Does the Iron Preemies Get Upset the Bacteria in Their Gut?

A plain-language look at a study (VanOrmer et al., 2024) on iron supplements and the premature baby's gut, and what newer research has added

Premature babies are almost always given a daily iron supplement, because being born early means missing out on the iron a baby normally stockpiles in the last months of pregnancy — and iron is essential for healthy blood and a developing brain. But parents and doctors have wondered whether that same iron might feed the "wrong" bacteria living in a baby's intestines and tip the balance toward illness. A 2024 study set out to check this directly, and its answer was reassuring: starting routine iron did not appear to disturb the mix of gut bacteria in the premature babies studied [1]. This article explains the worry, what the study found, and how even newer research has strengthened the reassurance.

Why iron matters so much for a premature baby

A baby growing in the womb collects most of its iron during the third trimester, the final stretch of pregnancy. A baby born weeks or months early simply misses much of that stockpiling, and then uses up what little it has through rapid growth and the small blood samples the hospital must take for testing. Without enough iron, a baby can become anemic and, more importantly over the long run, may face setbacks in brain development. For these reasons, pediatric guidelines recommend giving premature and low-birth-weight babies a daily iron supplement, usually starting at around one month of age [2]. Iron, in other words, is not an optional extra — it is a protective medicine.

The gut, its bacteria, and why they are so important early in life

Every person's intestines are home to trillions of bacteria, collectively called the gut microbiome. In a healthy baby, "friendly" bacteria gradually take over and help digestion, train the immune system, and keep harmful microbes in check. In a premature baby, this process is thrown off course. Because these infants are often born by cesarean, treated with antibiotics, fed slowly at first, and cared for in a hospital environment, their gut is colonized late and tends to be dominated early on by a group of potentially troublesome bacteria before the friendly ones establish themselves [3]. When that balance goes wrong — a state called dysbiosis — it can set the stage for necrotizing enterocolitis, a serious and sometimes fatal intestinal illness that is one of the gravest threats to the tiniest babies [4],[5]. Anything that might push a premature baby's gut in the wrong direction is therefore taken seriously.

Where the worry about iron came from

Here is the crux of the concern. Iron is not just food for us; it is food for bacteria too, and many of the harmful, "invasive" kinds are especially good at using it to multiply. In fact, the body naturally hides iron away during an infection, as a defense to starve invading microbes [6]. So it is reasonable to ask whether pouring extra iron into a baby's intestines could give harmful bacteria an advantage. Much of the early evidence for this worry came from studies in older children in parts of the world where infections are common: giving them iron did seem to shift their gut bacteria in an unhealthy direction [7]. But those children were much older than newborns and lived in very different circumstances, so it was never clear whether the same thing would happen to a tiny premature baby in a modern neonatal intensive care unit being fed breast milk. That gap is exactly what the 2024 study aimed to fill.

What the researchers did

Researchers at the University of Nebraska Medical Center in Omaha followed 14 premature babies, all born at 34 weeks of pregnancy or earlier and all fed breast milk [1]. They took advantage of their hospital's normal routine, in which iron is started when a baby is two weeks old. By collecting tiny stool (poop) samples during the first, second, and fourth weeks of life, they could compare the gut bacteria of each baby before and after iron was begun — with each baby serving as its own point of comparison. They then used genetic testing to identify which bacteria were present and in what proportions. The babies were quite premature, with a typical (median) gestational age of about 29 to 30 weeks, and the smallest was born at just under 24 weeks.

What they found

The main result was that starting iron did not cause any meaningful change in the babies' gut bacteria. The major groups of bacteria were present in about the same proportions before and after iron was begun [1]. The overall variety of bacteria did increase a little over the study, but this is exactly what naturally happens as a premature baby's gut matures week by week, and is not a sign of iron doing harm [8]. The most surprising finding had nothing to do with iron at all: babies born in the spring had a very different bacterial mix — with far more of the potentially troublesome group — than babies born in the winter, even though the two groups of babies were equally premature. In other words, the season a baby happened to be born in mattered more to their gut bacteria than the iron supplement did, a hint that the hospital environment and the time of year can shape these communities in ways researchers are still working to understand [9].

How much to trust a small study

It is important to be honest about the study's size. With only 14 babies, this was a small, preliminary study — what scientists call a pilot. A small study can easily miss a real effect, and it could not fully account for other things that strongly influence gut bacteria, such as antibiotics or whether a baby received mother's own milk versus donated milk. The researchers were careful to point this out and called for a larger study to confirm their findings [1]. So the honest summary of this study alone is not "iron is definitely harmless" but rather "in these babies, at the usual dose, we saw no sign of harm — and a bigger study is needed."

How families and doctors used to face this question — and what changed

For years, this uncertainty left neonatal teams in an awkward spot. Some worried enough about the gut that they leaned toward the lowest possible iron dose, even at the risk of leaving a baby short on a nutrient the brain needs. The evidence they had to go on came mostly from older children in other countries, or from a single earlier study of 80 premature babies that had raised a red flag — but only in babies given a high dose of iron, and even then the finding was tangled up with the fact that those high-dose babies were younger and sicker to begin with [10]. The 2024 pilot, finding no problem at the usual dose, was an important next step. Then a much larger and more rigorous study provided the clearest answer yet. In that trial, premature babies were randomly assigned to a lower or a higher iron dose, and their gut bacteria were examined in detail [11]. Among 151 babies, the higher dose did not harm the gut bacteria at all. In fact, babies on the lower dose showed slightly more signs of a "leaky" gut lining and were more likely to end up iron-deficient. The overall picture that emerged is that worrying about gut bacteria is not a good reason to skimp on the iron a premature baby needs.

What this means for your baby

If your premature baby is receiving iron, this research offers genuine reassurance. The best available evidence does not show routine iron upsetting the balance of gut bacteria, and iron is given for important reasons — to protect against anemia and to support the developing brain [2],[11]. The care team chooses the dose based on your baby's blood counts, any transfusions, and how feeding is going, and they monitor iron levels and how your baby tolerates feeds, adjusting as needed. It is always a good question to ask your baby's doctors or nurses about, and they can explain the plan for your child. The things that protect a premature baby's gut the most remain the familiar ones — breast milk whenever possible, careful use of antibiotics, and strict infection-prevention practices [9].

What researchers are working on next

Scientists still want to understand the finer details: how iron interacts with breast milk, with the probiotic supplements some units give, and with antibiotics; whether measuring iron directly in the stool could help fine-tune each baby's dose; and whether the gut-lining and iron-status advantages seen with a higher dose translate into better growth and development over time [11]. Larger, longer studies will help answer these questions. For now, the reassuring bottom line for families is that the iron premature babies receive appears to be doing its job — supporting healthy blood and brain development — without upsetting the delicate community of bacteria in their gut.

References

  1. VanOrmer M, Thompson M, Thoene M, Riethoven J-J, Natarajan SK, Hanson C, et al. The impact of iron supplementation on the preterm neonatal gut microbiome: A pilot study. PLoS ONE. 2024;19(2):e0297558. doi:10.1371/journal.pone.0297558 ↩
  2. Baker RD, Greer FR; AAP Committee on Nutrition. Diagnosis and Prevention of Iron Deficiency and Iron-Deficiency Anemia in Infants and Young Children (0–3 Years of Age). Pediatrics. 2010;126(5):1040–1050. doi:10.1542/peds.2010-2576 ↩
  3. Underwood MA, Sohn K. The Microbiota of the Extremely Preterm Infant. Clin Perinatol. 2017;44(2):407–427. doi:10.1016/j.clp.2017.01.005 ↩
  4. Pammi M, Cope J, Tarr PI, Warner BB, Morrow AL, Mai V, et al. Intestinal dysbiosis in preterm infants preceding necrotizing enterocolitis: a systematic review and meta-analysis. Microbiome. 2017;5(1):31. doi:10.1186/s40168-017-0248-8 ↩
  5. Baranowski JR, Claud EC. Necrotizing Enterocolitis and the Preterm Infant Microbiome. Adv Exp Med Biol. 2019;1125:25–36. doi:10.1007/5584_2018_313 ↩
  6. Weinberg ED. Iron availability and infection. Biochim Biophys Acta. 2009;1790(7):600–605. doi:10.1016/j.bbagen.2008.07.002 ↩
  7. Zimmermann MB, Chassard C, Rohner F, N'Goran EK, Nindjin C, Dostal A, et al. The effects of iron fortification on the gut microbiota in African children: a randomized controlled trial in Côte d'Ivoire. Am J Clin Nutr. 2010;92(6):1406–1415. doi:10.3945/ajcn.110.004564 ↩
  8. Korpela K, Blakstad EW, Moltu SJ, Strømmen K, Nakstad B, Rønnestad AE, et al. Intestinal microbiota development and gestational age in preterm neonates. Sci Rep. 2018;8(1):2453. doi:10.1038/s41598-018-20827-x ↩
  9. Rozé JC, Ancel PY, Marchand-Martin L, Rousseau C, Montassier E, Monot C, et al. Assessment of Neonatal Intensive Care Unit Practices and Preterm Newborn Gut Microbiota and 2-Year Neurodevelopmental Outcomes. JAMA Netw Open. 2020;3(9):e2018119. doi:10.1001/jamanetworkopen.2020.18119 ↩
  10. Ho T, Sarkar A, Szalacha L, Groer MW. Intestinal Microbiome in Preterm Infants Influenced by Enteral Iron Dosing. J Pediatr Gastroenterol Nutr. 2021;72(5):e132–e138. doi:10.1097/MPG.0000000000003033 ↩
  11. Gibbons JA, Nelson RM, Dabrowski CN, Narkhede A, Szalacha LA, Kneusel ML, et al. Enteral Iron Dose Effect on Iron Storage, Intestinal Barrier, and Gut Microbiome in Preterm Infants: A Randomized Clinical Trial. Am J Clin Nutr. 2026:101389. doi:10.1016/j.ajcnut.2026.101389 ↩