Feeding Premature Babies an All-Human-Milk Diet: What It Means for Health, Growth, and Development

A plain-language guide to what research — including two studies from Kaiser Permanente Southern California — tells families about the exclusive human milk diet

Why this matters for families

Babies born much too early, before 32 weeks of pregnancy, need very careful feeding while they grow in the neonatal intensive care unit, or NICU. Breast milk — either the mother's own milk or carefully screened donated milk from other mothers — is the recommended foundation for these babies [1]. But breast milk alone does not contain enough calories, protein, and minerals for such tiny infants, so the care team adds a "fortifier" to make it richer [2].

Here is where families face a choice that the medical team will often discuss with them. The fortifier can be made from cow's milk, or it can be made from human milk. When everything the baby receives — both the base milk and the fortifier — comes from human milk, this is called an exclusive human milk diet, often shortened to EHMD. This article explains, in plain terms, what researchers have learned about how this diet affects three things parents care about most: their baby's health complications, their baby's growth, and their baby's later development. It draws on the major studies in this field, including two from Kaiser Permanente Southern California (KPSC) [3],[4].

Health complications: protection against a dangerous gut disease

The clearest benefit of an all-human-milk diet involves a serious intestinal illness called necrotizing enterocolitis, or NEC. NEC is a condition in which part of a premature baby's bowel becomes inflamed and can be badly damaged; it is one of the most feared complications in the NICU. Early research showed that babies on an all-human-milk diet developed NEC less often than babies who received cow's-milk-based products [5]. In one study, only about 3 out of 100 babies on the human-milk diet developed NEC, compared with about 21 out of 100 fed premature formula [6].

It is important to understand the nuance here. The protection against NEC is biggest for the babies at highest risk — the smallest and sickest, and in hospitals where NEC is more common to begin with. In settings where NEC is already rare, the added benefit is smaller. In the KPSC studies, where NEC was uncommon, the babies who received the human-milk diet were actually the sickest babies in the unit, because doctors chose to give it first to those who needed protection most [3],[4]. This is worth knowing, because it means simple comparisons can look misleading until researchers carefully account for how sick each baby was to start.

Growth: how the fortifier is used makes the difference

Parents often hear that the all-human-milk diet might slow a baby's growth, and there is some truth to this — but the full picture is more hopeful and more in the family's and team's control than it first appears. The KPSC growth study, which looked at more than 1,600 babies, found that babies on the human-milk diet grew somewhat more slowly in length (height), while their weight gain and head growth were not meaningfully different overall [3].

The most useful discovery was about how strongly the milk was fortified at the start. When fortification began at a lower calorie level, babies grew more slowly. But when fortification began at a higher calorie level — what doctors call 26 kcal/oz instead of the more traditional 24 — those babies actually gained weight better than babies not on the diet, and their length and head growth caught up too [3]. In plain terms, much of the slower growth was not caused by the human milk itself, but by not adding enough calories early on. This is something the care team can adjust. Larger carefully designed studies have shown the same overall point: a human-milk base can slow weight gain a little, especially when calories are not pushed high enough [7],[6],[8],[9].

Development: the encouraging surprise

The biggest worry behind slower growth is whether it means a baby's brain development will suffer, because for many years doctors believed faster growth went hand-in-hand with a healthier brain. The newer research is genuinely reassuring on this point — and may even point to a benefit.

Breast milk in general is linked to better brain development in premature babies [10]. Studies looking specifically at the all-human-milk diet have found either no difference in development or, in some cases, better scores [11],[12],[13]. The KPSC team followed their babies until three years of age and looked at "motor function" — the development of movement skills like sitting, crawling, walking, and coordination. They found that babies who had received the all-human-milk diet were less likely to have a motor problem, even though that same diet had been linked to slightly slower length growth in the same group of children [4].

This is the surprising and important part: growth and brain development did not rise and fall together. A baby could grow a little more slowly in length yet do better with movement skills. After accounting for the babies' prematurity and other complications, the movement benefit was still there [4]. This suggests that something special in human milk itself — not just the calories it provides — may help protect the developing brain.

A separate large, carefully designed study supports the same general idea, though it did not test the full all-human-milk diet. That study compared donated human milk with formula and found that the donated-milk babies gained weight a little more slowly but developed just as well — another example of growth and brain development not moving in lockstep [7].

It helps to know why doctors chose to look at movement skills in particular. Movement development, like learning to sit, reach, and walk, tends to show up earlier than language or thinking skills, and it often lays the groundwork for those later abilities. Early movement difficulties can also be an early warning sign of broader developmental challenges, because the parts of the brain involved overlap. And unlike some other measures of development, movement skills are less affected by a family's income or home environment, which makes them a fairer and more reliable thing to measure across many different families [4]. For all these reasons, a lower risk of movement problems is a meaningful and encouraging finding, not a narrow technical one.

What might explain this

Scientists think the answer lies in the many special ingredients in human milk that formula and cow's-milk fortifiers do not fully reproduce. Human milk contains particular fats, proteins like lactoferrin, and unique sugars that nourish the gut and appear to support the brain through what researchers call the "gut-brain" connection [14]. One especially interesting component is the membrane that surrounds the tiny fat droplets in milk, which is rich in building blocks the brain uses to form the insulation around nerve fibers and to build connections between brain cells. Donated milk loses some of these ingredients during the pasteurization that keeps it safe, but fortifying with a human-milk product can restore some of them [15],[16]. The leading idea is that an all-human-milk diet preserves a richer mix of these brain-supporting ingredients than a cow's-milk-based approach.

What this means for your baby's care

If your NICU offers an all-human-milk diet, a few things follow from the research that are worth discussing with your care team. The diet's protection against NEC is most valuable for the smallest and sickest babies. If your baby is on this diet, asking the team about fortifying with enough calories early on can help protect growth — slower growth is not inevitable. And slower length growth, if it happens, has not been shown to harm development; in fact the movement outcomes may be better. These are reasonable questions to raise, and a good care team will welcome them. You might also ask whether your own milk supply can be supported and increased, since a mother's own milk remains the first choice whenever it is available, with donor milk and the human-milk fortifier filling in the rest.

Honest limits of what we know

It is important to be straight about the limits of this research. Most of these studies, including the KPSC ones, looked back at babies already cared for rather than assigning the diet in advance, which makes it harder to be certain the diet itself caused the results rather than which babies happened to receive it [3],[4]. The movement study used doctors' diagnoses from medical records rather than a single standardized test [4]. And the large carefully designed study compared donor milk with formula, not the full all-human-milk diet [7]. None of these studies is the final word.

What researchers are working on next

Scientists are now trying to learn whether starting with higher-calorie fortification helps not just growth but also development, whether the benefits are unique to human milk, and exactly which ingredients in human milk help the brain the most. Future studies that assign the diet in advance and measure development carefully will help answer these questions. For now, the all-human-milk diet is a reasonable, well-supported choice — especially when paired with attention to getting enough calories — and the news for families worried about development is, on balance, encouraging.

References

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  2. Brown JVE, Lin L, Embleton ND, Harding JE, McGuire W. Multi-nutrient fortification of human milk for preterm infants. Cochrane Database Syst Rev. 2020;2020:CD000343. doi:10.1002/14651858.CD000343.pub4 ↩
  3. Chou F-S, Zhang J, Nguyen C, Faison GM, Thompson LR, Villosis MFB, et al. The impact of exclusive human milk diet on short-term growth of very preterm infants. J Perinatol. 2024;44. doi:10.1038/s41372-024-01980-w ↩
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  5. Sullivan S, Schanler RJ, Kim JH, Patel AL, Trawöger R, Kiechl-Kohlendorfer U, et al. An exclusively human milk-based diet is associated with a lower rate of necrotizing enterocolitis than a diet of human milk and bovine milk-based products. J Pediatr. 2010;156:562–7.e1. doi:10.1016/j.jpeds.2009.10.040 ↩
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