Feeding a Premature Baby More Milk Sooner: Good for Growth, and Gentle on the Gut

What a randomised study of early breast-milk feeding tells families about their baby's growing "gut garden"

A new study of very premature babies offers a piece of reassuring news for families: giving a baby more of their mother's or donor breast milk in the very first hours of life helped babies reach full feeds faster and grow better, and it did not upset the community of helpful bacteria taking root in the baby's gut. Doctors in Birmingham, Alabama, carefully compared two feeding approaches in babies born about two to three months early, and when they examined the babies' gut bacteria at two weeks of age, the two groups looked essentially the same. In short, feeding sooner was good for the baby without any detectable downside for the gut [1].

Why the tiny world inside a baby's gut matters so much

Every human carries trillions of bacteria in the intestines, and in a newborn these microbes are just beginning to move in — a bit like the first seeds arriving in an empty garden. In babies born very early, this "gut garden" is especially delicate. When the wrong bacteria take over too quickly, doctors worry about a dangerous bowel illness called necrotising enterocolitis, or NEC, which can be life-threatening for premature infants. Researchers have found that an unhealthy balance of gut bacteria often appears just before NEC develops, which is why so much attention is paid to what these babies are fed and how their gut bacteria grow [2]. The gut community also does not appear all at once; it develops in a fairly predictable sequence over the first weeks, with different kinds of bacteria taking their turn as the baby matures [3].

The old worry: feed too much, too soon?

For a long time, doctors were understandably cautious about feeding premature babies quickly. The concern was that a tiny, immature gut might not handle milk well, and that pushing feeds could trigger NEC. So many hospitals started feeds slowly and built them up gradually over many days, meanwhile giving nutrition through a vein. Breast milk was always seen as protective — studies showed that babies who received more of their own mother's milk tended to have a healthier mix of gut bacteria [4], and that mother's own milk and donor milk each left their own signature on the gut community [5]. But there was a catch that made these findings hard to trust: the sickest babies are usually the ones fed the least, so it was never clear whether the milk itself was helping the bacteria, or whether healthier babies simply tolerated more milk. An earlier carefully designed study of an all-human-milk diet found only small effects on the gut bacteria [6], leaving families and doctors with a real question: would feeding more milk earlier help the gut, harm it, or make no difference at all [7]?

How the researchers found a fair answer

The best way to answer such a question fairly is a randomised trial, where a coin-flip — not the baby's illness — decides which feeding plan each baby receives. That way, the two groups start out similar, and any difference at the end can be credited to the feeding, not to how sick the babies were. In this trial, carried out at a single hospital — the University of Alabama at Birmingham (listed in the public trials registry as NCT04337710) — 102 babies born between 28 and 32 weeks of pregnancy (roughly two to three months early) who were stable enough to feed were randomly assigned to one of two plans in their first day and a half of life [7]. One group received a larger amount of breast milk — about 60 to 80 millilitres per kilogram of body weight each day — while the other received the more traditional smaller amount, about 20 to 30 millilitres per kilogram per day [1]. Both groups were fed mother's own milk or donor milk. Later, the team collected a stool sample from each baby at around two weeks of age and used genetic sequencing to identify which bacteria were present. In total, 95 babies' samples were compared — 47 from the larger-feeding group and 48 from the smaller-feeding group.

What they discovered

When the researchers compared the gut bacteria of the two groups, they found no meaningful difference. Measures of how many different kinds of bacteria were present, and how evenly they were balanced, were almost identical between the two feeding plans [1]. When they looked at the overall makeup of each baby's gut community, the two groups blended together rather than separating out. They also sorted the babies into four natural "types" of gut community, and these types were spread evenly across both feeding groups. Interestingly, the one thing that did seem to shape a baby's gut bacteria was how the baby was born — vaginal birth versus caesarean — which fits what scientists already know about where a newborn's first bacteria come from. The amount of milk, by contrast, made no visible difference to the gut at two weeks. It can help to picture the newborn gut as a garden that is still mostly bare soil at two weeks: the first plants to arrive depend heavily on which "seeds" the baby was exposed to at birth, and the community is still shifting week to week. Against that backdrop of rapid, natural change, adjusting how much milk a baby received in the first day and a half simply was not a strong enough push to redraw the garden's layout by the two-week mark. That does not mean feeding is unimportant — it clearly helped the babies grow — it means the gut's early makeup is driven by other forces at this particular stage.

Here is why that "no difference" is actually good news. In the main results of the same trial, the babies fed more milk sooner did better in ways that matter a great deal: they reached full feeds about three days earlier, built more lean body tissue by two weeks, were longer for their age by the time they went home, and cost less to care for [7]. So the extra early milk brought real benefits for growth and recovery — and the study shows it did so without disturbing the delicate gut garden. Feeding intolerance and serious infections were also uncommon during the trial, adding to the picture of a feeding approach that was both helpful and gentle.

What this means for families

If your baby is premature and the care team is starting or advancing breast-milk feeds sooner than you expected, this study is a reason for reassurance rather than alarm. Feeding more of your milk early helped babies grow and get to full feeds faster, and it did not throw off the balance of good bacteria in the gut. It is worth knowing that your own milk is especially valuable here; where a mother's milk is not available, donor milk is used, and the source of the milk appears to matter more for the gut than whether an added fortifier comes from cow's milk or human milk [8]. If you are pumping or working with a lactation consultant, that effort is directly connected to your baby's nutrition and gut health. And if you have heard about probiotics — supplements of beneficial bacteria — it is true that these are the tools that most clearly change the gut community in premature babies, and some units use them; they work differently from feeding and are a separate conversation to have with your baby's doctors [9], [10].

The honest limits of one study

No single study settles everything, and the researchers were upfront about this one's limits. It was done at one hospital with a modest number of babies, so it is better at ruling out big effects than tiny ones. It looked at the gut bacteria only once, at two weeks, so it is possible that differences appear earlier or later that this one snapshot missed — the early gut is a fast-changing, unpredictable place, and it tends to develop along its own timeline through a series of natural milestones [11]. And the genetic test used identifies which bacteria are present but not everything about what they are doing. So the fair conclusion is a careful one: within what this study could measure, feeding more human milk sooner did not disturb the gut, while clearly helping the baby grow.

What researchers are working on next

Scientists now want to follow babies' gut bacteria over many weeks rather than at a single moment, to combine bacterial "head counts" with tests of what those bacteria are actually doing, and to connect all of this to the outcomes families care about most — avoiding NEC and infections, and healthy development over time [12]. For now, the practical takeaway is encouraging and simple: for stable, very premature babies, giving breast milk early and building it up steadily supports growth and recovery, and the best evidence available says the gut garden takes it in stride.

References

  1. Salas AA, Stewart CJ, Young GR. Early gut microbiome composition of very preterm infants randomised to receive human milk volumes of 60 ml/kg/day or more within the first 36 hours after birth. Pediatric Research. 2026;99(5):1964–1969. doi:10.1038/s41390-025-04456-5 ↩
  2. Cuna A, Morowitz MJ, Ahmed I, Umar S, Sampath V. Dynamics of the preterm gut microbiome in health and disease. American Journal of Physiology-Gastrointestinal and Liver Physiology. 2021;320(4):G411–G419. doi:10.1152/ajpgi.00399.2020 ↩
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  6. Embleton ND, et al. Effect of an exclusive human milk diet on the gut microbiome in preterm infants: a randomized clinical trial. JAMA Network Open. 2023;6(3):e231165. doi:10.1001/jamanetworkopen.2023.1165 ↩
  7. Razzaghy J, Kissler K, Reeves A, et al. Early and exclusive enteral nutrition in infants born very preterm. Archives of Disease in Childhood - Fetal and Neonatal Edition. 2024;109(4):378–383. doi:10.1136/archdischild-2023-325969 ↩
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  9. Beck LC, Masi AC, Young GR, et al. Strain-specific impacts of probiotics are a significant driver of gut microbiome development in very preterm infants. Nature Microbiology. 2022;7(10):1525–1535. doi:10.1038/s41564-022-01213-w ↩
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