A fibre-rich dietary pattern with probiotics and less saturated fat, combined with reducing chronic stress, is the best-supported approach for improved immune function. The strength of evidence is, however, moderate and largely based on animal models and mechanistic studies, not on large clinical trials in healthy adults.
Probiotics (live lactic acid and bifidobacteria, found in fermented products or as a supplement) can have a beneficial effect on the immune system. They strengthen the gut barrier and, via so-called pattern recognition receptors, activate signalling pathways that regulate inflammatory responses. Researchers are clear about one caveat: the precise mechanism is not yet fully understood, and the strength of evidence is 'moderate' rather than strong (PMID: 23037511).
Prebiotics, which are certain dietary fibres that stimulate the growth of beneficial gut bacteria, are associated with measurable changes in immune markers. In infants, a lower risk of infection has also been observed. In adults, the immune effects are less well substantiated and have not yet been definitively proven. This means that eating a fibre-rich diet (vegetables, legumes, wholegrains) is a worthwhile step, but not a guaranteed remedy (PMID: 20920376).
A disrupted gut microbiome, also known as dysbiosis, appears to be linked to impaired immune regulation and has been associated with a range of conditions. An important limitation applies here, however: most of the evidence comes from animal studies. Direct causality in humans has not yet been sufficiently demonstrated (PMID: 40804450).
Chronic stress actively undermines the immune system. In mouse models, prolonged stress was shown to release stress hormones that modify small cellular vesicles (exosomes); these then push immune cells (neutrophils) into a pro-inflammatory programme. This mechanism has been studied primarily in the context of cancer spread and in animals, but it illustrates that persistent stress is not a neutral factor for immune defence (PMID: 39630109).
A diet high in saturated fat, and more specifically a high blood level of palmitic acid as occurs in obesity, drives immune cells (macrophages) towards chronic low-grade inflammation. This dysregulates the immune system rather than strengthening it, and can contribute to insulin resistance. A dietary pattern with less saturated fat is therefore relevant to good immune function (PMID: 31363792).
Finally, there is research into short-term fasting (16 hours) in combination with cancer immunotherapy. In mice and a limited number of cancer patients, this was found to improve the functioning of specific immune cells (CD8+ T cells). To be clear: this research is aimed at people already undergoing immunotherapy for cancer, not at strengthening general immune defence in healthy individuals. Translating these findings to everyday use is premature (PMID: 41720105).
All claims are based on moderate evidence. A large part of the mechanistic insights comes from animal models (mouse). Clinical evidence in healthy humans is absent or limited. No robust RCT data with long-term immune outcomes in adults are available in the sources provided.