One member in each of 15 parabiosed pairs of rats was fed twice its normal food intake as four tube-fed meals per day. Seven other pairs ate ad libitum. Partners of overfed rats ate approximately 90% of the intake of individual members of ad libitum pairs. After 46 days of overfeeding, blood samples were taken and the rats were killed for carcass analysis. Tube-fed parabiotic rats had gained a considerable amount of fat and some protein. Their partners had a normal lean body mass but very little fat. Serum corticosterone, reverse triiodothyronine, free fatty acids, and beta-hydroxybutyrate were the same in all parabionts. Serum triiodothyronine and insulin were increased and growth hormone was decreased in obese rats. Serum thyroxine and triiodothyronine were increased and glucose was decreased in their parabiotic partners. The results are discussed as evidence for a humoral factor that crossed the parabiotic union and acted as a "lipid-depleting" agent in the partners of overfed rats.
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