Extended exposure to dietary melatonin reduces tumor number and size in aged male mice

Several sets of male mice were given dietary melatonin over a series of experiments performed during a nine year period. Overall, melatonin-supplemented mice aged ≥26 months at sacrifice had significantly fewer tumors with lower severity than similarly aged control animals. The studies were originally designed to explore the potential of this agent for reducing the rate of onset of some genetic indices of brain aging. When these animals were sacrificed they were routinely examined for overt evidence of tumors and when these were found, a note was made of their occurrence, and of their size. Tumors are commonly found during senescence of several strains of mice. Since tumorigenesis was not the original intent of the study, these observations were recorded but not pursued in greater detail. In this report, these data have now been collated and summarized. This analysis has the disadvantage that tumor origin and morphology were not recorded. However, the study also has the advantage of being conducted over an extended period of time with many groups of animals. In consequence, many extraneous factors, which could be potential confounders, such as seasonal or dietary variations, are unlikely to have interfered with the analysis. The use of more than one mouse strain strengthens the possibility that the findings may have general relevance. Both aged and young animals were included in the original experiments but the tumor incidence in animals younger than 25 months was very low.