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The Story of Brown Fat: How Cancer Scans Revealed a Hidden Organ in Adults
Brown fat is the body’s heat-producing tissue, and every newborn mammal has it. The textbook story said that humans lose it in the first years of life. Then a group of radiologists noticed something that looked like a problem on cancer scans, and it changed the field.
Key takeaways
- Brown fat burns energy as heat through a protein called UCP1.
- PET scans used for cancer showed symmetrical glowing areas in the necks of some adults.
- Three papers in 2009 confirmed active brown fat in adults.
- Activity is higher in the cold and generally lower in people with higher body fat.
How brown fat works
White fat stores energy. Brown fat cells are packed with mitochondria containing uncoupling protein 1 (UCP1), which lets protons leak across the membrane instead of driving ATP synthesis, releasing energy as heat. It is the main way small mammals and infants stay warm without shivering.
The glowing necks
In whole-body PET scans, patients receive a radioactive glucose analog called FDG, which accumulates in tissues that consume a lot of glucose, notably tumors. Radiologists kept seeing symmetrical bands of uptake in the neck and shoulders, more common in women and in cold weather, that did not match tumors. They were frequently called an artifact and some centers gave patients a drug to suppress it before scanning.
2009: three papers
In April 2009, the New England Journal of Medicine published three independent studies. Aaron Cypess and colleagues, in Boston, used PET with CT scans to show that the neck fat that took up FDG had the density of fat and that its activity correlated with cold exposure. Wouter van Marken Lichtenbelt and colleagues, in the Netherlands, showed that cold exposure activated it in healthy volunteers, and that it was lower in people with more body fat. Kirsi Virtanen and colleagues, in Finland, confirmed UCP1 in biopsies. Together they showed that adult humans have functional brown fat.
Beige fat and plasticity
Researchers also found that white fat depots can develop brown-like cells, called beige or brite cells, under cold exposure and certain signals. This plasticity raised the possibility of changing energy balance by activating thermogenic tissue, and it is why brown and beige fat feature so often in metabolic research.
Lessons for study design
- Temperature changes results. Mice are normally housed at 22 degrees Celsius, below their thermoneutral point of around 30, which keeps brown fat switched on and can change responses to drugs (see measuring metabolism in animals).
- Imaging and cold exposure protocols matter. Scans after a warm room and a cold room differ.
- Mass versus activity. Tissue volume does not equal heat output.
Open questions
How much brown fat contributes to daily energy expenditure in adults is still debated, with estimates suggesting a modest share. Whether it can be recruited for weight control in people remains under study, and the mouse-to-human gap is a recurring theme (see the evidence ladder).
The bottom line
The brown fat story is a reminder that anomalies in data are sometimes discoveries. It began as an annoying artifact on a scan.
Related reading
- Measuring Metabolism in Research Animals: DEXA, EchoMRI and Indirect Calorimetry
- The Appetite Hormone Map: Ghrelin, Leptin, PYY, GLP-1, CCK, Amylin and the Brain
- The 10-Degree Rule: How Temperature Changes Peptide and NAD+ Stability
- Browse the full Research Library
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For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.