Fetal bone mineralization relies on calcium and vitamin D supply from the mother. Some evidence suggests fetal vitamin D concentrations have long lasting effects on bone health but studies are inconsistent.
When in the womb, fetuses rely on placental blood for a good supply of nutrients necessary for growth and development. Among the essential nutrients are calcium and 25-hydroxyvitamin D (vitamin D), needed for bone growth. About 30 g of calcium need to be transferred from the mother to the fetus by the end of pregnancy for appropriate bone growth, and adequate vitamin D is essential for maintaining calcium levels in the blood. However, previous studies are inconsistent regarding the effects of maternal vitamin D on long-term bone mass content of children, so a comprehensive study was conducted to determine its long-term effects.
5294 mother and child pairs were included in this study by Garcia et al., published in The Lancet. The study examined the association between vitamin D concentrations mid-pregnancy and at birth against bone health of offspring. Maternal vitamin D concentrations were measured via blood sample, and subjects were categorized into severely deficient, deficient and sufficient groups. At 6 years, the offspring were examined for total-body bone mineral density, bone mineral content (BMC), area-adjusted BMC, and bone area using dual-energy X-ray absorptiometry (DXA). Additional information was also gathered, such as gestational age, birth size, ethnicity, sunlight per day during pregnancy and alcohol use during pregnancy.
The study showed an inverse relationship between fetal vitamin D concentrations and bone mineral content and bone area at age 6. Those whose mothers were severely deficient during pregnancy actually had higher bone mineral content and larger bone area during childhood. Taking into account the child’s own vitamin D levels at 6 years, no significant relationship between concentration levels and bone mineral content remained. This can be explained by studies that suggest fetal calcium is obtained independent of maternal vitamin D levels and vitamin D levels are actually only relevant post-birth. Fetal calcium concentration has been shown to be more linked to fetal parathyroid hormone than maternal vitamin D concentration. Also, 25-hydroxyvitamin D is a precursor to the active form of vitamin D, meaning it may not correlate with vitamin D activity. It was used because it is easier to measure than 1,25-dihydroxyvitamin D (the active form).
Strengths of this study included a large sample size and detailed collection of information, such as bone mineral density, vitamin D concentration and various adjustments for confounding factors. It was, however, limited by missing information on certain parameters such as parathyroid hormone levels, and a lack of information on children’s diet and vitamin D levels during infancy prior to the 6-year follow-up. Regardless, the study showed no evidence of maternal vitamin D concentrations affecting long-term offspring bone outcomes.
Written By: Wesley Tin, BMSc