Recent studies have demonstrated that vitamin D, a nutrient found in fatty fish, plays a significant role in cancer prevention and treatment. Research found that mice on a vitamin D-rich diet had enhanced immune responses to transplanted cancers and better outcomes from immunotherapy. Vitamin D initiates a process in the gut that leads to the production of bacteria known to boost immune defence against cancer. This is because vitamin D affects the cells lining the intestine, altering the gut microbiome in a way that promotes the growth of these beneficial bacteria. The findings suggest potential new cancer treatments could be developed focusing on the gut microbiome. However, more research is needed to understand the precise mechanisms by which vitamin D influences the microbiome and immunity. The study also connects lower vitamin D levels with higher cancer rates, underlining the importance of maintaining adequate levels for overall health.