A quiet scientific discovery from Japan has highlighted an unexpected ally in the fight against ageing: a bacterium often found in fermented milk. According to researchers, a heat-killed strain of Lentilactobacillus kefiri identified from kefir may help slow down age-related immune system deterioration, which affects practically everyone over time. 

The study, led by scientists at Shinshu University, investigated how ageing causes long-term, low-level inflammation in the body. This syndrome, known as inflammaging, gradually reduces immunity and increases the risk of chronic diseases. Rather than studying a new medicine, the researchers chose a known food source. 

Lactic acid bacteria, which are found in fermented milk products like kefir, have long been thought to be healthy, though their exact role is unknown. To bridge this gap, the researchers tried heat-inactivated Lentilactobacillus kefiri YRC2606 on elderly mice for eight weeks. Even though the bacteria were no longer living, their biological components still interacted with the immune system. 

The findings were remarkable. Mice administered YRC2606 had decreased levels of inflammatory molecules, including IL-6 and TNF-alpha, which increase with age and harm tissues with time. The medication also lowered activity in the IL-6/STAT3 pathway, which is a critical inflammatory signal associated with age-related illnesses. Simply put, the immune system appeared to be more stable and balanced. 

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Ageing also causes organs such as the thymus, which is necessary for immune cell synthesis, to decrease. The researchers discovered that YRC2606 slowed this process while lowering markers like p16 and p21, which prevent cells from proliferating and signal biological ageing. 

Ms Hiroka Sasahara, a PhD researcher at Shinshu University, explained her reason, saying, "My interest in studying immunosenescence is driven by a deep curiosity about ageing, a universal biological process that everyone experiences." She went on to say that the purpose was to look into everyday foods as evidence-based anti-ageing treatments. 

The findings were first published online on October 10, 2025, and later in Volume 134 of the Journal of Functional Foods. Ms Sasahara pointed out, "To our knowledge, this is the first report demonstrating that inactivated L. kefiri attenuates immunosenescence by regulating the IL-6/STAT3 pathway and alleviating thymic atrophy." 

While the study is based on mice rather than people, the implications are substantial, particularly for nations such as India, where the elderly population is fast expanding. Functional foods that boost immunity may one day supplement healthy diets, providing a preventive rather than a medicinal solution. 

For now, the study lends support to a basic hypothesis: what we consume on a daily basis may silently impact how we age.