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Food Goes In, Poo Goes Out: Your Infant’s Digestive System

infant digestion, open gut, diarrhea, constipation, amylase, IgA, digestive development, breast milk, intestinal flora

Four months ago I began the daunting task of investigating infants and food introduction. I wanted to be prepared when my little girl was ready to start solid foods. I read books, articles, blogs and studies. When she turned five and half months I felt ready to take on the task of slowly introducing solid foods. I was wrong.

What I failed to do during my initial research was start with the right questions. The first question asked by most first time parents, including myself, is “when do I introduce solid foods?” The question that soon follows is, “what foods should I introduce first?” I am not saying these are the “wrong” questions, but there are questions that must be asked and answered before even discussing solid food introduction. I soon discovered that the two first questions parent’s should really ask is: “how does my infant’s immature digestive system effect her health?” and “what are my infant’s nutritional needs?” To start this series of articles, we are going to explore the basics of an infant’s digestive system.

The Digestive System

The digestive system is a symphony of organs and body parts working together to breakdown food into the necessary energy and nutrients needed for infants to thrive. The mouth, tongue, gums or teeth, esophagus, stomach, small intestine, large intestine, colon, pancreas, liver and gallbladder are all part of the digestive system. The delicate environment within these organs also involves microflora, enzymes, bile salt and hormones. And we cannot forget that the digestive system interacts with the nervous, endocrine and immune systems. The evolution of the unique digestive system is one that takes years and even then is constantly changing based on lifestyles, environment and general health.

New Beginnings: Fetal digestive development

Approximately 22 days after conception the digestive system rapidly forms, starting with the “primitive gut.” Within a handful of days, the primitive gut forms into three sections: foregut, midgut and hindgut. These sections quickly develop into the esophagus, stomach, liver, pancreas, and small and large intestines. By 20 weeks, the major components of the digestive system are formed, but it is far from complete.

A fetus’s digestive tract is sterile until birth. This means there are no microflora, or bacteria, present in a fetus’s digestive tract. Furthermore, a fetus begins suckling and swallowing around 16 weeks, but does not become more efficient at these reflexes until approximately 33 weeks. Digestive motility, intestinal mucous, hormones and enzymes are also immature even in a full-term fetus. All of this affects the efficiency of the digestive system at birth and contributes to the vulnerability of infants.

At Birth: Microflora

Infants developing a healthy microflora is vital to their overall health. Microflora contributes to the stimulation of tissue growth, the breakdown and absorption of nutrients, the development of a mucous lining, the production of certain vitamins (vitamin K and B12), and help fight off unwanted microbes, or germs.

The microflora , or bacteria, in the gastrointestinal tract (GI tract) get their beginnings at birth. When an infant is born, he is exposed to a plethora of germs and bacteria. Some of them find their way into the infant’s mouth and eventually take up residences in the GI tract. Although the types and amounts of microflora vary from infant to infant, some generalizations can be made.

The predominant microflora in infants’ oral cavity, or mouth, is Streptococcus salivarius . When teeth begin to appear, other microflora begin to take up residence in the oral cavity, as well.

For breast-fed infants, the predominant microflora in the intestines is Bifidobacteria . It is thought, that a carbohydrate in breast milk called Oligosaccharide promotes the growth of this particular bacteria. Whereas in formula-fed infants this carbohydrate is absent. Therefore, slowing down the growth of Bifidobacteria , making it less dominant. The significance of how this affects digestion and overall health is not completely understood. However, it is likely that an altered microflora may make infants susceptible to illness, allergies and prolonged “open gut,” which will be discussed further in the next section.

Other factors that shape infants’ microflora include: the type of birth – cesarean or vaginal, solid food consumption, environment, illness, diarrhea, constipation and antibiotics. Antibiotics, for example, kill bacteria and alter the balance of an infant’s gastrointestinal environment.

Open Gut Theory

One theory that is often used to explain an infant’s immature digestive system and the importance of breast milk is the Open Gut Theory. Unlike an adult’s intestinal wall, it is theorized that an infant’s intestinal wall cells are spaced farther apart. Hence the “open gut.” These gaps between the cells might allow larger molecules to pass through, such as antibodies present in breast milk and pathogens.

Also, the mucous lining that is normally a barrier for the intestinal wall is either absent or inadequate in infants. Thus further increasing the permeability. Fortunately, colostrum and breast milk appear to play a significant role in providing a protective barrier to infants’ intestinal wall. This gives the digestive system time to mature.

The timing in which infants’ intestinal wall “closes,” or becomes less permeable, is unknown. Based on the available research, the gut may close some time between six and 12 months of age. What prompts the closure of the intestinal wall is also unknown. Again research indicates that genetics, solid food introduction and hormones, such as cortisol, may contribute to the maturation of the intestinal wall. Like so much information about an infant’s digestive tract, the Open Gut Theory is based on research using animals, cell cultures and at times questionable methodology. Therefore, this information should be taken with “a grain of salt.”

Staying Healthy: Immunogloblin

The digestive system and immune system interact closely, and are both immature in infants. One way in which their immune system interacts with the digestive system is through immunogloblins , or antibodies. In normal adults, immunogloblins are found in abundance throughout the gastrointestinal and respiratory tracts. In infants, on the other hand, immunogloblin A (IgA) and immunoglobulin M (IgM) are produced at very low levels. This makes infants susceptible to illness of the digestive and respiratory systems.

Infants will not produce IgA and IgM in adequate amounts until 2 months of age and 2 years of age, respectively. Breast milk during this time provides infants with the IgA, IgM and other antibodies needed to boost the immune system and prevent digestive disruption among other ailments.

The Break Down: Enzymes and more

In general, the production, or activity of digestive enzymes and hormones are lower in infants when compared to adults. Digestive enzymes and hormones are responsible for the efficient break down and absorption of macronutrients: protein, carbohydrates and fat.

Proteolytic enzymes, such as pepsin, break down proteins and are at low levels until an infant is around two months old. Trypsin, another protein enzyme is near normal, or adult, levels; but because pancreatic proteolytic hormones are low its activity is limited, as well. Other enzymes include Chymotrypsin and Terokinase, which are produced at levels between 10 and 60 percent of adult levels. Regardless of these limitations in protein enzymes and hormones, full-term infants appear to efficiently digest and absorb proteins.

For the digestion of carbohydrates, amylase, disaccharidase and glucoamylase enzymes are produced. Salivary amylase levels are 30 percent of adult levels, and pancreatic amylase levels are less than five percent of adult levels. Most disaccharidase enzymes appear to be produced at adequate levels, with the exception of lactase. It is produced at levels two to four times higher than older children. This may be due to the high consumption of breast milk or formula, which are high in lactose. Lastly, glucoamylase is produced at approximately 50 to 100 percent of adult levels. Fortunately, mammary amylase, or amylase present in breast milk, is able to help compensate for a decrease in carbohydrate enzymes and help with the digestion and absorption of breast milk.

Pancreatic lipase enzyme, which breaks down fats, is also deficient in infants. It is produced at levels 10 to 20 percent of adult levels. Salivary and gastric lipases may help to breakdown fat within the stomach and compensate for low pancreatic lipase. An enzyme within breast milk - mammary lipase – assists with the digestion of fat as well.

Many factors, such as genetics, contribute to the rate in which enzyme production increases. It is, however, still unclear when enzyme production increases to normal, or adult, levels in infants. Current research suggests a healthy infant’s enzyme production will be equal or close to adult levels between the ages of 8 and 16 months.

Along with reduced enzyme and hormone production, bile salt secretion into the stomach and stomach acid are limited as well. Bile salts help with the digestion of fats and are half of adult levels. Since mammary lipase appears to remain active in the stomach, it may continue to help with fat digestion even in the midst of low bile salt production.

Stomach acid is inadequate at birth, causing a stomach pH that is either neutral (pH 7) or slightly alkaline. However, within a 24 hour period stomach acid production increases and acidity doubles by the time the infant is two months old. It is extremely important that stomach pH be acidic (pH of 1.5 to 3.5). The acidity helps kill food-borne pathogens, helps activate the pepsin enzyme and makes the stomach more efficient at breaking down food before it enters the small intestine.

Moving Along: GI motility and Reflexes

Esophageal, gastric and intestinal muscle tone is low, which decreases motility and slows down the digestion of food. Combine this with a small stomach and underdeveloped esophageal sphincter – a valve between the esophagus and stomach – and it leads to infants frequently spitting up, especially early on.

The suckling-swallowing reflex is an important reflex. Without it, infants cannot nurse and efficiently obtain the nutrients and calories they need for survival. Although infants begin practicing this reflex before birth, they normally still lack coordination and strength at birth. Therefore, as infants practice with frequent feedings, they become more efficient and stronger.

First to Last: Gums and Teeth

Infants are born without teeth, which is another element that contributes to the immature digestive system. As previously mentioned the microflora in the mouth diversifies as infants and toddlers get teeth, helping with food digestion. First molars tend to erupt in infants between the ages of 12 and 18 months. While all primary teeth normally erupt by the time a child is three years old.

Tooth eruption propels infants into the world of solid food. Prior to teeth, infants’ gums limit them to liquid, pureed foods, and soft, easily mashed foods. With teeth, infants, or toddlers, have the ability to efficiently chew food. Chewing is the body’s first mechanism of digestion called mastication. And as backwards as it may seem, the first mechanism for digestion is one of the last to mature.


I did not intend on writing an article on an infant’s digestive system. I merely wanted to make the best possible decision for my daughter when it came to her diet. However, this information I felt was valuable and needed to be more accessible to parents. Unfortunately, there is a lot of misleading information out there, and most parents do not have the time or energy to read hundreds of research articles and books. I wish I could say that this knowledge makes food introduction easier. It does not. However, having this information in what I hope is in a user-friendly format will allow parents, including myself, to ask the right questions and weigh the pros and cons before making any decisions on food introduction.

The digestive system is complex. The unknowns seem endless. The more we research and learn, the more we as a parents can provide the best diet for infants; and ultimately improve the health and wellbeing of our youth.

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