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Exclusively breastfed healthy, full-term infants from birth to six months who have adequate exposure to sunlight are not at risk for developing vitamin D deficiency or rickets. Rickets occurs because of a deficiency in sunlight exposure, not because of a deficiency in human milk.
"Vitamin D" is a steroid hormone
Very few foods naturally contain significant amounts of vitamin D. The biologically normal and most common means of obtaining adequate levels of vitamin D for human beings, including infants, is through casual exposure of the skin to sunlight. Because the skin has a large capacity to produce vitamin D, adequate levels can be developed from partial exposure of the body to small amounts of sunlight well before sunburn occurs. According to the World Health Organization and United Nations Children's Fund, small amounts of UVB radiation are beneficial for people and essential in the production of vitamin D. However, excessive sunlight exposure can cause sunburn and increase the risk of skin cancer.
Concerns about sunlight deficiency, vitamin D and breastfeeding have been raised by a new American Academy of Pediatrics (AAP) clinical report. The AAP now recommends that all infants have a minimum intake of 200 IU of vitamin D per day beginning during the first two months of life.