This review pooled four small trials of 256 children with ADHD, aged 2 to 18 across the trials. Three trials took place in Iran and one in Saudi Arabia. Boys outnumbered girls in every trial. All the children took the ADHD medicine methylphenidate, and one group also took vitamin D, at doses from 1,000 IU a day to 50,000 IU a week. The other took a placebo. The trials lasted 6 to 12 weeks.
On parent ratings, children given vitamin D did better on overall ADHD symptoms, attention, hyperactivity and behavior. The gains were small to moderate and unlikely to be chance. Oppositional behavior did not clearly change. Blood vitamin D levels rose. Side effects were mild and no more common than with placebo. In one trial, only children who started with low vitamin D improved.
The authors rated the evidence low to very low quality. On several outcomes, the trials disagreed a lot. Three trials were unclear on how children were put into groups. One trial lost about 12% of its placebo group without explanation. No trial studied adults, other regions, or vitamin D on its own.
The authors suggest these trials support vitamin D alongside methylphenidate in children with ADHD, but say larger trials across more ages and regions are needed before its benefit and safety are settled.
Talk to a clinician about your own situation or your child's.
The numbers Stats and what the terms mean
The statistics behind the plain-language claims above.
- Gan, 2019: overall ADHD symptom scores (3 trials, 221 children), SMD 0.39, 95% CI 0.12 to 0.65, I2 = 41%, p = 0.005.
- Gan, 2019: inattention scores (3 trials, 185 children), SMD 0.67, 95% CI 0.12 to 1.23, I2 = 67%, p = 0.02.
- Gan, 2019: hyperactivity scores (4 trials, 256 children), SMD 0.60, 95% CI 0.07 to 1.13, I2 = 75%, p = 0.03.
- Gan, 2019: behavior scores (2 trials, 125 children), SMD 0.54, 95% CI 0.18 to 0.90, I2 = 0%, p = 0.003.
- Gan, 2019: oppositional scores (2 trials, 89 children), MD 9.76, 95% CI -0.62 to 20.13, p = 0.07.
- Gan, 2019: blood vitamin D levels (3 trials, 221 children), SMD 2.26, 95% CI 1.46 to 3.06.
- One included study (cited in Gan, 2019): total side effects, vitamin D versus placebo (n = 54), OR 1.53, 95% CI 0.86 to 2.72. What the terms mean:
- SMD (standardized mean difference) is the size of the gap between groups on a common scale. Roughly, 0.2 is small, 0.5 is moderate and 0.8 is large.
- MD (mean difference) is the plain gap between the groups' average scores, in the scale's own points.
- OR (odds ratio) compares how likely something was in one group versus the other; 1 means no difference.
- 95% CI (confidence interval) is the range the true result likely falls in; if it crosses zero for a difference, or one for an odds ratio, the result may be due to chance.
- I2 shows how much the trials disagreed with each other, from 0% (none) upward.
- p is how likely a result this big would be if there were no real effect; below 0.05 is usually read as unlikely to be chance.
Source: Gan J, Galer P, Ma D, et al.. The Effect of Vitamin D Supplementation on Attention-Deficit/Hyperactivity Disorder: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of Child and Adolescent Psychopharmacology (2019). doi:10.1089/cap.2019.0059