For many years we thought of our intestines – also known as our gut – as a pretty boring place. Our gut's responsibility was solely to digest food and absorb nutrients – an important but rather mundane conveyor belt of digestion.
But it turns out that our gut is actually a command and control center for our immunity, representing an estimated 70% of our immune system. It also turns out that breast milk protects our intestines, priming them to protect us at our most vulnerable age: infancy.
Let's start at the beginning, with birth. Our babies are born with guts ready to absorb information about the specific threats that are present in their new environment. They are very vulnerable in this state, unable to produce one key antibody called SIgA – a key crime fighting immunoglobulin – for about 30 days after birth.
Breast milk is loaded with SIgA. It accounts for 90% of all immunoglobulins in breast milk, and is present in amounts 12 times higher in colostrum than mature milk. In fact, it's present in breast milk at rates from 10 to 100 times higher than in blood.
When you breastfeed, your baby's gut becomes coated with SIgA. This creates a barrier against the transfer of bad bacteria into the baby's bloodstream – an ingenious means of fighting infection.
But it does something else, too. When a breastfeeding mother encounters a pathogen by breathing or ingesting it, her lungs or intestines send a message to her breasts to produce milk with SIgA specifically targeting those pathogens. When the milk enters the baby's intestines SIgA recognizes and specifically blocks those pathogens from infecting the baby. In this way, SIgA helps the gut provide targeted protection against threats in our infants' specific environments.
Another component of breast milk, oligosaccharides, play a key role in the gut's immune function. These sugars were once thought to be a “filler” in breast milk because much is not absorbed by the baby. We now know that the hundreds of varieties of oligosaccharides in breast milk also coat the baby's intestines and fight the entry of bacteria. (An added bonus: they also contain sialic acid, which promotes brain development.)
But oligosccharides have yet another superpower. They are a prebiotic, feeding important probiotic bacteria in the baby's system and promoting healthy gut colonization. They play an important role in establishing the baby's intestinal microbiome – the community of bacteria that supports immune function and overall health. Oligosaccharides support, for example, lactobacillus bifidus – a healthy bacteria that creates an acid level inhospitable to harmful bacteria.
Our microbiome has such a broad influence on our health that it has been called a separate organ. And breast milk has a significant impact on the baby's microbiome as an estimated 30% of the beneficial bacteria in a baby's intestines come directly from breast milk!
Far from a boring assembly line processing food into energy, we now know that our intestines, and the complex microbiome that inhabits them, are key to our health and protection from disease. And, we also know that breast milk plays a key role in making our babies' guts healthy and strong.