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Breast milk vitamin D and 25OHD concentrations divided by season and region in 1989 and 2016–2017. Total vitamin D and Total 25OHD concentrations in summer (a) and winter (b). Solid bar: 1989, Gray bar: 2016–2017. Data are represented by the mean ± SD. Student’s t-test: * p < 0.05, ** p < 0.01.
