Age Affects the Generation of Serum Thyronine-Binding Protein by Rats Fed a Low Protein-High Carbohydrate Diet*

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Abstract
The feeding of a low protein-high carbohydrate (PM) diet to young (8-wk-old) rats induces the production of serum thyronine-binding globulin. The effect of age on this process was studied in older (5-mo.-old) rats. In contrast to the findings in 8-wk-old rats, no significant differences in the serum concentrations of T3 [triiodothyronine] and free T3 and the percentage of free T3 (equilibrium dialysis) were observed between the 5-mo.-old rats fed the control diet and those fed the PM diet. There were also no significant differences between the 2 diet grops in the distribution of tracer [125I]T4 and [135I]T3 among the serum proteins as assessed by gel electrophoresis. The changes in thyronine-binding globulin and subsequent alterations in T3 binding previously observed in young rats fed a PM diet do not occur in older rats, suggesting that the age and perhaps the weight of rats must be considered in studies related to diet-induced effects on thyroid hormone economy.