Serum hormones in diet-restricted gnotobiotic and conventional Lobund-Wistar rats

In order to clarify the reasons for life extension due to the germ-free state and mild dietary restriction, serum levels of thyroid stimulating hormone, thyroxine, triiodothyronine, prolactin, and testosterone were determined in conventional and gnotobiotic (single microbe contaminations) Lobund-Wistar rats fed either ad libitum or restricted to 12 grams (70% of ad libitum) of diet per day from weaning. Thyroid hormone levels were slightly higher in gnotobiotic rats, declined between 7 and 30 months of age in all rats, and were not affected by dietary restriction. Prolactin showed a significant rise with age within the ad libitum rats only. Testosterone levels declined significantly with age in all rats, but were significantly higher in restricted rats at all ages. These results indicate that mild dietary restriction extends life span without reducing circulating levels of thyroid stimulating hormone, thyroxine, triiodothyronine, or testosterone in mature Lobund-Wistar rats, and prevents the age-associated rise in serum prolactin. Life extension in germ-free rats is not related to differences in endocrine function.