During fetal development, all nutrients for growth are supplied by the placenta, which also removes wastes from amniotic sac. At birth that changes in one fell swoop, when the newborn digestive and excretory systems must take over.

The infant's gastrointestinal tract is also one of the key "surfaces" that are newly exposed to the environment. We are not used to thinking of it in this way, but the lining of a baby's digestive system, in a fundamental way, is actually on the "outside" of the baby. It is the boundary and interface between food taken in from the environment, whether that is breast milk or formula, and the organ systems this food will nourish. The gastrointestinal (GI) tract must therefore serve as a protective boundary against a wide variety of environmental microbes and other contaminants. It is a very dynamic but vulnerable surface.

And yet, the baby's stomach, and digestive system in general, is still immature. One significant difference is size, of course. A baby's stomach capacity is extremely small, which is why infants require frequent feeding to supply adequate amounts of nutrients for growth and development.

"It's an important point for new parents to keep in mind," says registered dietitian Julie Balay. "The big question that moms always ask is, 'Is my baby eating enough?' That's certainly the concern with newborns. And maybe even more so with breastfeeding, since there is no 'last drop' in a breast to gauge when a mom feels her infant has had enough. But it also extends to the second 6 months, when some solid foods and finger foods are being introduced."

The small size of the stomach and digestive system is another reason there is so much fat in breast milk. A gram of fat contains more than twice the caloric energy of a gram of either carbohydrate or protein. And that means the most efficient fuel for a small digestive system will include a lot of fat. "Most of what we eat is burned as fuel," says Professor Tom Brenna of Cornell University. "We use it for energy. We could track the components of a sandwich and after a week, two thirds of it would be gone, burned as energy. And even the components that are not burned for fuel are turned over as cells grow, die, and are replaced. After a month, there would be very little of that sandwich still in the body."

According to Susan Tucker Blackburn, in her book, Maternal, Fetal, & Neonatal Physiology, researchers have even calculated the overall energy bill for fetal development. In the second trimester of pregnancy, a woman needs an additional 300-340 kcal per day and up to 450 additional kcal per day in the third trimester to meet the energy and growth demands of the fetus. The total minimum energy cost of reproduction is 80,000 kcal and can be as high as 120,000 kcal for some women.

How much energy is that? In fast food terms, it's roughly equivalent to 100 double-cheeseburgers, 200 small orders of fries, and 90 hot fudge sundaes for good measure. And that's just the energy bill at birth. The energy needs of a 2-month-old, 11-lb baby girl is more than 500 kcal a day, or roughly the equivalent of a Big Mac.

The Immature Digestive System
An infant's digestive system, besides having a very limited capacity, is also immature. The infant's pancreas is not yet up to speed, which means that certain digestive enzymes are at much lower levels than they are in older children or adults. The infant compensates by relying on nonpancreatic enzymes such as those found in breast milk as well as in the baby's own salivary secretions. Another difference is that the esophageal valve, which is the opening at the top of the stomach, is not fully developed. That's why it is common for babies to have problems with spitting up.

Although infants can digest proteins, fats, and carbohydrates at birth, their ability to do so improves as they develop and the GI tract lining matures while levels of digestive enzymes and stomach acids increase. Meanwhile, the limitations of their immature digestive and renal systems can put them at risk for dehydration, malabsorption of nutrients, and electrolyte imbalance.

The infant's protective mucosal barrier is immature at birth. As a consequence, pathogens and other macromolecules can potentially penetrate the intestinal epithelium and enter the infant's circulatory system. This can increase the infant's risk of infection as well as the development of allergies. "There are antibodies and other large immune molecules present in breast milk," says Brenna, "that can help support the baby's immune system." The main antibody providing protection in breast milk is called secretory IgA. As the infant's digestive system continues to develop, it will become much less permeable to large molecules. This coincides, however, with the baby's increasing ability to make his or her own antibodies.

The infant's gastrointestinal system matures with the help of many factors, including vitamins and minerals, hormones, and other components of the diet. Breast milk is rich in intestinal growth factors and other components that help establish the colonization of the GI tract by beneficial bacteria, which in turn help the intestinal lining mature as well as prevent the spread of pathogenic microbes.

It is no surprise that an infant's digestive system encounters a learning curve at birth. What is remarkable is the extent to which the fetus has been rehearsing for this debut. Even sucking and swallowing, the behavioral and motor skills most fundamental to the success of the baby's nutritional changeover, have been practiced while the infant was still developing in the womb.

More on this topic

Infant Nutrition (VIDEO)
Fueling Growth & Development
Milk Enters the Stomach
Milk Enters the Small Intestine
Some Key Nutrients
Gut & Immune Development
Respiratory Health
Skeletal & Muscular Development
Skin & Hair Growth
Cardiovascular Development
The Importance of Fat
DHA & ARA
Nervous System Development
Good Nutrition Builds Healthy Babies


Related Health Centers:

Infant Nutrition Health Center, Mother-Baby Bond Health Center, Mother’s Milk Health Center, Monthly Infant Development Calendar Health Center,Weekly Pregnancy Calendar Health Center