A biphasic response of rats to cobalt.

The study was designed to confirm a previous, unexpected observation of a strong growth depressing effect of 1 microgram cobalt/g in rats fed lactalbumin based diets. The addition of 0.25, 0.5, and 1.0 microgram of cobalt/g to the basal diet containing 0.056 microgram/g depressed growth rates of rats progressively with increasing doses. This depression was overcome by increasing the cobalt supplement to 2 microgram/g, and additional weight gain was observed with 3 microgram/g. Higher concentrations were progressively toxic. Hemoglobin concentrations, hematocrits, and thyroid retention of intravenously injected sodium iodide all were lowest in rats fed the diet containing 1 microgram cobalt/g and increased with lower and higher concentrations of cobalt. The opposite was true for fasting serum glucose levels, which were elevated in rats fed the 1 microgram/g diet and low in rats fed the 3 microgram/g diet or control diet. This biphasic response to cobalt is consistent with the hypothesis that cobalt in low concentrations may have an essential function in the rat. However, an alternative explanation, an interaction of cobalt with a toxic constituent of the diet, has not yet been ruled out.