New meta­bolic BMI: When body weight doesn’t paint the whole picture

The classic body mass index (BMI) is robust, simple – and, as we know, rather crude. In our blog post on the Body Round­ness Index, we have already shown how much measu­re­ments of body shape can diverge and how little they some­times contri­bute to risk assess­ment in ever­yday clinical prac­tice. Now there is another chal­lenger: the meta­bolic BMI (metBMI) .

The metBMI is not based on body measu­re­ments, but on an AI-supported model of blood meta­bolites. It was deve­loped by rese­ar­chers at the Univer­si­ties of Leipzig and Gothen­burg. The starting point is a known weak­ness of the BMI: up to 30% of people of normal weight already have meta­bolic disor­ders, whereas some over­weight people remain meta­bo­li­cally normal. The BMI does not capture the func­tional quality of adipose tissue.

The new index is the result of a study published in Nature Medi­cine invol­ving almost 2,000 parti­ci­pants, in which exten­sive multi-omics data was analysed. From over 1,000 meta­bolites, the authors iden­ti­fied a clini­cally mana­geable panel of 66 markers that accu­ra­tely reflect obesity-related dysfunc­tions. An elevated metBMI was asso­ciated with a two- to five-fold higher risk of meta­bolic fatty liver, type 2 diabetes, visceral fat accu­mu­la­tion, insulin resis­tance and systemic inflamm­a­tion – regard­less of actual BMI. In addi­tion, indi­vi­duals with high metBMI lost around 30% less weight after baria­tric surgery, suggesting meta­bolic ‘therapy resistance’.

Parti­cu­larly exci­ting is the link with the gut micro­biome: as metBMI increased, the diver­sity of gut bacteria decreased, as did their ability to produce protec­tive short-chain fatty acids such as buty­rate – a possible mecha­ni­stic link between obesity, chronic inflamm­a­tion and insulin resistance.

What does this mean in prac­tice? While the Body Round­ness Index (BRI), Waist-to-Hip Ratio (WHR) and Waist-to-Height Ratio (WtHR) prima­rily quan­tify fat distri­bu­tion, metBMI addresses the biolo­gical quality of adipose tissue. It could help to iden­tify high-risk pati­ents earlier, perso­na­lise treat­ment decis­ions and break the ‘weight equals risk’ para­digm. The scales remain important – but they now have meta­bolic competition.

Chakaroun RM et al. Multi-omic defi­ni­tion of meta­bolic obesity through adipose tissue-micro­biome inter­ac­tions. Nat Med. Published online 2 January 2026.