Fasting during promotion, but not during initiation, enhances the growth of methylnitrosourea-induced mammary tumours

The purpose of this work was to investigate the effect of fasting on the induction and growth of chemically-induced mammary carcinogenesis. Female Sprague-Dawley rats were given methylnitrosourea (MNU) i.p. (50 mg/kg) at 50 days of age; a group of rats were exposed to 4 day fasting followed by 1 day of refeeding before the administration of the carcinogen, while another group was exposed to three cycles of 3 days fasting in 10 days, beginning 1 week after MNU injection. Fasting enhanced the development of mammary tumours only in rats fasted after carcinogen damage, while it did not affect the induction of tumours in rats fasted before MNU, if compared with full-fed controls. The enhanced growth of mammary tumours sustained by fasting during promotion was observed in the cervical-thoracic region. In addition, exposure to fasting made rats susceptible to the development of MNU-induced extra-mammary cancers. Different from the preventive effect of caloric restriction on tumor development, these data demonstrate that fasting affects the promotion phase of carcinogenesis by enhancing the growth of MNU-induced mammary tumours.