A Bigger Dose of Vitamin D for Premature Babies: What a New Study Found

How a trial in Thailand compared two daily vitamin D doses for premature newborns — and why the higher one worked better

Premature babies very often run short of vitamin D, the nutrient their growing bones need, and a 2025 study from Thailand tested whether giving a bigger daily dose helps. Researchers gave 38 premature newborns either the usual 400 international units (IU) of vitamin D each day or a doubled dose of 800 IU, then checked their blood after six weeks. The higher dose worked noticeably better: the share of babies still low in vitamin D dropped from about 9 in 10 to just 1 in 20, compared with a smaller improvement on the standard dose — and the bigger dose caused no signs of getting too much [1]. It is a small study, but a practical and reassuring one for families of premature infants.

Why Vitamin D Matters So Much for Premature Babies

Vitamin D is the nutrient that lets the body absorb calcium and phosphate and lock them into growing bones [1]. For a baby, the last three months of pregnancy are when most of that mineral stockpiling happens — the baby draws vitamin D, calcium, and phosphate across the placenta and stores them up for the fast bone growth to come. A baby born many weeks early misses much of that window. They arrive with very little vitamin D in reserve, their skin makes almost none because they are indoors under lights rather than in sunshine, and breast milk — which is the best food for them in nearly every other way — happens to contain very little vitamin D [2], [3].

When that shortfall is not corrected, it can lead to a condition called metabolic bone disease of prematurity: the bones become soft and thin, blood chemistry drifts out of balance, and in the worst cases the bones can even break or develop rickets [2], [4]. This is why almost every premature baby in a modern neonatal unit is given vitamin D drops. The open question — and it is a surprisingly old one — has been exactly how much to give.

The Problem That Motivated the Study

For decades, the "standard" dose of 400 IU a day was more a matter of habit and tradition than of firm proof in premature babies specifically. It was borrowed from what worked in full-term infants and from long experience preventing rickets, and it stuck. Over the last ten years, several teams of researchers began to actually test whether that familiar dose is enough for the smallest, earliest babies. Studies in the United States, India, and elsewhere compared different doses and generally found that higher amounts raised vitamin D levels more reliably [5], [6], [7]. But there was a catch: how a baby responds to any given dose depends a great deal on where they live, how much sunlight the region gets, and how much vitamin D their mother had during pregnancy. Results from one country do not automatically apply to another. In Thailand, where vitamin D shortage is common among pregnant women and there was no national guideline for premature babies, doctors were essentially making their best guess [8]. That gap is what this study set out to close.

What the Researchers Did

The trial took place in the neonatal intensive care unit of a university hospital in Chiang Mai, Thailand, between March 2024 and February 2025 [1]. It enrolled babies born at 32 weeks of pregnancy or earlier, or weighing 1,500 grams (about 3.3 pounds) or less. Thirty-eight babies took part. Which dose each baby received was decided at random — like a sealed-envelope coin toss — so that the two groups would be as similar as possible and the comparison would be fair. Nineteen babies got the standard 400 IU a day, and nineteen got the doubled 800 IU a day. Everything else about their care and feeding followed the same protocol. The laboratory staff who measured the vitamin D levels did not know which baby was in which group, which keeps the results honest. The team measured each baby's vitamin D level and several bone-related blood values before starting the drops and again at six weeks of age.

What They Found

The first finding was sobering on its own: before supplements began, 82% of these babies — more than four in five — already had too little vitamin D [1]. In other words, most premature babies start life already behind, which is exactly why catching up matters.

After six weeks, both doses raised vitamin D levels, but the higher dose raised them considerably more. The clearest way to see the difference is in how many babies were still short of vitamin D at the end. In the 800 IU group, only about 1 in 20 babies (5%) remained low, and none were in the deficient range. In the 400 IU group, roughly 1 in 3 babies (32%) were still low [1]. Put another way, for every four babies given the higher instead of the standard dose, one extra baby was spared from remaining vitamin D–short. That is a meaningful, real-world difference from a simple change in dose.

It helps to picture what these numbers mean. Vitamin D in the blood is measured in nanograms per milliliter (ng/mL), and doctors generally consider a level above 20 to be healthy. When the babies started, their average level was under 15 — below the healthy mark. After six weeks, the standard-dose group had climbed to an average of about 32, and the higher-dose group to about 47. Both improved, but the higher-dose babies not only reached healthier levels on average, they were far less likely to be left behind. The care team also looked at whether the type of feeding mattered — for instance, whether milk was fortified with preterm formula or with a human-milk fortifier — and found it made no meaningful difference to the vitamin D result, so the dose itself, not the feeding choice, was doing the work [1].

Just as important, the bigger dose appeared safe over the six weeks. No baby in either group developed signs of getting too much vitamin D [9]. The researchers did note one thing that sounds disappointing but is not: the bone-chemistry blood tests did not change much between the groups. That is expected, because vitamin D levels in the blood shift within a few weeks, while the bones themselves respond much more slowly — six weeks is simply too short a window to see a bone difference [2]. The vitamin D level is the early warning light; the bones catch up later.

What This Means for Families

If your baby was born prematurely, there are a few comforting things to take from this study. Low vitamin D is extremely common in premature babies — it is not a sign that anything was done wrong, and it is not caused by breastfeeding being a poor choice; breast milk is simply naturally low in this one nutrient, which is why the drops exist in the first place [3]. It is also easily corrected. This study suggests that in places where babies tend to start out very short of vitamin D, a higher daily dose of drops can close the gap more completely, without causing harm over the weeks that were studied. Care teams often re-check a vitamin D blood level around six weeks to make sure the dose is working and to fine-tune it for each baby [10]. Guidelines from expert groups already allow for higher doses when needed, so this is well within accepted practice [11]. If you ever have questions about your own baby's vitamin D drops, your neonatal team can tell you the exact dose your baby is on and why.

What Researchers Are Working On Next

This study answers a narrow question well — a higher dose corrects the vitamin D shortfall better — but it leaves the biggest questions open, mostly because it was small and short. It included only 38 babies at a single hospital, and it followed them for just six weeks, so it could not measure whether the higher dose ultimately means stronger bones, better growth, or fewer fractures down the road [5]. Those are the outcomes that matter most to families, and answering them will take larger studies across several hospitals, following babies for months or years, and also checking mothers' vitamin D levels during pregnancy, since a mother's vitamin D strongly shapes her baby's [1]. It is also worth remembering how this study fits into a longer effort. It is one of several trials from different parts of the world — the United States, India, Turkey, Poland, and now Thailand — that have each chipped away at the same question of how much vitamin D premature babies really need [5], [6], [7], [8]. No single small study settles it, but together they point in a consistent direction, and this one adds a careful answer for a setting where babies tend to start out especially low. For now, the message is modest but genuinely useful: for premature babies who tend to start out low, doubling the standard vitamin D dose is an effective and, over six weeks, safe way to help them catch up.

References

  1. Rueang-amnat N, Kittisakmontri K, Khuwuthyakorn V, et al. Improving Vitamin D Status in Preterm Newborns: A Randomized Trial of 800 vs. 400 IU/Day. Nutrients. 2025;17(11):1888. doi:10.3390/nu17111888 ↩
  2. Faienza MF, D'Amato E, Natale MP, et al. Metabolic bone disease of prematurity: diagnosis and management. Front Pediatr. 2019;7:143. doi:10.3389/fped.2019.00143 ↩
  3. Abrams SA. Calcium and vitamin D requirements of enterally fed preterm infants. Pediatrics. 2013;131:e1676–e1683. doi:10.1542/peds.2013-0420 ↩
  4. Munns CF, Shaw N, Kiely M, et al. Global consensus recommendations on prevention and management of nutritional rickets. J Clin Endocrinol Metab. 2016;101:394–415. doi:10.1210/jc.2015-2175 ↩
  5. Anderson-Berry A, Thoene M, Wagner J, et al. Randomized trial of two doses of vitamin D3 in preterm infants < 32 weeks: dose impact on achieving desired serum 25(OH)D3 in a NICU population. PLoS ONE. 2017;12:e0185950. doi:10.1371/journal.pone.0185950 ↩
  6. Natarajan CK, Sankar MJ, Agarwal R, et al. Trial of daily vitamin D supplementation in preterm infants. Pediatrics. 2014;133:e628–e634. doi:10.1542/peds.2012-3395 ↩
  7. Fort P, Salas AA, Nicola T, et al. A comparison of 3 vitamin D dosing regimens in extremely preterm infants: a randomized controlled trial. J Pediatr. 2016;174:132–138. doi:10.1016/j.jpeds.2016.03.028 ↩
  8. Kołodziejczyk-Nowotarska A, Bokiniec R, Seliga-Siwecka J. Monitored supplementation of vitamin D in preterm infants: a randomized controlled trial. Nutrients. 2021;13:3442. doi:10.3390/nu13103442 ↩
  9. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357:266–281. doi:10.1056/NEJMra070553 ↩
  10. Dawodu A, Tsang RC. Maternal vitamin D status: effect on milk vitamin D content and vitamin D status of breastfeeding infants. Adv Nutr. 2012;3:353–361. doi:10.3945/an.111.000950 ↩
  11. Embleton ND, Moltu SJ, Lapillonne A, et al. Enteral nutrition in preterm infants (2022): a position paper from the ESPGHAN Committee on Nutrition and invited experts. J Pediatr Gastroenterol Nutr. 2023;76:248–268. doi:10.1097/MPG.0000000000003642 ↩