Researchers in China tested vitamin D levels in over 1,100 young children and discovered something surprising: whether a child is considered to have enough vitamin D depends heavily on which measurement rules doctors use. The study found that vitamin D levels naturally change with the seasons and age, with babies having higher levels than older preschoolers. Most importantly, the same child could be labeled as having “enough” vitamin D using one set of rules but “not enough” using different rules. This means doctors and parents need clearer, more consistent guidelines for measuring vitamin D in children.

The Quick Take

  • What they studied: Whether different ways of measuring vitamin D in children’s blood give different results about whether kids have enough vitamin D
  • Who participated: 1,183 healthy children between newborns and 6 years old living in Quanzhou, China, tested between January 2022 and March 2023
  • Key finding: Using one measurement method, only 2.5% of children were considered vitamin D deficient, but using a different method, 20% were considered deficient—the exact same children got different results depending on which measurement rule was used
  • What it means for you: If your child’s doctor says they have low vitamin D, it’s worth asking which measurement standard they’re using, since different standards might give different answers. This suggests doctors need better, more consistent guidelines for checking vitamin D in children.

The Research Details

Researchers measured vitamin D levels in the blood of 1,183 healthy young children in China over about 15 months. They used a standard lab test called ELISA to measure a specific form of vitamin D in the blood (called 25-hydroxyvitamin D). They also collected information about each child’s age, sex, weight, height, and when the blood sample was taken (which season). Then they took all these measurements and sorted them by different groups—boys versus girls, different seasons, different ages, and different body weights—to see how vitamin D levels changed across these groups.

The key part of their study was comparing what happened when they used different diagnostic criteria (different measurement rules) to decide if a child had enough vitamin D or not. They tested at least two different sets of rules for what counts as “deficient,” “insufficient,” or “sufficient” vitamin D and compared how many children fell into each category depending on which rules they used.

This type of study is called “cross-sectional” because researchers took a snapshot in time of many children rather than following the same children over months or years.

This research matters because vitamin D is important for children’s bone health and immune system, so doctors need accurate ways to measure it. If different measurement methods give different answers about the same child, it creates confusion for doctors trying to decide whether to treat a child or give them supplements. By showing that the measurement rules matter this much, the study highlights that we need better, more standardized ways to check vitamin D in children.

This study has several strengths: it included a large number of children (over 1,100), it was conducted over a long time period (15 months) to capture seasonal changes, and it used a standard lab test method. However, the study only looked at children in one city in China, so the results might be different in other parts of the world with different climates and sun exposure. The study was also observational, meaning researchers just measured what was happening rather than testing whether a treatment worked.

What the Results Show

The average vitamin D level in the children’s blood was 73.02 nmol/L, which is a standard measurement unit. Interestingly, boys and girls had almost identical vitamin D levels—there was no meaningful difference between them. However, vitamin D levels did change depending on the season: children had the highest levels in summer (75.85 nmol/L) and the lowest in winter (69.00 nmol/L), which makes sense because sunlight helps our bodies make vitamin D.

The most striking finding was about age differences. Babies (infants) had higher vitamin D levels than toddlers, who had higher levels than preschoolers (3-6 year olds). This pattern was statistically significant, meaning it wasn’t just random chance.

The biggest surprise came when researchers applied different diagnostic criteria. Using one set of measurement rules (Criterion I), only 2.46% of children were classified as vitamin D deficient, 9.97% as insufficient, and 87.57% as having enough vitamin D. But when they used a different set of rules (Criterion II), the numbers changed dramatically: 20.29% were deficient, 29.41% were insufficient, and only 50.30% had enough vitamin D. This means the exact same children got completely different classifications depending on which measurement standard was used.

The study also looked at whether vitamin A status (another important nutrient) was related to vitamin D levels, but found no significant connection. When researchers looked at children’s weight and height measurements, these also didn’t show strong relationships with vitamin D levels. The agreement between the two different diagnostic criteria was very poor (measured by something called a Kappa value of 0.071), meaning the two methods almost never agreed on which children had vitamin D problems.

This research adds to existing knowledge by showing that the problem of inconsistent vitamin D measurement standards is real and significant in children. Previous research has suggested that vitamin D levels vary by season and latitude (how far north or south you are), and this study confirms those findings. However, this study uniquely highlights how much the choice of measurement criteria matters, which is an important practical problem that hasn’t received enough attention.

The study only included healthy children from one city in China, so the results might not apply to children in other countries with different climates, sun exposure, or diets. The study didn’t include children with health conditions that might affect vitamin D. The researchers didn’t measure how much sun exposure each child got or what they ate, which could affect vitamin D levels. Additionally, the study is observational, so it can show that vitamin D levels vary but can’t prove that these variations cause health problems or that treatment would help.

The Bottom Line

Based on this research, parents should be aware that vitamin D measurement standards vary and may need to ask their doctor which standard they’re using. The research suggests that doctors should consider using region-specific guidelines that account for local sunlight and climate. For children in areas with long winters (like northern China), the study suggests that higher vitamin D supplementation during winter months might be beneficial, though more research is needed to confirm this. Confidence level: Moderate—the research clearly shows the measurement problem exists, but more studies are needed to determine the best solutions.

Parents of young children (especially infants and toddlers) should pay attention to this research, particularly if they live in areas with significant seasonal changes or limited winter sunlight. Healthcare providers who test children’s vitamin D levels should care about this research because it shows they need clearer guidelines. Children with limited sun exposure, those living in northern climates, or those with dietary restrictions may benefit most from attention to vitamin D status. This research is less immediately relevant to families living in sunny climates year-round.

If a child is given vitamin D supplements based on these measurements, it typically takes several weeks to months to see changes in blood vitamin D levels. Seasonal changes in vitamin D naturally occur over months as sun exposure changes. Parents shouldn’t expect immediate results from supplementation—consistent supplementation over 8-12 weeks is usually needed to see meaningful changes in blood vitamin D levels.

Want to Apply This Research?

  • Track your child’s vitamin D test results over time, noting the date, the measured level (in nmol/L or ng/mL), which diagnostic criteria the doctor used, and the season when tested. This helps you see patterns and compare results across different seasons and years.
  • Set seasonal reminders to discuss vitamin D with your child’s doctor, especially before winter months. If your child lives in a northern climate, work with your doctor to plan appropriate vitamin D supplementation for the winter season based on their age and current levels.
  • If your child is on vitamin D supplements, retest blood levels every 3-4 months initially to ensure the dose is appropriate, then annually once stable. Keep records of which diagnostic criteria were used each time so you can accurately compare results over time and discuss with your doctor whether the measurement standard changed.

This research describes how vitamin D is measured in children and shows that different measurement methods give different results. It does not provide medical advice about whether your child needs vitamin D testing or supplementation. Always consult with your child’s healthcare provider before making decisions about vitamin D testing or supplementation. Your doctor can determine the best approach for your individual child based on their age, location, sun exposure, diet, and health status. Do not use this information to self-diagnose or self-treat vitamin D deficiency in your child.