Doubt cast on benefit of vitamin D in pregnancy

“Guidelines could be overstating the importance of taking vitamin D supplements during pregnancy,” The Independent has reported.

The newspaper reports on a large UK study that appears to contradict advice that vitamin D supplements in pregnancy can help strengthen babies’ bones.

Researchers assessed the link between maternal vitamin D levels and the mineral content of their child’s bones by measuring expectant mums’ vitamin D levels. They later measured the bone mineral content of the resultant children at 9-10 years of age. Vitamin D is important for strong bones and preventing rickets, and it is currently recommended that pregnant women take a 10 micrograms supplement of vitamin D every day. However, in this study the researchers found no association between maternal vitamin D levels and their child’s bone mineral content.

This study suggests that there is no strong evidence that pregnant women should take vitamin D to prevent low bone mineral content in their children. However, it doesn’t tell us about other reasons why boosting vitamin D levels may be beneficial.

Further randomised controlled trials of vitamin D supplementation in pregnancy are needed to see if it is truly beneficial.

“Guidelines could be overstating the importance of taking vitamin D supplements during pregnancy,” The Independent has reported.

The newspaper reports on a large UK study that appears to contradict advice that vitamin D supplements in pregnancy can help strengthen babies’ bones.

Researchers assessed the link between maternal vitamin D levels and the mineral content of their child’s bones by measuring expectant mums’ vitamin D levels. They later measured the bone mineral content of the resultant children at 9-10 years of age. Vitamin D is important for strong bones and preventing rickets, and it is currently recommended that pregnant women take a 10 micrograms supplement of vitamin D every day. However, in this study the researchers found no association between maternal vitamin D levels and their child’s bone mineral content.

This study suggests that there is no strong evidence that pregnant women should take vitamin D to prevent low bone mineral content in their children. However, it doesn’t tell us about other reasons why boosting vitamin D levels may be beneficial.

Further randomised controlled trials of vitamin D supplementation in pregnancy are needed to see if it is truly beneficial.

Where did the story come from?

The study was carried out by researchers from the University of Bristol and the University of East Anglia. It was funded by the UK Medical Research Council, the Wellcome Trust and the University of Bristol.

The study was published in the peer-reviewed medical journal The Lancet.

The results of this study were accurately reported by the media.

What kind of research was this?

This was a cohort study. It collected data from mother-and-child pairs (mothers who had given birth to a single child) who were participating in the Avon Longitudinal Study of Parents and Children.

It has been suggested that a mother’s vitamin D levels during pregnancy affect her child’s bone mineral content. The National Institute for Health and Clinical Excellence (NICE) recommends that “all women should be informed at the booking appointment about the importance for their own and their baby’s health of maintaining adequate vitamin D stores during pregnancy and whilst breastfeeding. In order to achieve this, women may choose to take 10 micrograms of vitamin D per day”.

Children who do not have enough vitamin D can develop rickets, which is a condition that can cause the bones to become painful and weakened.

This study aimed to investigate whether there was any link between a mother’s vitamin D levels during pregnancy and her child’s bone mineral content at 9-10 years of age.

A cohort study can find associations, but cannot prove causation. In this case, the study cannot prove that maternal vitamin D levels are responsible for differences in bone mineral content. This is because there could be other factors (calledconfounders) that could be responsible for the association seen.

A randomised controlled trial would be required to prove causation. Given the surprising results of this study, randomised controlled trials are hopefully being planned.

What did the research involve?

The researchers measured the vitamin D levels of 3,960 mothers during pregnancy. These samples were taken during routine antenatal screening, so the mothers were at different stages of their pregnancy. From these samples, the researchers predicted the women’s vitamin D levels in their third trimester. Vitamin D levels in this study were classified as sufficient (more than 50.00nmol/l), insufficient (49.99 to 27.50nmol/l) or deficient (lower than 27.50nmol/l).

Children had the bone mineral content of their total body (except the head) measured between nine and 10 years of age, usingdual energy X-ray absorptiometry (DEXA) scans (a special type of X-ray that measures bone density).

The researchers investigated whether there was an association between the mother’s vitamin D concentration and bone mineral content by trimester of pregnancy.

What were the basic results?

Among the mothers:

• 1,035 (26%) had vitamin D concentrations measured during the first trimester
• 879 (22%) had vitamin D concentrations measured during the second trimester
• 2,046 (52%) had vitamin D concentrations measured during the third trimester

Using the predicted third trimester vitamin D concentrations:

• 2,644 women had sufficient levels of vitamin D (67%)
• 1,096 women had insufficient levels of vitamin D (28%)
• 220 women were vitamin D deficient (6%)

Total body and spinal bone mineral content were the same for mothers who were vitamin D deficient or who had insufficient levels of vitamin D and mothers who had sufficient levels of vitamin D.

No link was seen between bone mineral content in children assessed by DEXA scanning and maternal vitamin D concentrations in any trimester of pregnancy.

The researchers then adjusted the results for the following confounders:

• maternal age
• child’s age and sex
• maternal education
• number of previous births
• smoking history
• body mass index

After adjusting the results the researchers calculated that for every 10.0nmol/l difference in maternal vitamin D concentration during the third trimester (calculated values), total body (less head) bone mineral content was reduced by 0.03g and spine mineral content increased by 0.04g. But neither of these results was statistically significant and they could well have been the result of chance.

How did the researchers interpret the results?

The researchers conclude that they found “no relevant association between maternal vitamin D status in pregnancy and offspring [bone mineral content] in late childhood”.

Conclusion

This large cohort study of almost 4,000 mother and child pairs has found no association between a mother’s vitamin D concentrations during pregnancy and the child’s bone mineral content at 9-10 years of age.

Vitamin D is known to be important for strong bones. Deficiency of vitamin D can lead to rickets in children, and in adults it is linked to weakening of the bones osteoporosis and risk of fractures.

However, this study raises questions about whether the routine universal supplementation of women during pregnancy in the UK with vitamin D is warranted.

It should be noted that, in this study, around two-thirds of women had sufficient levels of vitamin D. The results might have been different in other populations if vitamin D levels were lower, for example if the study had included more women with inadequate dietary intake of vitamin D or those not exposed to sufficient sunlight to generate vitamin D in the skin.

Although this study suggests that there is no strong evidence that pregnant women should receive vitamin D to prevent low bone mineral content in their children, it does not tell us about other beneficial effects vitamin D could have.

Randomised controlled trials of vitamin D supplementation are needed to see whether vitamin D during pregnancy is beneficial.

While this study casts some doubt on the beneficial effects of vitamin D on improving bone density in children, it is important to realise that it is very safe to take vitamin D supplements at the recommended levels (10 micrograms). If you are concerned about taking vitamin D talk to your GP or midwife.