Vitamin D, ageing, and the immune system

Changes occurring in the immune system along the ageing process increase the risk of infection, susceptibility to tumor development, and autoimmunity. Interventions such as physical exercise, supplements, and probiotics have been proposed in order to circumvent these conditions. Vitamin D supplementation could contribute to the immune system homeostasis in older adults since a large proportion of this population has low levels of circulating vitamin D. Additionally, observational studies have shown the association between vitamin D status and infections, chronic diseases such as cancer, diabetes, and cardiovascular disease. Recently it was observed that old patients with COVID-19 and vitamin D deficiency had enhanced severity of lung damage, longer stay at the hospital, and increased risk of death, suggesting that vitamin D plays an important role in the patient outcome from COVID-19. A high dose of vitamin D supplementation improved clinical recovery in a case-series report but in another study, no evident link between levels of vitamin D and risk of COVID-19 infection was found. Results also remain debatable for vitamin D supplements and improvement of immune response after vaccination, tuberculosis, pneumonia, and sepsis. It has been hypothesized that vitamin D could modulate the immune system and thus provide both efficacies in the immune response to pathogens/vaccinations and reduction of the inflammatory phenotype. This review will discuss vitamin D and homeostasis of the immune system; the literature-based clinical data on vitamin D and infections; and the possible link between vitamin D and immune response after vaccination.