Understanding the infant gut and microbiome

This is the foundation the rest of infant feeding rests on. If you understand what a baby's gut is expecting, then colic, reflux, intolerance and allergy stop looking like random misfortunes and start looking like predictable consequences.

Why babies are born unfinished

As female primate bodies adapted to walking upright, the shape of the pelvis changed. Combine that with the skull size needed to house a large human brain, and you get what's known as the evolutionary compromise: human babies must be born earlier, less physically capable and more dependent than virtually every other mammal.

Because they arrive "too early", much of their development happens outside the womb — the fourth trimester. The brain doubles in size and creates billions of synaptic connections in the first year, yet most children won't walk independently until the end of it.

Over millions of years, that meant babies were carried by family members and rarely left alone. That single fact explains a great deal — including why reflux is so common in babies who spend long periods lying flat.

What happens in the first days

During pregnancy, progesterone suppresses prolactin and with it the ability to make lactose. The body is capable of making milk but doesn't begin until after birth. From around 20 hours after the placenta is delivered, progesterone drops and milk production can begin — which is why retained placental products can delay it.

Before that, colostrum does specific jobs: it clears the sticky meconium from the gut, and provides antioxidants and immune protection.

Healthy babies don't need large fluid volumes immediately after birth. They're breaking down the extra blood volume needed during pregnancy, which liberates fluid to be excreted and also bilirubin — a powerful antioxidant that breastfed babies retain a little longer, because higher-protein formula binds it for immediate excretion rather than allowing it to recirculate via the liver as it evolved to do.

Volumes then climb steadily as lactose production increases — and the baby's lactase production rises in step:

Amounts vary enormously, and so does the caloric density of the milk. If a baby is gaining well, there is no need to know the amounts.

The microbiome

The first months are when the gut is colonised by hundreds of lifeforms forming a complex ecosystem. Put together, these would make a mass almost as big as the brain — which is why the microbiome is increasingly treated as an extra organ, sometimes described as a second controlling brain.

The microbes transferred in milk interact with immature gut cells, establish colonies, multiply, and direct actual gut development in ways that let them persist and crowd out harmful invaders.

Gut bacteria break food down into metabolites we can absorb — but they also communicate with and influence the developing brain, and go on doing so throughout life. This pioneer microbiome is probably the single biggest factor in lifelong health, and by age three it has settled and is not readily reshaped. Hence the emphasis on the first thousand days.

How the mother's body feeds it

A mother's milk is not a static product. Her body actively samples her environment through the air she breathes and the food she eats, and sensitised cells migrate from her gut and airways to the breast, where specific antibodies are made. These are the entero-mammary (gut to breast) and broncho-mammary (airway to breast) pathways.

More surprising: there is a small but significant backwash of infant saliva into the breast ducts during feeding, and the breast responds to it. A mother who kisses and nuzzles her baby is actively sampling the child's microbiome — which becomes a genuinely protective strategy when the child is also being cared for by others.

We saw how fast this works during the Covid pandemic: it takes scientists months or years to develop a vaccine; a breastfeeding mother produces specific antibodies within days, on autopilot.

Gut permeability

One of the biggest differences between infant and adult bodies is how permeable the gut lining is.

The period after birth is critical, because the gut needs to let some of breastmilk's gifts through to the rest of the body — notably pluripotent stem cells, which travel to other organs and are capable of repair work. Later in life a "leaky gut", whether from viruses, alcohol or food antigens, is associated with inflammation and disorders such as irritable bowel syndrome.

So a newborn's gut is intentionally leaky. That's the design.

It also means the first few days carry weight. Giving any food other than breastmilk in that window alters the oral and gut microbiome in all exposed children, and in susceptible ones increases the risk of creating sensitivities to those foods. This is the mechanism behind a great deal of what later presents as cows' milk protein allergy.

Gut acidity and infection

The first barrier to microbial infection is stomach acid. A breastfed baby's normal stomach pH of under 4 is enough to kill most pathogens swallowed.

A baby who isn't breastfed often has a less acidic stomach with a higher pH, which lets fungi and bacteria survive and go on to colonise the intestines. Some survivors can grow in the stomach itself and ferment food, producing gas — and alcohol — as by-products.

That lower pH isn't incidental. The Lactobacilli and Bifidus strains that dominate a breastfed baby's gut produce acetic and lactic acid, preventing pathogens like Cronobacter and Clostridia from establishing themselves.

This is also why acid-suppressing drugs deserve caution: anything that neutralises stomach acid for a long period increases the danger of infection. They are sometimes needed, but many haven't been researched in infants, and some manufacturers advise against giving them to children.

Transit time and feeding intervals

Watch: a lactation consultant's top tip for relieving trapped gas in babies.

Breastmilk is digested rapidly — half gone from the stomach in around 45 minutes, the stomach completely empty in well under two hours. Formula stays roughly twice as long, which in preterm infants has been shown to increase the risk of feed intolerance and sepsis.

So breastfed babies need to feed often and cannot be expected to go long periods between feeds. Routine or scheduled feeding was built on ignorance of infant biology, and has done enormous damage to normal breastfeeding.

Prescribed three and four hour intervals derive from less digestible foods. Slower transit might look like a benefit — fewer nappies, longer gaps — but it also means foreign proteins are in contact with the body for longer. The gut's alternative response is to push everything through fast, along with a lot of fluid, which we'd recognise as diarrhoea.

One historical note worth knowing: the very high-casein formulas of earlier decades had a sedating effect, because casein breaks down into opioid compounds that lengthen sleep beyond what is typical for a young baby. Modern formulas are not those formulas — composition is now regulated and casein levels are far lower.

It's still useful context when a family asks why a "Hungry Baby" or "Follow-on" milk seems to make their baby sleep longer: those products sit at the higher-casein end of the range, and longer sleep is a digestion effect rather than a sign the baby was previously underfed. Worth knowing when advising, not a reason to alarm anyone already using one.

The body's own defences

Infant defence mechanisms are largely self-regulating, and should not be suppressed unnecessarily. Pain, fever and increased secretions are indications that a cause needs identifying — blot out the indication and the damage continues.

Mucus and blood in stools

Mucus traps threats and carries them out of the body, and increases in response to irritation. Mucus in infant stools is normal and frequently over-interpreted — only large quantities warrant concern.

Blood in infant stools is different. It means mucus has failed to prevent damage to the cells lining the gut, and that is a cause for concern. Find the irritation and end it: infection, allergen or toxin. Sometimes a change in maternal diet to remove the likely allergen is all that's needed.

Fever

Most bacteria and viruses are temperature-sensitive and thrive only within a limited range. Raising body temperature is one of the body's first defences, and a fever simply signals the immune system has engaged — it can be triggered by an allergen as readily as an infection.

Fever should not be actively suppressed by medication unless it's high enough to raise concern about seizures. The common antipyretics all have side effects, including reducing the body's ability to fight infection. Temperature can be lowered by simple physical means that work immediately, pose no threat to immature kidneys, and don't down-regulate temperature for hours afterwards.

Ear infections

An infant's eustachian tubes sit lower in the back of the throat than they will in adulthood. For a breastfed baby that's protective — breastmilk washes over the opening at every feed. For a formula-fed baby the same anatomy works against them, as foreign proteins washing over the opening can provoke reactions that encourage pathogen growth. It's part of why otitis media is more common in children who aren't breastfed.

Epigenetics

The old nature-versus-nurture debate is settled: the genes we're born with can be expressed in different ways with different consequences, depending on physical and emotional environment.

What we eat — and what our parents and grandparents ate — is among the biggest influences, because diet feeds the microbiome that feeds us. Milk is food and passive protection, but it's also a signalling mechanism that drives development along the lines the genes dictate.

Changes to gene expression can be inherited: increased obesity risk, for example, can be built in before birth. But it isn't fixed forever — expression can revert to baseline over generations if conditions change.

Formula-fed babies still receive at least two to three times the protein intake of breastfed infants, even as manufacturers reduce levels in response to the known link between high cows' milk protein and obesity.

Common questions