A method of utilizing the physiological activity of a rare saccharide, wherein physiological-activity sensitive cells are treated with the rare saccharide to modify the function of the cells. A composition containing, as an active ingredient, a rare saccharide which is introduced into physiological-activity sensitive cells and has an effect of modifying the function of the cells. The cells are human cells. The composition is a functional food, a drug, or a cosmetic. The rare saccharide is a rare saccharide belonging to aldose and/or ketose. The aldose is D-allose, and the cells are selected from the group consisting of cancer-cell proliferation inhibitory activity sensitive cells and active-oxygen production inhibitory activity sensitive cells. The ketose is D-psicose, and the cells are selected from the group consisting of chemokine secretion inhibitory activity sensitive cells, microglia migration inhibitory activity sensitive cells, and hypoglycemic activity sensitive cells.