The Science Behind Infant Digestion

Moms who are breasfeeding need more water.
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A few of the many advantages to breastfeeding are the ease of feeding and the bonding that occurs. But what happens once the baby is latched on and feeding well? Each section of the digestive tract has specific functions that work in the transport and digestion of foods important for your baby's growth. The digestion of breast milk in your infant plays important functions ranging from the absorption of protective antibodies which fight bacteria and viruses to establishing healthy gut bacteria. What is the science behind infant digestion?

Anatomy and Physiology of the Infant Digestive Tract

Let's begin with looking at the anatomy of the infant digestive from the moment food enters the mouth until it passes into your baby's diaper and the functions which occur along the way. Accessory organs are extremely important for proper digestion and will be discussed below.

  • Mouth. Your baby's mouth plays the role of accepting food and is also the place where digestion of some nutrients begins. Some newborns may have difficulty with latching on or problems related to conditions such as cleft lip or cleft palate.
  • Esophagus. This esophagus is the tube that connects the mouth to the stomach and 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 stomach is responsible for storing the swallowed food, combining and disintegrating the food, and regulating excretion of the stomach contents into the duodenum, the first part of the small intestine. 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 small intestine is a tube-like organ separated into three parts—the duodenum, jejunum, and ileum. It has a huge job to do as it is in charge of the digestion and 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 breast milk proteins into amino acids; breast milk carbohydrates into glucose and other monosaccharides; and breast milk 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 four distinct layers—the serosa, muscularis, submucosa, and muscosa. 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. The colon curves upward from the end of the small intestine, across the abdomen 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.

Accessory Organs of the Infant Digestive Tract

In addition to the digestion tract itself, there are several accessory organs that are important in the digestion of food. These include:

  • Salivary glands. The salivary glands in the mouth produce salivary enzymes. The submandibular, sublingual and parotid glands produce saliva which contains amylase, an enzyme responsible for beginning the digestion of carbohydrates.
  • Liver. The liver 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. The liver is where toxins are captured and sometimes stored to protect the rest of the body.
  • Gallbladder. The gallbladder 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. Similar to the liver, the gallbladder aids in the composition, storage and elimination of bile and plays a role in fat digestion.
  • Pancreas. The pancreas 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 in the absorption of fats, proteins, and carbohydrates. While these digestive enzymes are made in the "exocrine" pancreas, many people are more familiar with the hormone insulin which is made in the "endocrine" glands of the pancreas.

Breast milk also contains enzymes which help in digestion, such as amylase, lipase, and protease. This is important in infants as digestive enzymes are not present at the levels found in adults until babies reach the age of six months.

Together, the parts of the digestive system work together to take in food, transport it to and through the GI system, mechanically and chemically break it down and absorb the nutrients, and then eliminate the excess material as waste.

Differences Between the Gastrointestinal System of Infants and Adults

There are several anatomical as well as functional differences between the digestive tract of infants and adults.

  • Head and neck differences. In the infant, the tongue is larger in relation to the oral cavity and extra fat pads are present on the sides of the tongue that help with sucking. In addition, the larynx, or voice box, is situated higher in infants than adults and the epiglottis lies over the soft palate to supply extra airway protection.
  • Esophageal differences. In a newborn baby, the esophagus is about 4 1/2 inches long (versus 9 1/2 inches long in adults) and the lower esophageal sphincter is around 1/2 inch in diameter. Quite often at birth, a thin suction tube is passed through the esophagus to guarantee that this sphincter is open. Esophageal defects that aren't uncommon include atresias (a condition in which the esophagus is completely closed) and fistulas (a condition in which there is a connection between the esophagus and another organ, such as the trachea).
  • Stomach differences. The newborn stomach can only hold between 1/4 and 1/2 cup 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.
  • Small intestine. There are anatomical differences in the small intestine as well. In the infant, it measures between 100 and 120 inches long and in the adult, from 240 to 315 inches.
  • Large intestine. The colon of an infant is sterile at first. Yet 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, a vitamin that is important in blood clotting. Since it takes a while for this to be produced after birth, infants are usually given a shot of vitamin K upon delivery.
  • Evacuation. 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 (the white cheesy substance present on a baby's skin), lanugo (the fine hairs present on a baby's skin, especially in premies), hormones, and carbohydrates. It is extremely necessary that a newborn baby passes stool within 24 hours of birth.

Healthy Gut Bacteria

In recent years we've been learning more about gut bacteria and their importance in everything from physical health to emotional well-being. Breastfeeding usually leads to colonization of the colon with the right balance of healthy bacteria.

Rather than being a job left up to enzymes alone in the digestive tract, we are learning that healthy gut bacteria are very important in the proper digestion of foods and resultant absorption of nutrients needed for growth and development. As we learn more about how the connection of the microbiome of the infant gut with breastfeeding, it's likely that current recommendations to breastfeed will become even stronger.

A Word From Verywell

A baby's digestive tract differs from an adult's in several ways and is a process involving many different organs and multiple steps. From providing digestive enzymes to establishing healthy gut bacteria, breast milk can get your baby off to a healthy start.

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