Infant Nutrition Chapter 4

Milk Enters the Small Intestine

The term metabolism comes from a Greek word meaning "to change." It is often used in a narrow sense, as a synonym for burning calories, especially in diet books that promise ways to speed up a person's metabolism. But metabolism actually refers to the entire range of biochemical processes involved in assembly and disassembly, transport and transformation, that are constantly taking place within any living organism.

The metabolism of a developing fetus is fueled in large part by glucose, though amino acids and lipids are also required for protein synthesis and tissue growth. At birth, this constant supply of glucose comes to an abrupt end. The newborn must now rely on breast milk or formula, which in turn triggers a series of changes in the infant's body. Fat becomes a critical energy source. The infant's body must balance its simultaneous needs for the biological building blocks necessary for tissue growth and the energy required to fuel this growth.

The small intestine is the site of most of the infant's absorption of nutrients. A newborn has lower levels of digestive enzyme activity than does an older child or adult, but the newborn is able to compensate for this shortfall by using other enzymatic pathways. This ensures that he or she is able to obtain all of the nutritional components necessary for healthy growth and development.


The recommended protein intake for infants from birth to 6 months is about 9.1 g per day (1.52 g per kg of body weight per day). From the age of 7 months to 12 months, it is about 11.0 g per day (1.2 g per kg of body weight per day). [Institute of Medicine, Dietary Reference Intakes.]

There are thousands of different proteins in the human body, but they are all combinations of fewer than 2 dozen common amino acids. Some of these amino acid building blocks can be made by tissues in the body from even smaller components. Nine of the amino acids, however, cannot be synthesized in the body and therefore must be obtained through the diet. These are called "essential amino acids." Developing fetuses, as well as preterm infants, require at least 2 other amino acids in their diet because their capacity to manufacture them is still undeveloped.

Proteins are required for the building, maintenance, and repair of tissues throughout the infant's body. They are also necessary as a source of building blocks for the synthesis of digestive enzymes, regulatory hormones, growth factors, and important components of the immune system such as antibodies. "Most people think of protein as just being muscle," says registered dietitian Julie Balay. "But protein is really the building block for all tissues. It's important for muscles, yes, but there are proteins circulating throughout our body, proteins that are part of our immune system. Our enzymes are actually little proteins that help us digest our food. Protein is really a critical component throughout our whole body."

Infants are able to digest proteins, though the range of dietary protein they encounter as infants is very limited. The process begins in the stomach, where the enzyme pepsin, with the help of acid, hydrolyzes, or breaks apart, large protein molecules. In the small intestine, enzymes such as trypsin and chymotrypsin enable these components of protein, small peptide fragments and amino acids, to be absorbed by the intestinal lining.

The predominant protein fractions in human breast milk are referred to as whey and casein and are present in mature milk at a ratio of about 60% whey to 40% casein. Both whey and casein proteins are important components of human milk and provide complementary amino acid profiles to meet the requirements of developing infants. In addition to providing building blocks, casein aids in the absorption of minerals in the infant's gut, and casein may also help promote the growth of beneficial bacteria and discourage the growth of pathogens. The protein content of colostrum is twice the level of protein in mature milk, but much of the additional protein in colostrum is in the form of immune factors such as antibodies and immunoglobulins, which help the infant resist infection.

A newborn's initial gastric pH is less acidic than that of an older child or adult and this reduces enzyme activity and the breakdown of proteins. The secretion of gastric acid in the infant's stomach begins to increase at birth and doubles by 2 months. Levels of many enzymes in the infant's digestive system are also lower, between 10% and 60% of adult levels, according to Susan Tucker Blackburn, in Maternal, Fetal, & Neonatal Physiology. Other enzymes, such as trypsin, are closer to adult levels, but their activity is initially reduced.


An infant's intake of carbohydrates from birth to 6 months is about 60 g per day. From the age of 7 months to 12 months this increases to about 95 g per day. Carbohydrates are important because they supply food energy for growth, body functions, and activity, as well as facilitate the efficient use of protein and fat in the diet. Carbohydrates are also building blocks for some essential body compounds. Nucleotides, for example, are compounds made up of a sugar (a simple carbohydrate) plus a nitrogen base (a component of protein) and phosphate group; nucleotides, in turn, are the building blocks of DNA and RNA, the genetic instructions that guide the cellular development, division and function of all living organisms. Free nucleotides are present in breast milk and they also play important roles in a baby's metabolism, including cellular signaling and important enzymatic reactions.

While the developing fetus depends on the blood glucose from the mother to supply its carbohydrate needs, a newborn relies primarily on lactose from breast milk. There are small amounts of other carbohydrates in breast milk, including indigestible components that promote the healthy development of beneficial intestinal bacteria that inhibit pathogens. Called human milk oligosaccharides (HMOs) or prebiotics (in formula versions), "a good way to picture the function of these compounds," says Professor Tom Brenna of Cornell University, "is to consider them as digestive fiber for the baby."

Adults digest carbohydrates with the help of salivary and pancreatic amylase, as well as carbohydrate-digesting enzymes secreted by the lining of the small intestine. At birth, however, infants have only a third of the amylase activity of adults, according to Blackburn, and levels of pancreatic amylase are less than 5% of adult levels. Salivary amylase, as well as mammary amylase from breast milk, help the infant compensate for the reduced activity of pancreatic enzymes. An infant is able to digest carbohydrates fairly easily, with the exception of complex carbohydrates a baby would have no reason to encounter until the introduction of cereals at about 6 months. And at that point, the baby has developed the digestive capacity to break down this nutrient.


An infant's intake of fat, or lipids, is approximately 31 g per day from birth to 6 months. From the age of 7 months to 12 months, with the introduction of complementary foods such as cereal, intake decreases to 30 g per day. Fatty acids are building blocks for lipids and can be used as a source of energy. There are two fatty acids in particular that are called "essential" because they cannot be manufactured in the body and therefore must be obtained through the diet.

About 50% of the calories in breast milk come from fat. The function of lipids is to provide energy; serve as insulation to reduce body heat loss and protect the infant from injury; facilitate the absorption of the fat-soluble vitamins A, D, E, and K; and provide fatty acids that are critical for normal brain and eye development, healthy skin and hair, and resistance to infection and disease.

Lipids are also critical components, both structurally and functionally, of cells in the brain. They are the main components of the myelin sheath, which facilitates the efficient transmission of impulses along nerve cells.

And although we generally associate the image of plump newborns with health, that chubbiness develops gradually. According to Blackburn, fetal fat content, as a percentage of body weight, increases from a miniscule 0.5% in early gestation to approximately 3.5% by 28 weeks and then to a pudgy 16% of body weight by term birth. The gain in body fat is a result of both the transfer of dietary fat components from the mother as well as the production of fat in the fetal liver and other tissues.

Fat digestion in adults relies on an enzyme also produced in the pancreas called lipase, and again, at birth an infant's lipase production is only a fraction of that in older children and adults. The activity of another key component of fat digestion, bile acid, is also considerably lower. Lipase found in breast milk helps close this gap, however, and enzyme activity quickly accelerates in healthy term infants. This allows babies to absorb up to 90% or more of fat, though infants born prematurely usually absorb only 70% or less.

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


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

The material on this site is for informational purposes only and is not intended as medical advice. It should not be used to diagnose or treat any medical condition. Consult a licensed medical professional for the diagnosis and treatment of all medical conditions and before starting a new diet or exercise program. If you have a medical emergency, call 911 immediately.