Vitamin D plays a vital role in fetal development. Worryingly, though, research suggests pregnant women are at higher risk of deficiency – especially in regions that don’t get much sunlight. And vitamin D deficiency during pregnancy is linked with premature birth, low birthweight and gestational diabetes. Current health guidelines recommend pregnant women get 600 IU of vitamin D per day. But is this dose enough?
- Pregnant women are at high risk of vitamin D deficiency
- Vitamin D deficiency has adverse effects on fetal development
- Health guidelines recommend 600 IU vitamin D – but these guidelines have been criticized
- A trial found higher doses (4000 IU daily) to be most effective for achieving optimal vitamin D status during pregnancy
Vitamin D deficiency
Vitamin D increases absorption of essential elements such as calcium, magnesium and phosphate. It’s needed to form healthy bones and plays a vital role in regulating the immune system. Deficiency can cause terrible health problems such as rickets and muscle weakness.
But as important as vitamin D is, it’s technically not a vitamin.
The reason for this is that vitamin D is not an essential dietary factor. Instead, it can be made within the body from cholesterol – when exposed to enough sunlight.
The trouble is our modern lifestyles often prevent us from getting enough sun exposure to produce adequate levels of vitamin D. Estimates suggest around 1/3 of the US population are deficient and this risk is even higher for people with dark skin, people living in northern latitudes and pregnant women.
Vitamin D in pregnancy studies
During pregnancy, demand for vitamin D is even higher than usual. Not only does the mother need enough vitamin D for herself, she also needs enough to support the development of a growing baby.
Unfortunately, this makes it very common for pregnant women to be vitamin D deficient. This study measured serum vitamin D levels in pregnant women during their first trimester and found 70% to be deficient:
“Vitamin D concentrations were obtained from 235 patients (mean age 24.3 years, range 18-40 years). Seventy percent of our study population was vitamin D insufficient with a serum concentration less than 30 ng/mL (mean serum concentration 27.6 ng/mL, range 13-71.6 ng/mL).”
– Vitamin D Deficiency in Early Pregnancy, PMCID: PMC4405493
What’s more, there are many negative health complications associated with vitamin D deficiency during pregnancy – both for baby and mother. These include:
- Premature birth1,2,3
- Small birth weight1,2
- Gestational diabetes (high blood sugar during pregnancy)1
- Pre-eclampsia (high blood pressure during pregnancy)1,2
Thankfully, you don’t need to spend hours in the sun to counteract vitamin D deficiency. Instead, vitamin D can be absorbed through diet, although food sources don’t contain much naturally.
Due to the scarcity of vitamin D in food, supplementation is perhaps the most efficient way to avoid deficiency. Randomized controlled trials have demonstrated that supplementation can effectively raise vitamin D levels in pregnant women:
“Low levels of vitamin D status, as measured by 25-hydroxyvitamin D [25(OH)D], are common in pregnant women. […] Randomized controlled trials (RCTs) indicate that vitamin D supplementation during pregnancy optimizes maternal and neonatal vitamin D status. […] Recent evidence supports that low maternal vitamin D status is associated with an increased risk of adverse pregnancy outcomes.”
So, we know that vitamin D deficiency can have negative health consequences during pregnancy. We know that deficiency is particularly common among pregnant women and that supplementation can reduce this risk. The next question to address is how much vitamin D is needed for optimal levels.
Optimal vitamin D dose during pregnancy
At present, the recommended dietary allowance (RDA) for vitamin D is 600 IU. This is the same regardless of whether you’re pregnant or breastfeeding.
But these guidelines have been heavily criticized in recent research. There is a large and growing body of evidence to suggest current vitamin D guidelines for pregnant women should be higher:
“The World Health Organization, was echoed by a recent Cochrane Review, stating that there are simply no requirements for vitamin D during pregnancy, are contrary to expanding published data […] that suggest otherwise.“
The report goes on to emphasise the importance of vitamin D “to protect the mother and fetus and impart genomic imprinting on the fetus to ensure long term health” and reiterates the risks of vitamin D deficiency described above.
An example of evidence that current vitamin D guidelines are insufficient for pregnant women is this randomized, double-blind study.
Here, pregnant women were randomly given either 400, 2000 or 4000 IU vitamin D3 daily during pregnancy. Researchers observed no side effects at any of these doses but found that 4000 IU was most effective at achieving adequate circulating levels:
“In this randomized controlled trial, women with a singleton pregnancy at 12–16 weeks’ gestation received 400, 2000 or 4000 IU vitamin D3/day until delivery. The primary outcome was maternal/neonatal circulating 25(OH)D at delivery […] Mean 25(OH)D by group at delivery and 1-month before delivery were significantly different (p<0.0001), and percent who achieved sufficiency was significantly different by group, greatest in 4000 IU group (p<0.0001) […] In summary, starting at 12–16 weeks of gestation, vitamin D supplementation with 4,000 IU/day was most effective in achieving vitamin D sufficiency throughout pregnancy, one month prior to delivery and at delivery in a diverse group of women and their neonates without increased risk of toxicity. These findings suggest that the current vitamin D EAR and RDA for pregnancy women issued in 2010 by the Institute of Medicine should be raised to 4,000 IU vitamin D per day so that all women regardless of race attain optimal nutritional and hormonal vitamin D status throughout pregnancy.”
– Vitamin D Supplementation during Pregnancy: Double Blind, Randomized Clinical Trial of Safety and Effectiveness, PMCID: PMC3183324
It is important to point out that getting less than 4000 IU vitamin D per day does not automatically mean vitamin D deficiency.
However, what this research does show is that higher doses mean mother and baby are far more likely to achieve optimal vitamin D status. It also shows that it’s possible to hit the recommended dietary limit (600 IU) and still be vitamin D deficient.
An excess of vitamin D can lead to high concentrations of calcium in the blood, a condition known as hypervitaminosis D. Symptoms include dehydration, nausea, fatigue, constipation and muscle weakness.
But vitamin D toxicity is rare.
This study notes that all published cases of hypervitaminosis D resulted from intakes of over 40,000 IU per day and found no evidence of adverse effects with dosages up to 10,000 IU per day:
Except in those with conditions causing hypersensitivity, there is no evidence of adverse effects with serum 25(OH)D concentrations <140 nmol/L, which require a total vitamin D supply of 250 microg (10000 IU)/d to attain. Published cases of vitamin D toxicity with hypercalcemia, for which the 25(OH)D concentration and vitamin D dose are known, all involve intake of > or = 1000 microg (40000 IU)/d.
– Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety, PMID: 10232622
So, how much vitamin D should pregnant women take?
There are many factors that influence vitamin D levels and needs: sun exposure, diet, genetics and so on. This makes it difficult to give a one-size-fits-all answer.
For some women, the current guideline of 600 IU per day will be sufficient. For others, this dose risks vitamin D deficiency and its associated health problems.
Pregnant women are especially at risk of vitamin D deficiency – especially those in northern latitudes. And the evidence linking vitamin D deficiency with health issues for both baby and mother is beyond doubt. Given the low incidence of side effects, many mothers may feel it preferable to opt for higher doses around 4000 IU per day as administered in the randomized trial discussed previously.
And while this may conflict with current health advice, there is good evidence to suspect this advice is outdated.