Infant Digestion

Caucasian baby girl playing with blocks on bed
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One of the many advantages to breastfeeding is the ease and protection of breast milk on your baby's gastrointestinal system. But what happens once the baby is latched on and feeding well? Let's learn about the anatomy of the gastrointestinal tract of a baby from top to bottom...

  • Mouth
  • Esophagus: This has two principal jobs--to push food or fluid from the mouth to the stomach and to stop backflow, or reflux, of the contents of the stomach.
  • Stomach: This is responsible for storing the swallowed food, combining and disintegrating the food, and regulating excretion of the stomach contents into the duodenum. Digestion takes place in three phases--cephalic (initiated by the vagus nerve when someone sees and smells any food), gastric (caused by the taking in of food and controlled by gastrin) and intestinal (regulated by hormones released in the small intestine).
  • Small intestine: This is a tube-like organ separated into 3 parts--the duodenum, jejunum, and ileum. It has a huge job to do as it is in charge of digestion and most of the absorption of nutrients, vitamins, trace elements, fluids and electrolytes. Essentially, the acidic partially digested food from the stomach is combined with the basic secretions from the pancreas, liver and intestinal glands. The digestive enzymes from those secretions are in charge of most of the digestive process in the small intestine--they break down proteins into amino acids; carbohydrates into glucose and other monosaccharides; and fats into glycerol and fatty acids. The intestinal wall must be very strong to handle the work it does. Its strength comes from the fact that it has 4 distinct layers--the serosa, muscularis, submucosa and muscosa¬†--and top of that, the surface of the intestine is increased considerably by the existence of villi and microvilli by which the final products of digestion are absorbed.
  • The Large intestine, or colon: This curves upward, across and down to the rectum. It is mostly in charge of the absorption of water and electrolytes.
  • Rectum: The "sphincter of O'Beirne" regulates the flow of waste from the sigmoid colon into the rectum, which is a holding area for the waste products of digestion. The internal and external anal sphincters regulate the flow of fecal matter from the rectum.

    There are also accessory organs of the gastrointestinal tract involved in digestion which sprout from the foregut and midgut. They are:

    • Salivary glands: Produce salivary enzymes. Not a lot of digestion or absorption occurs in the mouth. The submandibular, sublingual and parotid glands produce saliva which contains amylase, which begins the digestion of carbohydrates.
    • Liver: This is actually the largest organ in the body. It is in charge of protein and carbohydrate metabolism and the storage of glycogen and vitamins. It also aids in the formation, storage and elimination of bile and plays a role in fat metabolism.
    • Gallbladder: This organ is a tiny sac that rests on the bottom area of the liver. Bile (which consists of salts that are essential for the digestion and absorption of fats) from the liver is collected here. The "sphincter of Oddi" regulates the flow of bile into the duodenum. So, just like the liver, the gallbladder aids in the composition, storage and elimination of bile and plays a role in fat digestion.
    • Pancreas: This creates alkaline (or neutral) secretions which take part in offsetting the acidic partly digested food (also called chyme) from the stomach. These secretions hold enzymes that are essential to the creation of lipase, amylase and protease and plays a role in the absorption of fats, proteins, and carbohydrates.

      What are some of the roles of the GI tract on infant digestion?

      The science of digestion is amazing because it has so many different functions on food--It takes it in, propels it through the GI system, mechanically and chemically breaks it down, and absorbs the digested materials. After that, it eliminates the waste products.

      Are there actual anatomical differences in the GI system between infants and adults?

      Yes! Let's start with the differences in the head and neck. In the infant, the tongue is larger in relation to the oral cavity and they have extra fat pads on the sides of the tongue that help with sucking.

      Also, in an infant, the larynx, or voice box, is situated higher and the epiglottis lies over the soft palate to supply extra airway protection.

      In a newborn baby, the esophagus is about 11 centimeters long (versus 9 1/2 inches long in adults) and the lower esophageal sphincter is around 1 centimeter. Quite often at birth, a thin suction tube is passed through the esophagus to guarantee that it is open. Esophageal defects can be atresias and fistulas.

      Now, the newborn stomach can only hold about 60 to 90 milliliters of fluid (versus about 14 cups in adults!) The digestive activity of the stomach is the same in both babies and adults--The gastric glands of the stomach include parietal cells, which produce hydrochloric acid and intrinsic factor. The chief cells in these glands secrete pepsinogen, which is changed into pepsin, breaking down proteins in the gastric juice. Amazingly, bowel sounds are already existent one hour after birth and the parietal cells start to work directly after birth. The gastric pH is less than 4 for the first 7 to 10 days of life.

      There are anatomical differences in the small intestine as well--it measures 250 to 300 centimeters in newborns and 600 to 800 centimeters in adults.

      And how about the colon? It is sterile at birth, but within a few hours E. Coli, Clostridium, and Streptococcus are established--the gathering of bacteria in the GI tract is essential for digestion and formation of Vitamin K.

      Finally, the first stools passed are called meconium. Meconium is thick, sticky and tarlike. It is black or dark green in color and made up of mucus, vernix, lanugo, hormones, and carbohydrates. It is extremely necessary that a newborn baby passes stool within 24 hours of birth.


      McCance KL and Huether SE. Pathophysiology: The Biologic Basis for Disease in Adults And Children. Mosby. 2005.

      Lawrence RA. Breastfeeding: A Guide for the Medical Profession. St. Louis: Mosby, Inc. 1999.

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