Researchers in Finland wanted to know if giving babies vitamin D supplements in their first two years would help their brains develop better. They followed nearly 400 children and tested their thinking skills and problem-solving abilities when they were 6 to 8 years old. The amount of vitamin D the babies received didn’t seem to make a difference in how smart they were. However, the study found something interesting: mothers who had either too little or too much vitamin D during pregnancy had children with slightly lower IQ scores, suggesting that getting the right amount of vitamin D while pregnant might matter for brain development.

The Quick Take

  • What they studied: Whether giving babies vitamin D supplements in their first two years of life would help their brains develop better, and whether the amount of vitamin D mothers had during pregnancy affected their children’s thinking skills.
  • Who participated: Nearly 400 healthy children from Finland who were part of a vitamin D study. About half received one dose of vitamin D (400 IU daily) and half received a higher dose (1200 IU daily) until age 2. Researchers tested their brain skills when kids were 6 to 8 years old.
  • Key finding: The amount of vitamin D babies received in their first two years didn’t affect how well they did on thinking and problem-solving tests. However, mothers who had vitamin D levels that were either too low or too high during pregnancy had children with slightly lower IQ scores compared to mothers with moderate vitamin D levels.
  • What it means for you: If you’re thinking about giving your baby vitamin D supplements, this study suggests the amount probably won’t directly boost their brain power. However, if you’re pregnant, maintaining a healthy vitamin D level (not too low, not too high) might be important for your baby’s brain development. Talk to your doctor about the right vitamin D level for you.

The Research Details

This was a carefully designed experiment called a randomized controlled trial, which is considered one of the best ways to test if something actually works. Researchers split nearly 400 babies into two groups: one group got 400 IU of vitamin D daily, and the other got 1200 IU daily for the first two years of life. Neither the parents nor the researchers knew which babies were getting which dose (called a “double-blind” study), which helps prevent bias.

The researchers measured vitamin D levels in mothers during pregnancy and in babies at 12 and 24 months old. When the children were 6 to 8 years old, they took special tests to measure their intelligence and executive functioning (the ability to plan, organize, and solve problems). The tests included the Wechsler Intelligence Scale for Children and other standardized brain function assessments.

The researchers also looked at whether the vitamin D levels in mothers during pregnancy were connected to how well the children did on these brain tests years later.

This type of study is important because it helps separate what actually causes changes in brain development from what just happens to be related. By randomly assigning babies to different vitamin D doses, researchers can be more confident that any differences in brain development are actually caused by the vitamin D, not by other factors like diet or family background. Following children from infancy to school age also helps show whether early vitamin D really affects long-term brain development.

This study has several strengths: it was a randomized controlled trial (the gold standard for testing treatments), it included a large number of children, and it followed them for several years. The researchers measured actual vitamin D levels in blood rather than just guessing. However, all participants were from Finland and identified as white, so the results might not apply equally to all populations. The study was also well-registered and published in a peer-reviewed journal, which means other experts reviewed it before publication.

What the Results Show

The main finding was that babies who received the higher dose of vitamin D (1200 IU) did not perform better on brain tests than babies who received the lower dose (400 IU) when tested at ages 6 to 8. Their IQ scores were essentially the same between the two groups, and their executive functioning (ability to plan and solve problems) was also similar. This was true whether researchers looked at formal testing or parent reports about how well kids could organize and manage tasks.

Interestingly, the amount of vitamin D in babies’ blood at 12 and 24 months old also didn’t predict how well they would do on brain tests later. This suggests that having more vitamin D circulating in a baby’s body during the first two years doesn’t automatically lead to better thinking skills in school-age children.

However, the study found a different pattern with mothers’ vitamin D levels during pregnancy. The relationship wasn’t straightforward—it wasn’t simply “more vitamin D is better.” Instead, mothers with either very low or very high vitamin D levels during pregnancy had children with slightly lower IQ scores. Mothers with moderate vitamin D levels (around 76.6 nmol/L was the sweet spot) had children with the highest IQ scores. This suggests there may be an optimal range for vitamin D during pregnancy.

The study looked at executive functioning in two different ways: through formal testing and through parent questionnaires. Neither approach showed differences between the vitamin D groups, which strengthens the conclusion that the vitamin D dose didn’t affect brain development. The researchers also found that maternal vitamin D levels during pregnancy were not associated with executive functioning specifically, only with overall IQ scores.

Previous research on vitamin D and brain development has been mixed and confusing, with some studies suggesting vitamin D is important and others finding no connection. This study adds to the evidence that vitamin D supplementation in babies might not be as directly important for brain development as some people thought. However, the finding about pregnancy vitamin D levels is new and interesting—it suggests that the timing of vitamin D exposure (during pregnancy rather than infancy) and the amount (moderate rather than very high or very low) might be what matters most.

The study only included children from Finland who identified as white, so we don’t know if these results apply to children from other backgrounds or countries with different sun exposure and diets. The vitamin D doses tested (400 and 1200 IU) are both relatively modest, so we don’t know what would happen with much higher or lower doses. The study also couldn’t prove that vitamin D during pregnancy causes the IQ differences—it only shows they’re connected. Other factors during pregnancy could explain the pattern. Additionally, the differences in IQ scores related to maternal vitamin D, while statistically significant, were small in real-world terms.

The Bottom Line

Based on this study, giving babies extra vitamin D supplements beyond standard amounts probably won’t boost their brain development (low to moderate confidence for this specific claim). However, if you’re pregnant or planning to become pregnant, maintaining a healthy vitamin D level is likely important—not too low and not too high. Talk to your doctor about what vitamin D level is right for you during pregnancy. Standard vitamin D recommendations for babies (400 IU daily) appear to be adequate based on this research.

Pregnant women and parents of young children should pay attention to this research. It’s especially relevant if you’ve been considering high-dose vitamin D supplements for your baby thinking it would boost brain development. The findings about pregnancy vitamin D are important for women who are pregnant or planning to become pregnant. However, this study doesn’t apply to children with diagnosed vitamin D deficiency, who may need treatment for other health reasons.

If you adjust your vitamin D intake during pregnancy, you wouldn’t expect to see changes in your child’s brain development immediately. Brain development is a long process, and this study measured effects years later (at ages 6-8). If you’re considering changes for a future pregnancy, the effects would only be measurable once your child is school-age.

Want to Apply This Research?

  • If pregnant or planning pregnancy, track your vitamin D blood levels at regular intervals (as recommended by your doctor) and note the dates. Record any vitamin D supplements taken and dietary sources. This creates a record to discuss with your healthcare provider about maintaining optimal levels.
  • Work with your doctor to determine your target vitamin D range during pregnancy, then use the app to log daily vitamin D intake from food and supplements. Set reminders for vitamin D-rich foods (fatty fish, fortified milk, egg yolks) and supplement timing if recommended by your doctor.
  • For pregnant users: monthly vitamin D level checks (if recommended by doctor) with app reminders. For parents of young children: document any vitamin D supplements given and note that standard 400 IU daily doses appear adequate based on current evidence. Track child’s developmental milestones separately to monitor overall growth and development.

This research summary is for educational purposes only and should not replace professional medical advice. Vitamin D needs vary by individual, location, diet, and health status. If you are pregnant, planning pregnancy, or considering vitamin D supplements for your child, consult with your healthcare provider to determine appropriate vitamin D levels and supplementation for your specific situation. Do not start, stop, or change any supplements without medical guidance. This study was conducted in Finland with a specific population and may not apply equally to all groups.