Published in The Australia & New Zealand Journal of Psychiatry, the findings come from the Odense Child Cohort (Odense Børnekohorte), a joint study between Odense University Hospital, the Psychiatric Service of the Region of Southern Denmark, Odense Municipality, and the University of Southern Denmark.
For the study the team of researchers looked at 1,233 children from Odense Municipality, measuring the levels of vitamin D in umbilical blood and later asking mothers to complete the Child Behavior Checklist (CBCL) when their child was 2½ years old. Although an ADHD diagnosis cannot be made at the age of 2½ years of age, the CBCL questionnaire can be used to identify early symptoms of the condition.
The team found that there was a clear link between vitamin D and indications of ADHD, with the children of mothers who had taken vitamin D while pregnant, and who had a vitamin D level (25OHD) in their umbilical blood over 25 nmol/L, producing lower ADHD scores.
In addition, the results also showed that, "For every 10 nmol/L increase in the vitamin D concentration in umbilical blood, the risk of a being among the 10% highest score on the ADHD symptom scale fell by 11%," added one of the study's authors, Professor Niels Bilenberg.
The findings still held even after many other factors had been taken into account, including the mother's age, alcohol intake, obesity, education, number of children, whether she smoked, psychiatric disease in the parents, child's sex, age and seasonal variation.
Although links between vitamin D during pregnancy and a decreased risk of other conditions in children have been found before, a link between vitamin D and early ADHD symptoms is a new finding, with two of the study's other authors, medical students Jens Bull Aaby and Mats Mossin, commenting that, "We were very surprised that the link was so clear, as there was no previous awareness that this link could be identified at such an early age."
They added that while it would be impossible to say with certainty which children would develop ADHD later on, it would be "interesting to further follow up those children who were at the highest end versus the normal range of the ADHD scale," they added.
Although the results of the study do not indicate why vitamin D could have a protective affect against ADHD, only that there is a possible link, previous research has shown that the vitamin D could be important in the early development of the brain.