Blockbuster obesity drug may burn calories by activating brown fat, study finds

Researchers reported at ENDO 2026 that tirzepatide increased brown fat activity in women with obesity. The finding suggests the drug may aid weight loss beyond appetite control, though experts say more study is needed.

In Short

• Clinical trial tracked premenopausal women with obesity over 24 weeks
• PET/CT and MRI scans measured fat changes before and after treatment
• Detectable activity rose from 41.2 percent to 64.7 percent after therapy

A blockbuster obesity drug may be helping people lose weight in a way scientists had not fully understood before. New research has found that tirzepatide, a medicine already known for reducing appetite and body weight, can also activate the body's calorie-burning “brown fat”, opening a new front in the fight against obesity.

The findings were presented at ENDO 2026 recently, the annual meeting of the Endocrine Society, and are being seen as an important step towards understanding how modern anti-obesity medicines work beyond simply reducing food intake.

Tirzepatide, under the brand name Mounjaro was launched in India in March last year by US-based pharma giant Eli Lilly and has been the country’s top-selling medicine brand by value since October 2025.

Last month, the drug also became the highest selling medicine brand globally.

The molecule belongs to a comparatively new class of medicine called GLP-1 based therapy, like semaglutide, which was first developed against type 2 diabetes but has gained immense popularity due to rapid weight-loss effect.

These drugs work by mimicking gut hormones, slowing digestion and reducing appetite and are now being studied for a number of conditions including cardiovascular and metabolic conditions, kidney and brain diseases and even cancers.

BROWN FAT BURNS ENERGY

The latest evidence shows that unlike white fat, which stores excess calories in the body, brown adipose tissue, commonly called brown fat, helps burn calories to generate heat. Scientists have long known that brown fat plays a role in energy expenditure, but its activity is usually reduced in people living with obesity.

Researchers conducted a randomized, placebo-controlled clinical trial involving premenopausal women with obesity. Participants received tirzepatide treatment for 24 weeks, while a separate group received a placebo. Advanced imaging techniques, including PET/CT scans and MRI scans, were used to measure brown fat activity before and after treatment.

The results showed a significant increase in both the activity and volume of brown fat among those taking tirzepatide. Detectable brown fat activity rose from 41.2 percent of participants before treatment to 64.7 percent after treatment.

No similar improvement was observed in the placebo group.

Researchers also found signs that the drug may help convert ordinary white fat into “beige fat”, a more metabolically active form of fat that can burn energy in a way similar to brown fat.

Back home too, specialists expressed excitement. “This is early data..Tirzepatide activates the metabolically active brown fat..No drug so far has been shown to increase calorie burning..If proven, this opens up an exciting new area,” veteran endocrinologist Dr Ambrish Mithal wrote on X.

BEYOND APPETITE CONTROL

Tirzepatide, like semaglutide, is already widely used for obesity and type 2 diabetes because it helps people eat less and lose substantial amounts of weight. However, the new findings suggest the drug may have a direct impact on the body's metabolism by increasing calorie burning.

Scientists say this could explain why the medicine has been so effective in helping patients achieve significant weight loss. Earlier laboratory and animal studies had hinted at a similar effect, but the latest trial provides evidence of brown fat activation in humans.

But experts also caution that the findings are still early and more research is needed to determine how much of the molecule's weight-loss benefits come from brown fat activation. They also want to explore whether the effect occurs in different age groups and among men.

Nevertheless, researchers describe the discovery as a significant milestone in obesity science.

If confirmed by larger studies, it could pave the way for new treatments that target the body's natural calorie-burning mechanisms, offering fresh hope to millions struggling with obesity worldwide.

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