Mice subjected to high dose ultraviolet light for about 6 months while fed a diet supplemented with 0%, 0.1%, 0.5%, or 1.0% (by dry weight of feed) of niacin showed 68%, 60%, 48%, and 28% rates of skin cancer respectively, indicating a protective effect. Elevated levels of NAD found in niacin supplemented mice. NAD modulates the function of DNA strand scission surveillance proteins p53 and poly(ADP-ribose) polymerase, two proteins critical in cellular responses to UV-induced DNA damage.