Gut Microbiome Metabolism Links High Fat Diets to Immunotherapy Efficacy

Patients with high body mass index (BMI) often respond better to immune checkpoint inhibitors (ICI) in several cancers, including lung cancer, a paradox given obesity's association with metabolic dysfunction and gut dysbiosis. To isolate the contributing mechanism, Desharnais and colleagues systematically modeled a diverse range of 12 diets in mice that reflect variation in human dietary patterns, spanning high fat, low fat, Mediterranean, Japanese, and other Western patterns, and assessed immune changes across models. Obesogenic diets were associated with fewer T cells and more monocytes in the blood, with a higher proportion of PD-1+ T cells. In a lung cancer mouse model, this systemic pattern did not predict outcome: obesity had no effect on tumor growth, and ICI efficacy was highest in mice fed high fat diets, independent of body weight or metabolic status. This suggested that the immune changes associated with obesity and the mechanism driving ICI response were independent. High fat diet mice shared a distinct gut microbial signature, particularly enrichment of Lactobacillus, previously linked to favorable ICI response. Switching nonresponsive mice to a high fat diet conferred treatment sensitivity, while switching responsive mice away from this diet reversed the effect. Antibiotic treatment eliminated the benefit, confirming gut bacteria, rather than diet or metabolic status, were required for this effect. Lactobacillus johnsonii were identified as a key species, and its effect was traced to a bacterial metabolite, desaminotyrosine (DAT), produced during microbial breakdown of tyrosine—an unexpected role for this bacteria that may be diet- or context-specific. Administering DAT to nonresponsive mice restored ICI sensitivity. In human lung cancer patients, a related metabolite was elevated among ICI responders, and transplanting gut bacteria from high, but not low, BMI donors restored ICI response in mice. Collectively, these findings identify a diet-driven microbial pathway, separate from obesity's metabolic effects, as the basis of the obesity paradox in ICI response, suggesting dietary intervention as a potential therapeutic strategy.Desharnais L, Swaby A, Messaoudene M, Doré S, Yu MW, Fiset B, et al. Diet–microbiome synergy underlies obesity-associated immunotherapy efficacy. Nature 2026 Jul 8 [Epub ahead of print].Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at https://aacrjournals.org/cdnews.