High fiber dietary intake improved progression-free survival in patients.Fiber-rich diet delayed tumor growth in mice.Fiber-rich diet had no impact on tumor response in germ-free mice.Higher propionate levels were observed in mice receiving a fiber-rich diet.Fiber-rich diet increased the number of tumor-infiltrating CD4+ cells and IFNγ+ cytotoxic T cells. Here, CD4 is linked to neoplasm.