Researchers looked at 18 studies involving over 10,000 people to see if vitamin D supplements could help prevent asthma in children. They found that giving pregnant women higher doses of vitamin D may slightly reduce wheezing in their kids, but the evidence isn’t strong enough to say vitamin D definitely prevents asthma. The results were mixed for other conditions like eczema and infections. Scientists say we need more research before recommending vitamin D supplements specifically to prevent childhood asthma.

The Quick Take

  • What they studied: Does giving vitamin D supplements to pregnant women, breastfeeding mothers, or young children help prevent asthma and related breathing problems?
  • Who participated: 18 different studies from around the world involving 10,611 healthy pregnant women, breastfeeding mothers, and children under five years old
  • Key finding: High-dose vitamin D given during pregnancy may slightly reduce wheezing in children (about 21% lower risk), but there’s not enough solid evidence to say it prevents asthma. Results for other conditions were unclear or showed little benefit.
  • What it means for you: While vitamin D is important for health, taking extra supplements specifically to prevent asthma in kids isn’t proven to work yet. Talk to your doctor before starting any supplements during pregnancy or for your child.

The Research Details

This was a systematic review, which means researchers looked at all the best quality studies they could find on this topic and combined the results. They searched medical databases through October 2023 and found 18 randomized controlled trials—the gold standard type of study where some people get vitamin D and others get a placebo (fake pill) to compare results fairly.

The studies looked at different approaches: some gave pregnant women vitamin D, others gave it to breastfeeding mothers or young children. The vitamin D doses varied widely, from small daily amounts (200 units per day) to large quarterly doses (100,000 units at once). Studies lasted anywhere from 28 days to two years.

Researchers carefully evaluated each study for quality and bias, then combined the results using statistical methods. They looked at whether vitamin D affected asthma, wheezing, eczema, infections, and allergic reactions.

By combining results from many studies, researchers can see patterns that might not be obvious in single studies. This approach is more reliable than looking at one study alone because it reduces the chance that random luck affected the results. However, the strength of conclusions depends on how many studies exist and how good they are.

The researchers used strict methods to avoid bias and assessed how confident we should be in each finding. Most findings were rated as ’low’ or ‘moderate’ certainty, meaning we can’t be very sure about the results yet. The studies came from different countries and used different doses and schedules, which made it harder to compare them directly. More large, well-designed studies are needed to give clearer answers.

What the Results Show

When pregnant or breastfeeding women took any amount of vitamin D compared to no vitamin D, there was a suggestion it might reduce early childhood asthma risk, but this was based on only one study with 236 people, so it’s not reliable yet. It probably didn’t help prevent airway infections based on three studies with 1,564 people.

When infants and young children took vitamin D compared to no vitamin D, the results showed little to no benefit for wheezing, eczema, infections, or allergic reactions. The evidence was too unclear to say anything about asthma prevention in this age group.

When comparing high-dose vitamin D to regular-dose vitamin D in pregnant women, high doses likely reduced wheezing in children by about 21% (based on three studies with 1,439 people). However, for actual asthma diagnosis, the benefit wasn’t clear enough to be certain. High-dose vitamin D in pregnancy probably didn’t help prevent eczema, infections, or allergies.

When infants and children received high-dose vitamin D compared to regular doses, there was a small reduction in airway infections, but the effect was tiny and based on low-certainty evidence. Other outcomes showed little to no benefit.

The research examined several related conditions beyond asthma. For eczema (atopic dermatitis), vitamin D showed little to no protective effect across most comparisons. For allergic sensitization (developing allergies), results were inconsistent and unreliable. For airway infections, most comparisons showed little benefit, though high-dose vitamin D in children showed a very small reduction. Airway inflammation was rarely measured in these studies, so conclusions couldn’t be drawn.

This review is more comprehensive than earlier research because it included studies comparing different vitamin D doses, not just vitamin D versus placebo. Previous reviews focused mainly on pregnancy studies, but this one also examined infants and young children. The findings align with growing scientific uncertainty about whether vitamin D supplements are a reliable asthma prevention tool, despite vitamin D’s known importance for bone health and immune function.

The studies used very different vitamin D doses and schedules, making direct comparisons difficult. Most studies were small, and many had design issues that could affect results. Very few studies looked at children under one year old or measured airway inflammation. Most studies took place in wealthy countries, so results might not apply everywhere. The evidence quality was generally low to moderate, meaning we can’t be very confident in the findings. Researchers need larger, better-designed studies, especially in infants and young children, to give clearer answers.

The Bottom Line

Based on moderate-certainty evidence, high-dose vitamin D during pregnancy may slightly reduce wheezing in children, but this is a modest benefit. There is insufficient evidence to recommend vitamin D supplements specifically for asthma prevention in children. Vitamin D remains important for overall health, so ensure adequate intake through diet, sunlight, and supplements as recommended by your doctor for general health—not specifically for asthma prevention. (Confidence: Moderate for pregnancy and wheezing; Low for other outcomes)

Pregnant women and parents of young children interested in asthma prevention should know that vitamin D supplements aren’t proven to prevent asthma yet. People with known vitamin D deficiency should still take supplements as recommended by their doctor for general health. This research doesn’t apply to people with existing asthma or other medical conditions. Anyone considering supplements during pregnancy or for children should consult their healthcare provider.

If vitamin D does help prevent asthma, benefits would likely appear years later in childhood, not immediately. Any effect on wheezing might be noticeable within the first few years of life. Don’t expect quick results, and remember that prevention is about reducing risk, not guaranteeing asthma won’t develop.

Want to Apply This Research?

  • Track vitamin D intake (daily dose in IU units) and any respiratory symptoms like wheezing, coughing, or breathing difficulties in your child. Record monthly to see patterns over time.
  • If your doctor recommends vitamin D supplementation, use the app to set daily reminders for taking supplements and log when doses are taken. This helps ensure consistency and makes it easy to discuss adherence with your healthcare provider.
  • Log respiratory health monthly using a simple scale (none, mild, moderate, severe symptoms). Compare patterns over seasons and years to see if vitamin D supplementation correlates with any changes. Share this data with your doctor during checkups to inform decisions about continuing supplementation.

This research summary is for educational purposes only and should not replace professional medical advice. Vitamin D supplementation decisions during pregnancy, breastfeeding, or for young children should be made in consultation with your healthcare provider who knows your individual health situation. The evidence reviewed here is still developing, and recommendations may change as new research emerges. Do not start, stop, or change any supplements without discussing with your doctor first.