Protective effect of beta-carotene against colon tumors in mice.

The effect of dietary beta-carotene on colon carcinogenesis induced by 1,2-dimethylhydrazine [(DMH) CAS: 540-73-8] was studied in female inbred Swiss Webster (ICR) mice. At age 10 weeks and continuing throughout the experiment, mice received diets consisting mainly of natural foods (laboratory chow) and containing 2 or 22 mg beta-carotene/kg. At age 15 weeks they received 7 weekly sc injections of DMH (total dose: 196 mg DMH X diHCI/kg body wt). When autopsied 31 weeks after the first DMH injection, the incidence (percent of mice with tumors) and multiplicity (number of tumors/tumor-bearing mouse) of colon tumors were reduced by half in the mice supplemented with beta-carotene. There was a much greater decrease in adenocarcinomas than in adenomas. Mice observed for 13 additional weeks revealed that the mortality rate, due largely or wholly to colon cancer, was only about half in supplemented mice. Mice sacrificed 12 weeks after the first dose of DMH (i.e., well before tumors appeared) showed mild colon mucosal hyperplasia. beta-Carotene supplementation, however, did not alter this, indicating that the protective effect against colon cancer may have occurred at a late stage of carcinogenesis.