Does vitamin D in pregnancy prevent ADHD?
"Sunbathing mothers guard against hyperactive babies," The Daily Telegraph reports – a headline that achieves the dubious dual distinction of being both inaccurate and irresponsible.
The study the news is based on never looked at sunbathing, which can actually be harmful during pregnancy.
Danish researchers took umbilical cord blood samples from babies soon after birth, and then asked parents to complete a behaviour checklist for symptoms associated with attention deficit hyperactivity disorder (ADHD) when the children were aged two to three.
They found that, generally, lower cord vitamin D levels were associated with higher ADHD symptom scores.
But this does not prove that low vitamin D directly causes ADHD symptoms – there may be various health, lifestyle and environmental factors that this study has not been able to take into account.
Even if there is a link, nobody needs to sunbathe to get vitamin D, especially pregnant women. Sunbathing is well known to be associated with the risk of skin cancer.
Pregnant women may be at even greater risk because their skin is more sensitive. It can also put them at risk of dehydration and overheating, which may be harmful to both mother and baby.
Vitamin D supplements (10 micrograms per day) are recommended for pregnant women and children between the ages of one and four, and birth to one year if they are breastfed.
Where did the story come from?
The study was carried out by researchers from the University of Southern Denmark and published in the peer-reviewed Australian and New Zealand Journal of Psychiatry.
It received several sources of funding, including from the Region of Southern Denmark, the University of Southern Denmark, the National Board of Social Services, and the Mental Health Research Fund of Southern Denmark.
The Daily Telegraph's reporting of the story was very poor. We are used to seeing headlines that are inaccurate. Less commonly, there are headlines that are irresponsible. But it is rare that we see both, as with this story.
Furthermore, the paper does not discuss this research's limitations, or give advice on more appropriate and safer sources of vitamin D during pregnancy, such as supplementation.
What kind of research was this?
This analysis of a Danish population-based birth cohort study aimed to look at the link between vitamin D levels in the umbilical cord and the child later developing attention deficit hyperactivity disorder (ADHD).
ADHD is fairly common in children, particularly in boys, but the causes are not known. It is also well established that vitamin D deficiency is quite common during pregnancy in women worldwide.
Low vitamin D levels in children have been linked to ADHD before, and researchers have hypothesised that low levels of vitamin D in the mother during pregnancy could cause ADHD.
Prospective cohort studies are often used to look at the possible link between an exposure and outcome.
But the main limitation of this approach is the possibility of confounding – in other words, health, lifestyle and environmental factors associated with low vitamin D levels could be independently associated with a child having ADHD, and not a direct cause.
What did the research involve?
This study population was drawn from the Odense Child Cohort (OCC), which recruited 2,549 pregnant women in the Odense region of Denmark between 2010 and 2012.
Upon enrolment, the women were asked to donate umbilical cord blood, from which vitamin D levels could be measured.
When their child was two to four years old, parents also filled in the Child Behaviour Checklist.
This checklist measures emotional and behavioural symptoms, and contains 100 questions with answers given on a three-point scale: 0 (not true), 1 (somewhat/sometimes true) and 2 (very true/often true).
Six questions with a maximum score of 12 covered ADHD symptoms:
- can't concentrate, can't pay attention for long
- can't sit still, restless or hyperactive
- can't stand waiting, wants everything now
- demands must be met immediately
- gets into everything
- quickly shifts from one activity to another
This analysis looked at the link between cord blood vitamin D levels and ADHD problems in 1,233 mothers and their children who had full data available. They represented 18% of all women eligible for this study who were pregnant during the study years.
In their analyses, the researchers adjusted for numerous potential confounders, including:
- mother's age
- child gender
- prematurity
- birth weight and season of birth
- mother's smoking history, alcohol use and educational level
- use of vitamin D supplementation
What were the basic results?
Children were assessed at 2.7 years on average, and the average ADHD problem score across the sample was also 2.7.
The highest ADHD scores (above the 90th percentile) were associated with several factors, including lower cord vitamin D, lower maternal age and educational level, and the mother smoking and drinking alcohol during pregnancy.
Split by vitamin D cut-offs, the researchers found that, generally, those with lower cord vitamin D levels had higher ADHD scores.
For example, those with vitamin D levels above 25nmol/L had lower ADHD scores than those with levels below 25nmol/L, and scores were lower for those with vitamin D above 30nmol/L compared with below 30nmol/L.
The odds of being in the highest ADHD scores (above the 90th percentile) decreased with every 10nmol/L increase in vitamin D levels.
How did the researchers interpret the results?
The researchers concluded that, "An inverse association between cord [vitamin D] and attention deficit hyperactivity disorder symptoms in toddlers was found, suggesting a protective effect of prenatal vitamin D."
Conclusion
This Danish birth cohort found an association between lower levels of vitamin D in umbilical cord blood and higher ADHD scores in the young child. But this should be interpreted with some caution.
This observational study does not prove that lower vitamin D levels in pregnancy have directly and independently caused ADHD symptoms in the child:
- The researchers have attempted to take account of various potential confounders, but they may not have been able to fully account for all confounding factors.
- The study looked at the association with ADHD scores on a problem scale, but did not get official medical diagnoses of ADHD.
- The sample is only a small proportion of all the women eligible to take part in the study. There may have been key differences between the mothers who agreed to take part and those who did not.
- As this is a Danish cohort, the results may also not be representative of the UK.
The Daily Telegraph has taken the rather irresponsible line that women should sunbathe during pregnancy. We do get vitamin D from sunlight, but most people get all they need from just normal daylight exposure – not sunbathing.
Sunbathing and excess UV exposure is well known to be associated with the risk of skin cancer. Pregnant women may be at even greater risk from excess UV because their skin is more sensitive.
It can also put them at risk of dehydration and overheating, which may be harmful to both mother and baby. Women in hot climates should take care to cover up in the sun, wear sunscreen and avoid the sun at its hottest.
Vitamin D can also be found in food sources like red meat, egg yolks and oily fish, although pregnant women need to limit their intake of oily fish.
Vitamin D supplements (10 micrograms a day) are recommended for pregnant women and children between the ages of one and four years, and from birth to one year if the child is breastfed.
