Health Insights
Recent findings reveal that body fat is a dynamic organ influencing everything from bone strength to emotional well-being, prompting a reevaluation of our understanding of the human body.
By Linda Geddes
Imagine melting down the average adult in the UK; you’d uncover about 22 kilograms (48.5 pounds) of creamy yellow fat. This amount is comparable to 88 blocks of lard, enough to fill two-thirds of a small suitcase or create 446 dinner candles. Melted, it could coat a large bedroom with a translucent, waxy layer.
For centuries, body fat was seen as a passive, lard-like substance. We carry it everywhere, often with disdain. However, this pale, oily tissue is now recognized as a vibrant organ, capable of influencing appetite, metabolism, fertility, mood, and immunity.
Fat isn’t uniform; it exists in white, brown, beige, and even pink forms, each with unique functions and locations. It contains immune cells, nerves, and blood vessels that enhance its capabilities.
“Today, no one would dispute that fat is an organ, much like the lungs or liver,” says Paul Cohen from The Rockefeller University. This new perspective is reshaping our understanding of body fat and obesity, challenging traditional methods of fat reduction, and inspiring scientists to explore ways to reprogram fat for better health.
The Evolutionary Role of Fat
Historically, body fat, or adipose tissue, was viewed as a storage depot for excess calories, insulation against cold, and padding. These functions were crucial for human survival, aiding migration from Africa and adaptation to colder climates. Even today, a bit of extra weight can reduce mortality risk in older adults during illness.
“Storing fuel was a significant evolutionary step,” says Randy Seeley from the University of Michigan. “Without it, you’d be like a filter feeder, constantly needing to consume food.”
In mammals, fat has evolved beyond a simple energy reserve. It plays a role in regulating blood glucose, body temperature, and physiological functions, including bone health.
Understanding Hunger and Communication
The 1990s discovery of leptin, a hormone secreted by fat cells, revealed fat’s role in suppressing appetite and boosting energy expenditure. When fat is lost, leptin levels drop, signaling the brain to increase hunger and conserve energy.
This discovery unveiled a complex communication network between fat and the body. Fat cells release various hormones and signaling molecules, known as adipokines, which interact with nearby tissues and distant organs.
Communication isn’t just chemical; it’s also electrical. Nerve fibers within adipose tissues create a direct, two-way communication line between the brain and fat.
“Nerve supply in adipose tissue allows fast, bidirectional communication with the brain,” says Kristy Townsend from The Ohio State University. Nerves convey messages about energy, metabolism, and fat health, such as injury or inflammation.
Fat’s Role in Immunity and Mood
Immune cells within fat tissue communicate about inflammation or injury, releasing molecules that support nerve survival and growth. “Fat is also an immune organ,” says Townsend, highlighting the presence of diverse immune cells within adipose tissue.
Fat’s impact extends beyond energy balance, influencing mood and mental health. Obesity, particularly metabolically unhealthy obesity, is linked to mood disorders like depression and anxiety. Inflammation in adipose tissue may trigger brain inflammation, altering neurotransmitter balance and behavior.
Fat is crucial for fertility, too. Without sufficient body fat, menstruation may not start or may cease, as entering pregnancy without enough energy for fetal development could be catastrophic.
“Fat is metabolically vital,” says Louise Thomas from the University of Westminster. “It affects hormonal control, infection, and immunity.”
The Dark Side of Fat
Despite its importance, fat often gets a bad reputation due to its location. White fat, comprising over 95% of total fat, is found under the skin (subcutaneous) and around internal organs (visceral). “Our organs are often surrounded by a sea of fat,” says Thomas.
Excess visceral fat is linked to higher risks of type 2 diabetes, high blood pressure, heart attacks, and certain cancers. It may also affect brain function and contribute to conditions like Alzheimer’s disease.
Research focuses on what causes fat to shift from a cooperative organ to a rogue state. While white fat cells can expand and contract, those around internal organs are particularly vulnerable to excess fat’s harmful effects.
In obesity, fat cells enlarge and may die when they reach a critical size. Their blood supply can’t keep up, leading to stress and inflammation, attracting immune cells to clear dead cells.
These immune cells intensify inflammation, affecting insulin regulation and increasing type 2 diabetes risk. They are also linked to cognitive changes in obesity, such as memory and attention issues, and may foster tumor growth. Obesity is a risk factor for many cancers, often leading to worse outcomes.
Dying fat cells release fatty acids, or lipids, which can be toxic in excess. This lipotoxic stress can damage nerves within fat, a condition known as adipose neuropathy. Obesity, type 2 diabetes, and aging are linked to this nerve loss, disrupting metabolism by impairing brain-fat communication.
Bone Health and Fat
Misfiring fat signals can also affect bones. Normally, estrogen from adipose tissue protects against excessive bone resorption. However, excess fat, especially visceral fat, can impair bone quality and increase fracture risk. Inflammatory cytokines from adipose tissue stimulate osteoclasts, promoting bone loss.
Despite dysfunctional fat’s downsides, adipose tissue is essential. Efforts to remove it can backfire. Liposuction studies show that removed fat may reappear elsewhere. “Removing subcutaneous fat may lead to more visceral fat, worsening the situation,” says Seeley.
Not all obesity is unhealthy. Between 10 and 30 percent of people classified as obese based on BMI avoid typical health issues like insulin resistance and high blood pressure. This “metabolically healthy obesity” intrigues researchers like Matthias Blüher from the University of Leipzig.
Blüher’s research shows that fat location and behavior are crucial. People with more visceral and liver fat tend to be less healthy, while those with smaller fat cells and healthier adipokine secretion are more protected.
Exploring Fat Diversity
Recent studies analyze gene activity in different fat deposits, revealing that not all visceral fat is equal. “Even within the visceral cavity, fat location matters,” says Blüher. Fat outside the intestine poses the highest risk, though the reasons remain unclear.
In healthy obesity, fat cells are metabolically flexible, releasing fewer inflammatory signals and hosting fewer immune cells. Visceral fat contains mesothelial cells, which can transform into other cell types, allowing smoother fat expansion without excessive inflammation. Genetics and lifestyle factors like diet and exercise may influence these traits.
Blüher believes these insights could help identify high-risk individuals and tailor treatments accordingly.
Reprogramming Fat for Better Health
Blüher’s long-term goal is to restore fat’s healthy function, potentially transforming “unhealthy” obesity into a more benign form. Encouragingly, this may not require significant weight loss. Modern weight-loss drugs and bariatric surgery benefits often stem from improved fat distribution and function, not just weight loss.
Achieving this could revolutionize our understanding of healthy body shapes.
If fat can be reprogrammed to behave healthily, many could live longer, healthier lives without focusing on size.
Whether obesity starts in adipose tissue or the brain is debated, but when communication between them falters, the system becomes unbalanced.
Seeley compares it to an orchestra: “All organ systems communicate with the brain, and if the conductor isn’t effective, the symphony suffers.”
Fat isn’t the problem; it’s an instrument slightly out of tune. Understanding and harmonizing this talkative organ with the body is key to overall health.
Not all fat is the same. Most is white fat, composed of large, round cells storing energy as triglycerides. It also plays a signaling role and provides insulation and cushioning.
Brown fat, found around the neck and shoulders, is packed with mitochondria that burn fatty acids to generate heat. When activated, it burns more heat per gram than any other tissue.
Both types of fat have shaped human history, says Aaron Cypess from the US National Institutes of Health. “White fat contributed to civilization by freeing time for other activities, while brown fat helped us adapt to different environments.”
Fat also comes in other hues. Beige fat cells, found in white fat, can adopt brown-like characteristics after exercise or cold exposure. During pregnancy and lactation, white fat in breasts transforms into pink fat, supporting milk production.
Saverio Cinti from Marche Polytechnic University argues that this network of fat forms a single, integrated system. “The old concept of adipose tissue as connective tissue is obsolete,” he says. “The adipose organ is a unified structure managing energy storage and heat production.”
Weight loss is challenging, and maintaining it is even harder. Even with drugs like Wegovy, Ozempic, and Mounjaro, weight often returns after treatment, suggesting fat has a stubborn memory.
This makes evolutionary sense. “In food-scarce environments, staying hungry after losing fat is beneficial,” says Ferdinand von Meyenn from ETH Zurich. Obesity and its complications were rare historically, so there was little evolutionary pressure to counter excessive food availability.
Von Meyenn’s research explores the “yo-yo” effect seen with dieting, suspecting that epigenetic markers on fat cell genes may be key.
Studies of fat samples from bariatric surgery patients revealed gene activity differences that persisted after weight loss.
Experiments on mice showed similar patterns. Their epigenomes, chemical markers on genes, were influenced by obesity.
Mice with these patterns gained weight faster on a high-fat diet, and their fat cells stored more fat and glucose.
Von Meyenn cautions that these changes haven’t been proven to cause the yo-yo effect. Other organs, including the brain, might also store obesity memories.
It’s unclear how long fat cells retain these memories. Experiments suggest changes last up to six months, but longer-term effects are unknown. “The longer animals remain obese, the more pronounced the epigenome effects,” says von Meyenn.
If epigenetic memory is part of the problem, erasing these marks might help prevent weight regain. “It might not stop obesity, but it could reduce the urge to overeat,” he says.
