Vitamin D in pregnancy may aid child's grip

"Higher levels of maternal vitamin D during pregnancy have been linked to better muscle development in children," BBC News reports.

The headline is prompted by a UK study involving more than 600 mothers and their children. It found that at the age of four, the children of women who had higher levels of vitamin D in late pregnancy had stronger hand grips than those whose mothers had lower levels of the vitamin.

Pregnant women are known to be at risk of not getting enough vitamin D. The Department of Health currently recommends that all pregnant and breastfeeding women should take a daily supplement containing 10 micrograms (0.01mg or 400 international units) of vitamin D. In the current study, less than 10% of the women were taking these supplements in late pregnancy.

However, this study did not follow these children up past the age of four, so we don't know for sure whether these differences remain as they get older. These results would need to be confirmed by other studies before they are seen as conclusive.

Despite this, the current study reminds us that it's important for pregnant women to get enough vitamin D. If you are pregnant, talk to your GP or midwife about ensuring you get the right nutrients and supplements.

Where did the story come from?

The study was carried out by researchers from the University of Southampton and other research centres in the UK. It was funded by the Medical Research Council and several other UK charities and research institutions.

The study was published in the peer-reviewed Journal of Clinical Endocrinology and Metabolism.

BBC News' reporting of the study is accurate as it generally covers the study reasonably. However, the Daily Mirror makes a fundamental error when reporting the recommended levels of vitamin D supplementation for pregnant women.

Pregnant women are recommended to take an extra 10* micrograms* of vitamin D a day, *not *10 grams (a million times higher) as the Mirror advised.

Taking such a high dose of vitamin D regularly could lead to side effects such as dehydration, nausea, vomiting and kidney damage. But this is unlikely to happen in the real world, as a 10 gram vitamin D supplement shouldn't be available to buy.

What kind of research was this?

This study was part of a prospective cohort study called the Southampton Women's Survey. The current analysis looked at the link between mothers' vitamin D levels in pregnancy and their children's muscle strength in early childhood.

The researchers report that other studies have found links between maternal vitamin D levels in pregnancy and child body composition outcomes such as bone and fat mass. There is little information about the potential effect of maternal vitamin D levels in pregnancy on child strength, so researchers wanted to see if there was a link.

This study design is the best way to assess the link between a naturally occurring exposure and an outcome. The main limitation is that women who differ in one characteristic (vitamin D levels in pregnancy) may also differ in other ways, for example, unmeasured health, lifestyle and socioeconomic factors. This makes it difficult to tell for certain which of these factors is affecting the outcome.

What did the research involve?

The study measured the vitamin D levels of women in pregnancy by taking samples of their blood. Their children's grip strength was assessed once they reached the age of four. The researchers then looked at whether a mother's vitamin D levels in pregnancy were related to how strong the child was.

The Southampton Women's Survey enrolled more than 15,000 young women aged 20 to 34. The current study only included women who became pregnant during the study and did not have twins or triplets.

The women reported on their lifestyles and had characteristics such as their height and weight measured at the start of the study and during pregnancy. At 34 weeks of pregnancy, they also had a blood sample taken and vitamin D levels were measured. After birth, the researchers asked the women about how long they breastfed their baby.

The children had their height, weight and body composition assessed at the age of four. They also had their hand grip strength measured as an indication of their muscle strength. A subset of the children also had their levels of physical activity measured by wearing monitoring devices for seven days.

The researchers analysed data for the 678 mother-child pairs that had all the necessary data. They then assessed whether the children's hand grip strength was related to their mother's vitamin D levels during pregnancy.

They took into account factors that could influence the results, known as confounders. These included:

the child's gender
exact age
height
current milk intake
duration of breastfeeding
the mother's smoking status in late pregnancy
walking speed in late pregnancy
late pregnancy body fatness (assessed either using triceps skinfold thickness or body mass index)
age at delivery
how many children they had
social class

They also assessed whether children's physical activity levels or the season the different measurements were taken in affected the results.

What were the basic results?

The researchers found that only 9.2% of women were taking a vitamin D supplement in late pregnancy (34 weeks of gestation). The women's average (median) intake of vitamin D in late pregnancy was 136 international units (IU) per day (3.4 micrograms).

Higher maternal levels of vitamin D in pregnancy were associated with greater hand strength in the child at the age of four, even after taking into account potential confounding factors.

They found that maternal levels of vitamin D in pregnancy were also associated with some of the measures of the child's lean mass, but not others. The associations with lean mass were not statistically significant once potential confounding factors were taken into account.

How did the researchers interpret the results?

The researchers say that their results suggest that exposure to higher levels of vitamin D in the womb may influence a child's muscle development. This seems to act by increasing strength, rather than the amount of muscle.

They say that vitamin D supplementation in pregnancy may improve child muscle development, but that this needs to be confirmed in an intervention study before any recommendations are made.

Conclusion

This study suggests that a mother's vitamin D levels in pregnancy may affect their child's muscle strength in early childhood. The study's strengths include the fact that it collected data prospectively as well as taking standardised information and measurements from the mothers and children.

The researchers note that some limitations of their study are the difficulties in carrying out body composition scans and hand grip tests in children. The study also only measured vitamin D levels at one point in pregnancy, and levels may have differed at various points in the pregnancy. Also, only one measure of child strength was taken (hand grip) at one time point, and this may also vary if multiple measurements were taken.

The current study didn't follow these children up past the age of four, so we don't know for sure whether these differences remain as they get older. Ideally, these results would be confirmed by other studies before they are seen as conclusive.

It's also not possible to say to what extent the differences in grip strength have any impact on the child's general health or wellbeing.

Pregnant women are known to be a group at risk of not getting enough vitamin D. The Department of Health currently recommends that all pregnant and breastfeeding women should take a daily supplement containing 10 micrograms (0.01mg or 400IU) of vitamin D. Worryingly, less than 10% of the women in the current study were taking such a supplement in late pregnancy.

Overall, while the study does not provide conclusive proof of a direct link between vitamin D in pregnancy and child muscle strength, it appears to reinforce the importance of vitamin D intake in pregnancy. Women who are pregnant can talk to their healthcare professional to ensure they are getting the right nutrients and supplements.

Read more about vitamin supplements in pregnancy here.