Vitamin D Effects on Bone Health
Skeletal growth is a dynamic process that requires 25 to 30 g of calcium to be transferred from the mother to the fetus during pregnancy. This calcium transfer is achieved through increased intestinal calcium absorption, which increases from 33% to 36% before pregnancy to 54% to 62% in the third trimester. Vitamin D facilitates the intestinal absorption of calcium, but its effects reach a plateau at 32 ng/mL, beyond which there is no further rise in calcium absorption. At suboptimal levels, vitamin D's availability limits the physiologic control of calcium transport across the mucosal membrane. In severe cases of vitamin D deficiency, secondary hyperparathyroidism or osteomalacia may result. To date, no studies have investigated the optimal level of 25(OH)D during pregnancy.
When pregnant, a women's body will put the physiologic needs of the baby before her own. For example, a vitamin D–deficient mother who develops secondary hyperparathyroidism will leach nutrients from her bones and tissues and transfer them to her fetus. This will have an adverse effect on maternal bone metabolism but will ensure that the pregnancy survives. The net amount of vitamin D transferred to the fetus from a vitamin D–deficient mother will be reduced, which may affect fetal bone mineralization and growth. A fetus that was vitamin D deficient during pregnancy will most likely be born with normal serum calcium concentrations and normal skeletal morphology, but risks developing osteomalacia or rickets within the first few weeks or months of life. These infants may also exhibit severe muscle weakness, which may alter their lung function. In one study, low maternal vitamin D intake during pregnancy was associated with decreased bronchodilator response and increased risk of wheeze symptoms in 5-year-olds. If the lower limbs are weakened or deformed, there may be adverse effects on the development of ambulation in childhood.
Rickets is a persistent global health concern among infants and children and is associated with anatomic deformities, including pelvic distortion. In this case, the anteroposterior diameter of the pelvis is shorter, causing the pelvis to be contracted. If the child is a female, the contracted pelvis may pose a problem during future childbirth. In general, African American women have a narrower pelvic inlet and a narrower pelvic outlet than European American women. Whether these anatomic differences are caused in part by vitamin D status has not been experimentally determined. However, one study found that 5% of white women and 30% to 45% of black women are vitamin D deficient during gestation. These differences result because highly melanized skin acts as a filter that increases the length of exposure to UVB light needed to synthesize vitamin D3. For this reason, health care providers should take extra caution about monitoring serum 25(OH)D concentrations of African American women, because they are at a greater risk of vitamin D deficiency.
Pregnant women with serum 25(OH)D concentrations below 15 ng/mL are four times as likely to have a caesarean birth than women with 25(OH)D concentrations above 15 ng/mL, even after correcting for confounding variables such as race. Vitamin D deficiency severely impairs muscle function and causes myopathy even before biochemical signs of bone disease develop. As a result, vitamin D–induced myopathy, which often goes undetected because the loss of bone strength persists for months before muscle weakness develops, may affect the strength of muscle contractions needed during labor and contribute to higher rates of caesarean births. Supplementing women with vitamin D may help to prevent bone disease, myopathy, and improve reproductive success.
J Midwifery Womens Health. 2010;55(6):550-560. © 2010
Elsevier Science, Inc.
Cite this: Relationship Between Vitamin D During Perinatal Development and Health - Medscape - Nov 01, 2010.