Zinc has an essential role in growth and developmental processes. Therefore, zinc deficiency in infants can result in various disorders including growth restriction, skin lesions, and alopecia. Zinc concentrations in breast milk are considerably higher than those of the maternal serum to meet infant's requirements. Thus, effective mechanisms ensuring secretion of large amounts of zinc into the milk operate in mammary epithelial cells during lactation. Recently, the zinc transporter ZnT2 was found to play an essential role in the secretion of zinc into breast milk. Several missense mutations of ZnT2 have been identified in the mothers, who secreted low zinc breast milk, causing their breast-fed infant to develop transient neonatal zinc deficiency. This paper reviews current knowledge of transient neonatal zinc deficiency and low milk zinc concentrations caused by ZnT2 mutations, discussing the molecular basis of ZnT2 functions.

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Sprouts of Kaiware radish (Raphanus sativus) were hydroponically cultivated in an environment with exposure to zinc at a level of 0, 20, 50, 100 or 200 μg /mL as zinc sulfate. Sprouts were exposed to zinc by two methods; one was performed during the hydroponics with a solution containing zinc sulfate (method 1) and another was performed by using seeds soaked in a solution containing zinc sulfate for 24 h before the hydroponics with a distilled water (method 2). In both methods, growth of sprouts was inhibited by the exposure to zinc at a level of 50 μg/mL or more. However, lower inhibition was observed in sprouts prepared by the method 2 than those prepared by the method 1. Zinc contents of sprouts were increased dependently to the zinc exposure levels up to 100 μg/mL; zinc contents of sprouts exposed to 100 μg/mL were 83.3 and 87.8 μg/g fresh weight in the method 1 and 2, respectively. In the case of exposure to 200 μg/mL, zinc contents of sprouts were saturated in the method 2. Based on these results, we decided to prepare zinc-enriched Kaiware radish sprouts by using the method 2 at a zinc exposure level of 100 μg/mL. To evaluate nutritional availability of zinc contained in the zinc-enriched sprouts, Male weanling Wistar rats were fed a low zinc basal diet (zinc content, 6.0 μg/g) or the basal diet supplemented with 20 μg/g of zinc as zinc sulfate or the zinc-enriched sprouts for 4 weeks. Irrespective of the zinc source supplied, the dietary zinc supplementation increased tissue zinc concentration in kidney, tibia and serum. There was no difference in apparent zinc absorption estimated from the zinc intake and the fecal zinc excretion between rats supplemented with zinc sulfate and the zinc sprouts. These results obtained by the animal experiment indicate that zinc in the zinc-enriched sprouts is nutritionally available equally to zinc sulfate.

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