Nutritional Studies with the Hyperthyroid Rat

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Abstract
Experiments designed to improve the assay for substance(s) in liver and other natural materials which partially reverse the growth suppression of hyperthyroid rats have been reported. A three-week curative assay which gives more reproducible results than the 5-week preventive assay has been developed and used. The assay involves feeding weanling rats a typical synthetic-type sucrose-casein-corn oil diet for one week and then a sucrose-casein-Crisco diet containing 0.243% iodinated casein for two weeks. These depleted, hyperthyroid rats are then used for the assay of liver or other natural products. Cholesterol, corn oil or crystalline aureomycin stimulated growth slightly under these assay conditions. A combination of these three substances was almost as effective as 10% liver residue alone. The liver residue plus these three substances supported growth and survival nearly equivalent to that obtained by removing the iodinated casein from the diet. Casein and many other proteins or protein-containing supplements were not active. Beef spleen, kidney and lung exhibited some activity. The active substance(s) in pork liver was not soluble in a mixture of chloroform and methanol. Removal of iodinated casein from the diet lowered the basal metabolic rate and caused immediate resumption of growth. A liver residue active in stimulating growth did not alter the basal metabolic rate.