Tirzepatide boosts brown fat, independent of weight loss

Clinical takeaway: This is an early mechanistic signal that suggests tirzepatide may act on brown fat directly thereby supporting metabolism.

Brown fat produces heat and burns calories, but it is suppressed in obesity and no drug reliably activates it. The weight-loss benefit of tirzepatide, meanwhile, has been pinned mostly on reduced appetite and smaller portions. A new trial examined whether the drug also works at the tissue level to boost metabolic activity.

Long thought to vanish after infancy, brown fat was only confirmed in adults through imaging in the late 2000s. Until now, moderate cold exposure has been its strongest known activator. A drug that switches it on would be a different lever to use.

Among women taking tirzepatide, the share with detectable brown fat activity on PET/CT rose to 64.7% from 41.2%, while the placebo group showed no change. Metabolic activity and volume also rose on the active drug, and MRI scans tracked with the PET/CT findings. Critically, the brown fat changes weren't associated with how much weight women lost, and median weight loss was 13.5%.

Known as TABFAT, the trial randomized 34 premenopausal women with obesity to once-weekly tirzepatide or placebo for 24 weeks. Brown fat was measured before and after treatment with cold-stimulated PET/CT, using cooling tailored to each woman's shivering threshold, and confirmed with MRI. Resting energy expenditure was tracked by indirect calorimetry.

"We found that tirzepatide significantly increased brown adipose tissue activity and volume, and it also showed potential signs of converting white subcutaneous fat into more metabolically active 'beige' fat," said Rok Herman, MD, a resident at University Medical Centre Ljubljana in Slovenia.

“This adds a new layer to how we understand the new generation of anti-obesity medications,” he continued. “They are not only appetite suppressants— tirzepatide also appears to modulate energy expenditure at the tissue level, opening a plausible path toward future therapies that combine appetite regulation with thermogenic activation.”

Source: Herman R, et al. ENDO 2026. 2026 Jun 15. Tirzepatide stimulates brown adipose tissue activity independent of weight loss in women with obesity: TABFAT trial