlarge beyond sustainable size, leading to cell death when blood supply is insufficient. The mechanisms that cause fat to shift from a supportive organ to contributing to disease remain a focus of ongoing study.

Scientists are exploring how diet, reprogramming of fat cells and other interventions might address these effects.