Rejuvenating effects of 10-week underfeeding period on estrous cycles in young and old rats

The effects of providing 50% of normal feed intake for 10 weeks followed by 16 weeks of ad lib feeding on estrous cycles and mammary tumor incidence were studied in female rats initially 4 months and 15–16 months old. Initially all young rats exhibited regular or irregular estrous cycles and only about 41% of the older rats cycled regularly or irregularly; the remainder of the older rats did not cycle. During underfeeding, both the young and older rats lost body weight and ceased to cycle. After refeeding 100% of both young and old rats resumed cycling, the young rats for a much longer period than the old rats, and more of both groups continued to cycle than their ad lib-fed controls. Upon refeeding, the young and old rats reached the body weights of the ad lib-fed controls in about 3 weeks. Mammary tumors were initially present only in old rats and regressed during underfeeding; they rapidly reached control size upon refeeding. Plasma PRL levels declined during underfeeding but rebounded to higher than control values upon refeeding in both young and old rats. In young but not in old rats, plasma LH levels fell during underfeeding but returned to control values upon refeeding. These results demonstrate that a relatively short period of underfeeding, followed by refeeding, can delay the decline in reproductive cycles in young rats and reinitiate estrous cycles in older rats. These effects appear to be mediated via the neuroendocrine system.