
They can be classified in several categories, according
to their actions. Since many of these bioactive sub-
stances present multiple roles and act synergistically
with each other, a particular component may be pre-
sent in more than one category. A broad category is
that of the defense agents, comprising cellular elem-
ents (leukocytes) and a variety of heterogeneous sol-
uble compounds, which may act directly or indirectly
against pathogens (antiinfectious agents) and/or alter
manifestations of infection due to their antiinflam-
matory properties (antiinflammatory factors), and
modulation of the immune responses (immunomodu-
lators). Other categories of beneficial compounds
present in human milk are growth factors and hor-
mones, which promote growth and maturation of
organ systems; binding proteins for hormones, vita-
mins, and minerals, which protect and increase the
bioavailability of the ligands; and several enzymes,
which ensure a better utilization of nutrients and act
as defense agents.
Nutritional Components of Human Milk
Protein
0006 Proteins are present in human milk as whey proteins
(50–90% of total), casein (10–50%) and milk fat
globule membrane proteins (1–3%). Whey proteins
include a-lactalbumin, the major protein in this frac-
tion (10–20% of total milk protein), serum albumin,
immunoglobulins, and lactoferrin. Casein is mainly
composed of two proteins: b-casein (85%), with dif-
ferent phosphorylated forms, and k-casein (15%), a
highly glycolysylated protein containing 40–60%
carbohydrate residues (hexoses, hexosamines, and
sialic acid). Casein micelles are formed by aggrega-
tion of caseins, calcium phosphate, and other ions,
depending on both hydrophobic interactions and
electrostatic binding. Human milk casein micelles
are considerably smaller (30–75 nm in diameter)
than bovine casein micelles (about 50–600 nm, mean
120 nm).
0007 The total protein concentration in milk is high
during early lactation (colostrum) and gradually de-
clines to a low level in mature milk (Table 2). Protein
concentration during early lactation is usually higher
in preterm than in term milk. There are pronounced
changes in the protein composition of milk as lacta-
tion progresses. Whey proteins decrease while casein
increases in milk during the first few days of lactation.
The ratio of whey proteins to casein is high (about
90:10) in colostrum and declines to 50:50 in mature
milk. The pattern in caseins is different for premature
milk, colostrum, and mature milk. In general, k-
casein is detectable in human milk only 3–4 days
postpartum and may not be detectable in preterm
colostrum. b-Casein concentration is higher in colos-
trum and mature milk of mothers with term com-
pared to preterm infants.
0008Milk proteins are a source of peptides, amino acids,
and nitrogen for the growing infant. In addition, milk
proteins serve several nonnutritive roles. Proteins in
human milk are not completely nutritionally available
to the infant. Proteins with nonnutrient functions must
remain intact in the infant gastrointestinal tract and
may be largely unavailable for nitrogen metabolism,
particularly during early lactation. However, at
6 weeks postpartum, 95% of breast milk protein is
considered nutritionally available in term infants.
Casein in human milk is an easily digested protein
which provides amino acids, calcium, and phosphorus
to the newborn. Casein phosphopeptides in human
milk have been shown to keep calcium (and probably
also bound iron, zinc, copper, and manganese) in sol-
uble form in the infant intestinal tract, thus favoring
absorption. a-Lactalbumin in human milk has a very
high nutritional value (protein quality) and its amino
acid composition is similar to the estimated amino acid
requirements of young infants.
Nonprotein nitrogen
0009The nonprotein nitrogen (NPN) fraction of human
milk comprises 20–25% of the total nitrogen and
remains relatively constant during lactation. This
fraction consists of more than 200 compounds,
including free amino acids, urea, uric acid, ammonia,
carnitine, choline, taurine, amino sugars, amino alco-
hols, nucleic acids, nucleotides, and polyamines.
Many of these components, such as taurine, choline
and nucleotides, are considered conditionally essen-
tial for the young infant. For example, human milk is
a source of purine and pyrimidine bases as well as
preformed nucleotides that are required by young
infants due to their high need of nucleic acid forma-
tion but limited capacity for de novo synthesis of
nucleotides.
0010Components of human milk NPN that contribute
to infant protein metabolism include some peptides,
free amino acids and, mainly, urea. Urea nitrogen is a
significant part (30–50%) of total NPN in human
milk, and it may contribute to infant protein synthe-
sis. Free amino acids represent 3–5% of the total
amino acids in human milk with higher levels in
colostrum than in mature milk. Glutamate/glutamine
and the nonprotein sulfonic acid, taurine, are the
predominant free amino acids in mature human
milk. Levels of urea and free amino acids in human
milk seem to reflect protein quality and protein intake
in the maternal diet. Increased urea and free amino
acid levels have been observed in women receiving
LACTATION/Human Milk: Composition and Nutritional Value 3451