Since no foods are vitamin D supplemented in Greece, vitamin D status was assessed in mothers at birth and their infants up to the first 6 months of life, while they were exclusively breast-fed. This was a prospective study. Full-terms (n =35) born during the summer-autumn months and their mothers were assigned to the summer group and the remainder (n =31) to the winter group. One week after birth, serum 25-hydroxyvitamin D (25OHD) was significantly lower in the winter-born than in the summer-born neonates (6.7+/-0.7 vs. 10.1+/-0.9 ng/ml, P <0.01). The respective levels of parathyroid hormone (iPTH) were 64.9+/-13.4 and 33.9+/-4.4 pg/ml (P <0.01). The mothers had serum 25OHD levels of 10.8+/-1.0 ng/ml and iPTH levels of 15.2+/-3.5 pg/ml in the winter and 12.9+/-1.3 ng/ml and 24.8+/-4.8 pg/ml in the summer. During the 6-month follow-up, a steady increase in circulating 25OHD (up to 19.4+/-2.8 ng/ml, P <0.0001) and a decrease in iPTH (to 26.8+/-3.5 pg/ml, P =0.10) were observed in the infants born in the winter. In the summer-born infants, serum 25OHD did not change but iPTH had increased significantly by the 3rd month (59.4+/-21.8, P <0.05). Serum calcium (Ca) increased within normal limits during the study period in both groups. Serum phosphorus (Pi) started higher in the winter group (7.43+/-0.38 vs. 6.27+/-0.23 mg/dl, P <0.01) but thereafter, it was similar in both groups. Total alkaline phosphatase (ALP) increased in both groups during the study (164+/-15 vs. 219+/-17 IU/l, P <0.05 and 189+/-14 vs. 288+/-35 IU/l, P <0.001, respectively). Serum osteocalcin (OC) decreased in the winter-born neonates (32.0+/-3.4 vs. 21.5+/-3.4 ng/ml, P <0.05) and did not change in the summer group (28.9+/-3.5 vs. 26.5+/-2.8 ng/ml).
Conclusion: Neonates who are breast-fed exclusively during the first 6 months of life are in need of vitamin D supplementation irrespective of the season even in a sunny country like Greece where foods are not supplemented.