Leading health organisations such as the World Health Organization recommend babies be exclusively breastfed for the first 6 months, and then for breastfeeding to continue alongside suitable complementary foods for 2 years and beyond.
Indeed, breastfeeding is important for the health of mothers and their babies.
Babies who are not breastfed are more likely to acquire infections (e.g. gastrointestinal, respiratory) compared to breastfed babies.
Breastfeeding is not magic, though.
It doesn’t provide certainty that a baby won’t get sick.
However, if a breastfed baby was to get sick, the severity of the infection is often less (or greater if the baby is not breastfed).
So how does formula increase the risk of infection and why doesn’t breastfeeding provide total protection from illness?
Breastfeeding And Immunity
Here are 6 important, science-backed facts:
#1: Formula Lacks Many Important Factors That Help Fight Against Infection
Formula lacks many factors which are found in breastmilk (e.g. various proteins, fats, sugars, prebiotics, probiotics and cells) which help support a baby’s immune system and fight against infection.
It’s important to note that many factors in breastmilk work synergistically — not individually — to help protect against infection. Just like no single instrument in a symphony works on its own, various factors in breastmilk often work together to create concerted protection against infection. For example, secretory IgA (Immunoglobulin A) in breastmilk binds cell surfaces of some pathogens (e.g. bacteria or viruses), then lysozyme (another anti-infective factor in breastmilk) destroys the pathogen by disrupting its cell walls.
#2: Formula Lacks Secretory IgA
One of the most important anti-infective factors in breastmilk which is lacking in formula is an antibody called secretory IgA (sIgA). SIgA helps protect a baby from pathogens he is most likely to encounter in his environment. Hence, sIgA provides ‘targeted protection' from pathogens.
Here’s how sIgA gets into breastmilk: a type of cell called B lymphocytes (B cells for short) originate from lymphoid tissue. Lymphoid tissue is located at sites that are most vulnerable to infection (eg gastrointestinal and respiratory tracts).
Once B cells are sensitised by pathogens, they enter the blood and travel to a mother’s breasts. Once there, they secrete sIgA that enters breastmilk. When a baby consumes the breastmilk, the sIgA travels to the part of the baby’s body (depending on where it originated in the mother’s body). For example, if B cells originated in the mother’s gastrointestinal tract, then the sIgA will travel to the baby’s gastrointestinal tract and help protect that part of the body from infection.
The B cells themselves can also enter breastmilk so when the baby consumes breastmilk, the B cells can travel through the baby’s body and secrete sIgA where it might be needed.
#3: Formula Fed Babies Have Different Bacterial Species In Their Gut
It’s becoming clearer that a diverse and resilient gut microbiome is very important for our health. Formula fed babies have significantly different bacterial species colonising their gut compared to breastfed babies. Early disruption (e.g. due to even small amounts of formula) to the normal healthy colonisation pattern of an exclusive breastfed baby’s gut could affect long-term health.
Breastmilk also contains prebiotics (oligosaccharides) that the beneficial bacteria in the gut use as food. Oligosaccharides are the third most abundant substance in breastmilk. Lack of oligosaccharides hinders the growth of beneficial bacteria and also hinders their ability to compete with pathogens for colonisation in the gut.
#4: Formula Lacks Oligosaccharides
Many formulas don’t contain oligosaccharides, which the beneficial bacteria use as food. Even formula which has added prebiotics are from a non-human source, so they cannot function in the same way. The lower population of these beneficial bacteria results in a higher intestinal pH of formula fed babies, which also encourages the growth of pathogenic bacteria.
#5: Formula Fed Babies Have A Delayed Reduction In Gut Permeability
Gut permeability (leakiness) is high at birth and then declines rapidly (a process known as ‘‘gut closure’’). Formula-fed babies have a delayed reduction in gut permeability.
Increased gut permeability could mean increased chance for pathogens to get through the gut which in turn could increase the risk of infection.
The exact timing of gut closure is not known but before it does occur, exclusive breastfeeding is likely to be even more important.
#6: Breastfeeding Does Not Provide Total Protection
Breastfeeding is important and helps protect against infection, but it’s not magic.
When we get sick (or receive a vaccination), our bodies make IgG antibodies specific to that pathogen. This is called ‘active immunity’ and helps protect us in the long-term against illnesses.
A mother passes IgG antibodies onto her baby through the placenta before the baby is born (an example of passive immunity). As maternal IgG levels in a baby’s blood gradually disappear (around 6 months), a baby starts to make his own in response to pathogens in his environment. IgG antibodies in breastmilk are only in minute amounts – not enough to pass on active immunity to the baby.
There are several ways not breastfeeding increases the risk of illness in a baby. This is not to say that all babies who are not breastfed get sick, there’s just an increased risk. You can read here about other ways breastmilk is important for the immune system.