Scientists have long puzzled over a strange pattern in cancer treatment: patients with obesity often respond better to immune checkpoint inhibitors, a class of drugs that unleash the immune system against tumors, than patients of normal weight. This is despite obesity's well-known links to inflammation and poor metabolic health. A new study published in Nature suggests the answer lies not in metabolism at all, but in the gut microbiome shaped by diet.
Researchers designed 12 distinct mouse diets to more realistically mirror the variety of human eating patterns, rather than relying on the standard high-fat "obesity" diet used in most lab studies. Alongside conventional Low Fat, High Fat and Western diets, they built versions meant to approximate Mediterranean, Japanese, vegan, American and ketogenic eating patterns, varying protein sources (from casein to pea protein), carbohydrate sources, fats, and fiber types such as psyllium, inulin and pectin.
After 15 weeks, the diets produced a wide range of body weights, body composition, glucose tolerance, and insulin and leptin levels, allowing the team to calculate a composite metabolic dysfunction score for each diet group. Consistent with earlier research, mice with worse metabolic scores had fewer CD4+ and CD8+ T cells in their blood, more PD-1-positive CD8+ T cells (a marker linked to T-cell exhaustion), and higher levels of certain monocytes — immune changes relevant to how tumors respond to checkpoint inhibitors.
Obesity and the gut, not metabolism
When the researchers tested these diet groups in a mouse model of lung cancer, they found no correlation between tumor size and body weight, glucose intolerance, insulin, leptin or the overall metabolic score. That result echoed clinical observations that, unlike in many other cancers, obesity does not drive lung cancer progression in these models. Yet immune cell composition still varied substantially between diet groups, both in the blood and in tumors at the end of the experiments.
The findings feed into a broader argument the authors make: that obesity-associated benefits for immunotherapy are tied to a "diet–gut axis" rather than to metabolic dysfunction itself. Obesogenic diets, they report, foster a resilient community of gut microbes that can restore sensitivity to checkpoint inhibitors, even after short-term diet changes or fecal microbiota transplants from mice that otherwise do not respond well to treatment. Introducing specific bacteria such as Lactobacillus johnsonii into germ-free mice on an obesogenic diet promoted tumor regression, associated with an increase in microbial metabolites derived from aromatic amino acids.
The team also transplanted fecal microbiota from human donors into mice: transplants from donors with high BMI improved immunotherapy effectiveness compared with transplants from normal-BMI donors, and an obesogenic diet restored treatment sensitivity after a transplant from a patient who had not responded to therapy.
The results offer a possible explanation for a long-standing epidemiological puzzle and point toward diet and microbiome-based strategies, including fecal transplants, as potential tools for improving how patients respond to cancer immunotherapy.