But taking into account the sporadic reports of subclinical rickets, vitamin D deficiency and hypocalcemia induced seizure 14- 16), it is necessary to investigate the probability of vitamin D deficiency or insufficiency of even healthy-looking infants. In case of South Korea, there are few reports of vitamin D state of the pediatric population and measuring value of vitamin D, particularly of asymptomatic infants or children, except a recent report that presented 29.8% of Korean children under 2 years old for vitamin D deficiency 12, 13). In the case of East Asia, according to small-scale studies of Asian nations, 31.2% to 57.8% of Chinese adolescent girls, and 76% of Mongolian children were in a state of vitamin D insufficiency 10, 11).

Recently, a large study in the United States (US) noticed that 9% of the pediatric population were vitamin D deficient and 61% were 25(OH)D insufficient 9). However there are not any generalized statistical data of vitamin D measurement in normal babies and few comparing studies of vitamin D level between breastfed and formula-fed groups, so it is difficult to give straight answers for vitamin D deficiency of breast-fed or formula milk fed infants and an efficient method of supplementation for them. Currently, previous reports that presented a high prevalence of vitamin D deficiency of exclusively breastfed infants have aroused public opinions about how aggressive vitamin D supplement needs to be for breastfed infants 1, 4- 8).

Therefore, there has been a growing interest in vitamin D deficiency and its supplementation 1- 3). In particular, vitamin D deficiency induced disorders such as rickets or osteomalacia are able to arrest normal growth and the development of infancy and childhood, and vitamin D has been currently known to influence the extraskeletomuscular system as well as the immune system 1, 2).

Vitamin D is a fat-soluble vitamin that performs a key role in calcium homeostasis and bone metabolism 1).

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