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We assessed the prevalence of vitamin D deficiency and insufficiency in 2-year-old Japanese children using data from a large ongoing birth cohort study.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>Data for analysis was obtained from the Japan Environment and Children\u2019s Study (JECS) and a Sub-Cohort Study (SCS) of JECS. We evaluated the children\u2019s serum 25(OH) D levels by 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles, and the rates of vitamin D deficiency or insufficiency. We also presented a weighted prevalence rate for vitamin D deficiency or insufficiency among all children in JECS.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>After excluding children with missing 25(OH)D2 or 25(OH)D3 data, we analyzed 4655 remaining children, of whom 24.7% (95% CI, 23.5\u201326.0%) had vitamin D deficiency (&lt;\u200920\u2009ng\/mL), and 51.3% (95% CI, 49.8\u201352.7%) were at risk of vitamin D insufficiency (20\u201330\u2009ng\/mL). The estimated prevalence of vitamin D deficiency and insufficiency among all children in JECS were 25.4% (95% CI, 24.1\u201326.7%) and 50.9% (95% CI, 49.4\u201352.4%). Vitamin D deficiency was found in 22.9% of boys and 26.5% of girls. Median serum 25(OH) D concentrations were lower among participants measured during winter and spring than among those measured in summer and autumn. The highest rate of vitamin D deficiency was observed in Hokkaido, the northernmost prefecture of Japan.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>We analyzed data on serum 25(OH) D levels from a birth cohort study and found that vitamin D deficiency and insufficiency are very common among 2-year-old Japanese children. Sex, season, and latitude affect serum 25(OH) D concentrations.<\/jats:p><\/jats:sec>","DOI":"10.1186\/s12887-021-03005-3","type":"journal-article","created":{"date-parts":[[2021,12,2]],"date-time":"2021-12-02T07:03:29Z","timestamp":1638428609000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["25-Hydroxyvitamin D levels among 2-year-old children: findings from the Japan environment and Children\u2019s study (JECS)"],"prefix":"10.1186","volume":"21","author":[{"given":"Limin","family":"Yang","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Miori","family":"Sato","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mayako","family":"Saito-Abe","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Makoto","family":"Irahara","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Minaho","family":"Nishizato","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hatoko","family":"Sasaki","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mizuho","family":"Konishi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kazue","family":"Ishitsuka","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hidetoshi","family":"Mezawa","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kiwako","family":"Yamamoto-Hanada","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yukihiro","family":"Ohya","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"name":"for the Japan Environment and Children\u2019s Study (JECS) Group)","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michihiro","family":"Kamijima","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shin","family":"Yamazaki","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yukihiro","family":"Ohya","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nobuo","family":"Yaegashi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Koichi","family":"Hashimoto","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chisato","family":"Mori","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shuichi","family":"Ito","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zentaro","family":"Yamagata","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hidekuni","family":"Inadera","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Takeo","family":"Nakayama","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hiroyasu","family":"Iso","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Masayuki","family":"Shima","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Youichi","family":"Kurozawa","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Narufumi","family":"Suganuma","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Koichi","family":"Kusuhara","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Takahiko","family":"Katoh","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Reiko","family":"Kishi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2021,12,2]]},"reference":[{"issue":"1","key":"3005_CR1","doi-asserted-by":"publisher","first-page":"44","DOI":"10.1111\/nyas.13968","volume":"1430","author":"DE Roth","year":"2018","unstructured":"Roth DE, Abrams SA, Aloia J, Bergeron G, Bourassa MW, Brown KH, et al. 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All procedures involving human participants were approved by the Japan Ministry of the Environment\u2019s Institutional Review Board on Epidemiological Studies (No. 100406001) and the ethics committees of all participating institutions. Written consent to the SCS protocol is signed separately from that of the Main Study and obtained from all participating mothers and fathers.","order":2,"name":"Ethics","group":{"name":"EthicsHeading","label":"Ethics approval and consent to participate"}},{"value":"Not applicable.","order":3,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent for publication"}},{"value":"The authors have no conflicts of interest to declare.","order":4,"name":"Ethics","group":{"name":"EthicsHeading","label":"Competing interests"}}],"article-number":"539"}}