Editors note: My "scientist" hat is on for this blog post (don't worry, there are still a few pics at the end!) I have been doing a lot of research on when to feed Tess solid foods and thought I would share what I've learned. This is just information sharing - I am not trying to tell anyone how they should or should not raise their kiddos!
When I was pregnant I took a
“How to Breastfeed” course and the instructor urged everyone in the class to
breastfeed for at least 1 year. I
had read this in some articles and books too, but it was unclear to me
what advantages there were to breastfeeding for so long.
I took the opportunity to
ask this bona fide breastfeeding guru
teaching our class what biological advantages there were to breastfeeding for a
year, rather than say, 6 months.
She gave the usual soliloquy about fewer ear infections and digestive
problems, but I already knew the benefits of breastfeeding; I wanted to know the
benefits of long-term breastfeeding. She could not give me a straight answer.
More recently our pediatrician encouraged us to start feeding Tess solid foods,
specifically rice cereal. As of last year the American Academy of Pediatrics
recommends that infants be exclusively breastfed for at least 6 months.
Previously it had been 4-6 months. Again, being the inquisitive scientist that
I am, I wanted to know why. What change
occurs at 6 months that suddenly enables a baby to properly chew, swallow and
digest something other than breastmilk?
This time I sought out some
answers on my own.
As we all know, food consists
of proteins, fats, and carbohydrates
that provide our body with the necessary support it needs to function. Enzymes in the digestive system help
break down a piece of broccoli or a Big Mac into the basic building blocks the
body uses to operate.
WHERE’S THE PROTEIN?
Pepsin is an
enzyme produced in the stomach that helps breakdown dietary proteins and is not
fully developed until anywhere between 3 and 8 months old in full-term infants
– in premies expression is even lower.
Trypsin,
another enzyme used for protein degradation, is normal at birth but chymotrypsin and carboxypeptidase are both very low. Elastase does not reach adult levels until 2 years of age. At the very start of life infants use something
called chymosin to digest protein,
an enzyme that specifically breaks down the proteins found in milk.
While some enzymes turn on early and others later, infants can’t fully and
properly digest most dietary protein until they have (if not all then, at least) most of the necessary digestive enzymes.
CHEWING THE FAT
Fats are broken down in the
gut by another set of enzymes called lipases.
Each individual lipase is specially designed to break apart specific types of
fat. Surprisingly, babies do not make an enzyme that breaks down the fat found
in breastmilk. However, Mom’s mammary glands secrete a lipase into her
breastmilk that enables digestion of the specific kinds of fats contained in
that milk. Yet another reason why breastmilk is so amazing: this lipase only
becomes active at the pH present in the gut, so the fat isn’t digested until the
milk is in baby’s belly.
There are different kinds of
lipases produced in our digestive system: lingual, gastric, pancreatic, and
epithelial. Lingual and gastric
lipase are at adult levels at birth, whereas pancreatic lipase is very low. While lingual and gastric lipases can
largely make up for a lack of pancreatic lipase, fat absorption is still incomplete
without it.
BREAKING BREAD
Once infants are weaned,
their diet usually shifts from a very high-fat milk-based diet to a high-carb rice
cereal diet. Rice cereal is rich
in starch. Starches are complex
carbohydrates, long chains of sugar molecules. Breakdown of starch in the gut is a multi-phase process and amylase is responsible for the first
phase: breaking down complex carbs into disaccharides. In other words, amylase
breaks up the long chain of sugar molecules into pairs of individual sugar
molecules – “di” meaning two, “saccharide” meaning sugar.
Other enzymes, like maltase, isomaltase and sucrase break
down the disaccharides even further. Some sources say these enzymes exist at
birth, other sources say not until 7 months. There does not appear to be a
definitive conclusion on these enzymes.
As far as amylase goes, the
body produces one in the saliva and another in the pancreas (which is then secreted
to the small intestine). Even 16-week
old fetuses produce salivary amylase, but infants less than 6 months do not
have pancreatic amylase. Much like
elastase, pancreatic amylase does not reach adult levels until 2 years of age.
Some argue that the salivary
amylase that makes its way to the stomach after swallowing is sufficient to
breakdown starches. However, salivary amylase can’t digest all starches, only those
of a particular molecular structure, and is inactivated at low (acidic) pH. The
stomachs of newborns do have a more neutral pH than adults, so salivary amylase
could potentially be effective in the stomach of a newborn. As they age, however, the pH becomes
more acidic, which will inactivate salivary amylase. A baby beyond the newborn
stage but who has not started to make pancreatic amylase will not be able to
digest complex carbohydrates like those found in rice cereal.
Interestingly, the carbohydrates
found in breastmilk (called Human Milk Oligosaccharides) are not digestible by
the enzymes produced in an infant’s GI tract. Recent studies have shown that Human Milk Oligosaccharides
facilitate the formation of “good bacteria” in the gut called bifidobacteria. As it turns out, the bifidobacteria themselves produce the
enzyme needed to break down the carbohydrates in breastmilk. Not only that, but Human Milk
Oligosaccharides also prevent formation of infectious (aka bad) bacteria in the
gut and help development of the immune system. More on bifidobacteria
later…
WHAT’S THE BIG DEAL?
Hopefully I have convinced
you that digestive enzymes develop slowly in an infant and all aren’t fully
active in the first few months - or even first few years - of life. You still might be wondering why you
should care.
In some cases, introduction
of certain foods can trigger the body to start producing the enzymes needed to
digest that food. However, introducing a food that the body isn’t ready to
digest can also have deleterious consequences. For example, one study showed that infants who were
genetically susceptible to celiac disease (gluten intolerance) and fed
gluten-containing foods prior to 1 year of age developed the disease more often
than at-risk infants who were not given glutens until after 1 year. Early introduction of glutens promoted
gluten intolerance in this particular population.
Another study showed that
giving iron fortified foods (including formula) to infants less than 7 months
old reduces the efficiency of iron absorption later in life. At 1 year, babies who were exclusively
breastfed until at least 7 months had higher hemoglobin iron content than those
who received iron fortified foods prior to 7 months. Early introduction of iron
fortified foods impairs the efficiency of iron absorption later in life.
Finally, when food is not
properly digested it begins to ferment in the gut. Excessive fermentation of undigested carbohydrates, for
example, produces the metabolites acetic acid, propionic acid and butyric acid.
These can decrease the pH of the gut, inhibit bacterial enzymes necessary for
digestion, damage the colon, decrease absorption of fatty acids and cause
diarrhea.
Propionic acid also can
affect the brain… more on that later.
A LEAKY GUT AND AN INFLAMMED BRAIN
Infants have what is called
an “open” or “leaky” gut, which allows large macromolecules in the digestive
tract to easily pass into the bloodstream. In an adult, the epithelial cells
that line the wall of the gut form tight junctions that act as a barrier. In a
baby these junctions are a bit “loose” so macromolecules can leak out of the
gut between the epithelial cells and into the blood.
A leaky gut is a good thing
in an infant, since this is how Mom’s antibodies are able to get into their
system to protect them at such a vulnerable stage of their lives. At around 6 months, when babies
start to develop their own immune system, the gut begins to close.
Anything that is
disagreeable to a baby’s tummy can cause inflammation. Inflammation is how the body responds
to a foreign substance that shouldn’t be there, like bacteria or a virus. Research is beginning to show how damaging
inflammation is to our bodies, and we have only touched the tip of the
iceberg. Inflammatory factors like
cytokines and activated immune cells can easily pass into the circulatory
system when the gut is still “open,” where they can potentially get into any
organ in the body, including the brain.
Neuroinflammation – or
inflammatory factors that are present or active in the brain – is the hottest
new topic in neuroscience. There
is evidence of neuroinflammation in stroke, ALS, MS, cancer, tuberculosis,
psychiatric diseases – the list goes on. Of particular note to parents of developing kids is the
central inflammation evident in autism. While we are far from even understanding what autism really
is, let alone what causes it, one hypothesis is that the neuroimmune system is
critically involved in the development of the disease.
Cells in the brain called
microglia and astrocytes are activated during an immune response, as are certain
cytokines and chemokines, things like IL-6, IL-1, TNF-α, and IFN-γ). All of these are elevated in the brains
and spinal fluid of autistic individuals. Lipopolysaccharides, or LPS, known to
induce an immune response, are not only increased in those suffering from
autism, but the levels measured correlate with the severity of the
disease. In other words, more LPS means
the autistic symptoms are worse.
Animal studies support this
hypothesis as well. When
pro-inflammatory cytokines are injected into a mouse they can reduce social
exploration, increase repetitive movements and impair learning, behaviors often
seen in autistic humans. Signs of
neuroinflammation are also seen in a mouse genetically engineered to exhibit
“autistic-like behaviors” (again, reduced sociability with other mice and
increased repetitive movements).
Autism is also linked
specifically to inflammation that originates in the gut. There are reports of a greater
incidence of chronic constipation or diarrhea, abdominal pain, food allergies,
GERD, colitis and other gut disorders in autistic people. Treating the GI
disturbances in autistic patients with antibiotics can improve both their bowel
symptoms and the cognitive and behavioral symptoms of autism as well.
There are also biological
markers of enhanced gut inflammation associated with autism. Biopsies taken
from the large and small intestine of autistic individuals show elevated levels
of active immune cells compared to both healthy non-autistic individuals and even
those with bowel disorders, like Irritable Bowel Syndrome.
Pro-inflammatory cytokines,
like IL-4,-5,-12,-13; TNF-α and Th1 and -2 are also increased in autism
spectrum disorders. There are lower levels of bifidobacteria (the good bacteria increased by breastfeeding) in
the gut of people with autism. Similarly,
toxin-producing bacteria, Clostridia,
are increased in autistic patients.
Gut bacteria can also translocate from the gut into the blood stream,
and there is evidence of an increased concentration of these bacteria in the
circulatory system of severely autistic people.
There is some evidence to
support that a gluten and milk-free diet can improve not only the GI
disturbances but also the social, behavioral and cognitive symptoms of autism.
The brain is protected from
the rest of the body by something called the Blood-Brain Barrier, a collection
of specialized cells that prevent things in the blood from passing freely into
the brain. Not only can some key inflammatory
cytokines and immune cells cross this barrier, but they can also weaken the
barrier making it even easier for toxins and other inflammatory factors to get
into the brain.
Propionic (also called
propanoic) acid is one of the metabolites of undigested carbohydrates that
ferment in the gut. Propionic acid
easily passes in and out of the gut and across the blood brain barrier. Not only is propionic acid increased in
autistic patients, but when it is injected into the brains of rodents, it
induces neuroinflammation, impairs social behavior, impairs learning and
induces repetitive movements, all clinical manifestations of autism spectrum
disorders. In fact, it is so effective at inducing autistic symptoms that
propionic acid is used as a model of autism in rodents.
BREAST IS BEST
The good news on inflammation and the gut is that breastmilk contains
anti-inflammatory factors, like IL-10 and TGF-β, and suppresses formation of pro-inflammatory
factors. Breastmilk is heavy in
omega-3 fatty acids, which are also thought to be anti-inflammatory.
Nursing itself can also be
beneficial in counteracting the behavioral effects of inflammation. In rats treated with pro-inflammatory
factors known to induce autistic-like symptoms, those that received more time nursing
developed healthier social behaviors as adults than those that received less
nursing as pups.
DISCLAIMER
To be clear, I am NOT trying
to say feeding your baby rice cereal at 6 months or 4 months or even 4 weeks
causes autism!
There are many theories
about what causes autism, disruptions in the serotonin system, genetic
mutations, etc. Most studies do
not suggest a causal relationship between gut problems and the disease, only a
correlation. However, there is strong
evidence of a relationship between what’s going on in the GI tract and what is
going on in the brain – particularly in the case of autistic individuals.
In fact, there are many
studies across various disciplines that highlight the relationship between the
gut and the brain. This is a
burgeoning new area of neuroscience.
There is evidence that links gut bacteria to mood disorders, anxiety and
depression, for example. Gut
bacteria likely influence development of key brain areas involved in stress and
anxiety, and probiotics (good bacteria) can reportedly improve stress, anxiety
and depression.
Every day we learn more and
more: We are what we eat.
What I intended to do here
is share all of the information I have gathered in making my decision on when
and what to feed my own baby. For
me, the evidence says to hold off on feeding her solids until I am sure she is
ready to digest them. Furthermore,
continuing to breastfeed while introducing solids is going to be hugely
beneficial in counteracting any inflammation that may result from those new
foods. While it is always important to look after our gut health, in light of
new research on the link between the gut and the brain, it is particularly important during this critical period of brain development.
And these are just because I can't do a blog post without photos, or I'd lose my biggest fans...
Trying to show off her shiny new tooth.
Her way of trying to convince me to feed her some "real food?"
Her first taste of water. Unconvinced.
"This milk tastes funny, Mom."
Sitting in her big girl high chair at a restaurant.
She's all set for when I am ready to give her something other than breastmilk.
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