Brown fat spotted on 100,000 old scans tracks with less diabetes and depression

Brown fat is the body's heater. Instead of storing energy, it burns energy to make heat. For a long time, adults were thought to have very little of it, and checking who does usually takes a dedicated scan. A team writing in Nature found another way: scans that already existed in a hospital archive.

Memorial Sloan Kettering Cancer Center uses PET-CT scans to look for tumors. The PET part relies on a tracer that shows where tissue is taking up sugar, and active brown fat takes it up too. The researchers assembled scan records from 101,243 patients. About 8.8% of them, or 8,897 people, had detectable brown fat; an AI-assisted method measured its activity in 8,709 of those patients.

Then they compared those scans with patients' medical records. After accounting for age, sex, body mass index, the outdoor temperature on the scan day and other factors, people with brown fat had less than half the odds of type 2 diabetes (an odds ratio of 0.42). Their odds of chronic ischemic heart disease, where narrowed arteries starve the heart of blood, were about half (0.53). The pattern extended to mental health: lower odds of depressive episodes (0.68) and anxiety disorders (0.78). Of 511 diagnoses tested, 98 stayed significant after adjustment, and 92 of those were less common in people with brown fat.

One unexpected lead came from the medication lists. People taking losartan, a common blood pressure drug, were more likely to have detectable brown fat. In cultured cells and in mice, losartan increased brown fat's heat-making activity. That is a reason to test the drug, not evidence that it does anything for brown fat in people. The mouse work used only males, and no human trial has been done.

The design has blind spots. Every patient came from one cancer center, and cancer or its treatment could shape both brown fat and other diagnoses. The scans weren't done after cold exposure, which normally wakes brown fat up, so many people with it were probably missed. The tracer tracks sugar uptake, which may not match how much heat the tissue can make. An outside researcher quoted in Nature's coverage said more work is needed to know whether brown fat causes any of these benefits. Healthier bodies might simply be more likely to keep active brown fat, and these scans can't tell the two possibilities apart.