Development of mechanisms for intestinal absorption of vitamin B₁₂ in growing rats

¹ Department of Physiology, Harvard Medical School, Boston, Massachusetts

Intestinal uptake of vitamin B₁₂, with and without gastric intrinsic factor (IF), was studied with isolated tissue from rats at various stages of growth. The intestine from fetal and newborn animals showed extremely high uptake of the vitamin in the absence of IF. This IF-independent B₁₂ uptake fell rapidly with time, approaching adult levels at about 14 days. Stimulation of B₁₂ uptake by IF was detectable in the intestine of newborn rats in spite of the high vitamin uptake in its absence. The per cent stimulation due to added IF increased during the 1st 2 weeks because of drastic reduction of the IF-independent B₁₂ uptake. The development of IF and pepsinogen in the stomach of growing rats was also investigated. The stomach wall of fetal and newborn rats contained less than 1% of the IF found in the adult. After the 1st week the amount increased rapidly, reaching adult levels at weaning. A rapid increase in pepsinogen content did not occur until the 3rd week. As the IF-independent transport system decreased in activity, intrinsic factor appeared in the stomach and the adult mechanism became the predominant system for B₁₂ absorption.