Vitamin D (vitD) deficiency is widespread and is associated with pregnancy complications. VitD’s role in pregnancy and the placenta is unclear as vitD’s best known role is in calcium (Ca) homeostasis. Pregnancy alters many aspects of physiology and potentially vitD metabolism. We investigated the effects of vitD and Ca deficiency in pregnant and non-pregnant mice.

Female C57BL6 mice were fed one of 8 assigned diets from 3 weeks old. The control diet contained a standard 1000 IU vitD + 1% Ca, while deficient diets were modified for Ca (1% or 0.1%) and vitD (1000, 500, 100 or 0 IU/kg vitD). At 12 weeks of age mice (n=120) were mated, then killed at day 18.5 post-coitum for body composition, fetal and placental analyses. Serum was analysed for vitD (25D) and Ca (n=80).

58% of these mice were pregnant and had greater relative liver and parametrial fat weights compared to non-pregnant mice (↑31%, ↑21%, respectively; P<0.001; organ weight/body weight minus fetal and placental weights). Mice consuming a 0.1% Ca diet had similar serum 25D levels whether they were pregnant or not and irrespective of dietary vitD. In non-pregnant mice, 25D increased with increasing vitD in mice consuming 1% Ca diets. However, 25D only increased in pregnant animals fed the control diet, although this was not significant due to low n assayed to date. Serum Ca levels were similar across diets but were increased by 17% in pregnant compared to non-pregnant mice (P<0.001).

Pregnancy alters both maternal physiology and body composition. Pregnant mice had higher serum Ca and lower 25D levels than non-pregnant mice, as seen in human pregnancies. Our data suggest that both a vitD and Ca replete diet is required to prevent 25D deficiency in late pregnancy. VitD on its own may not be sufficient unless the mother is calcium replete.