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Utilization of the Iron of Egg Yolk for Hemoglobin Formation by the Growing Rat¹

Vitamin and Mineral Nutrition Laboratory, United States Department of Agriculture, Human Nutrition Research Division, Agricultural Research Service, Beltsville, Maryland 20705

The maintenance of hemoglobin in the growing rat was used as a bioassay index to determine the relative biological value (RBV) of the iron in egg yolk. Weanling rats were fed a casein-based purified diet that contained a reference salt or egg yolk as iron source. The response to ferrous ammonium sulfate as the reference iron compound was set at 100. The RBV for different egg yolk preparations was: spray dried 20, raw 29, cooked 32, and whole egg 30. Fifty parts per million copper in addition to the 23 ppm in the basal diet produced only a slight increase in hemoglobin response. The yolk diets contained approximately 0.2% more phosphorus than the reference diet, but the addition of this amount of phosphorus did not suppress the response to the reference salt. Ascorbic acid at 0.125% of the diet increased the RBV of 20 ppm yolk iron to 100 and as little as 0.03% resulted in a statistically significant increase in hemoglobin. Other reducing agents without complexing potential did not improve the RBV of yolk iron, but those with complexing potential did. In vitro gel permeation chromatography experiments showed that ascorbic acid frees the iron from the yolk phosphoprotein. The chemical nature of food iron is a determining factor in its biological utilization.

KEY WORDS: • iron availability • egg yolk • ascorbic acid

¹ Presented in part at the meeting of the American Institute of Nutrition, Chicago, Ill. April 1971, Federation Proc. 30: 461 (abstr.).

Manuscript received 22 November 1971.