The Importance of Mother’s Milk 
Among its other significant ingredients, human milk contains a hormone, 
known as epidermal growth factor, that stimulates cell division 
by Graham Carpenter 
Reproduction of the species is con- 
sidered to be the driving force in the 
evolution of biological systems, and na- 
ture has a large investment in a new- 
born organism. This is particularly true 
of humans and other large mammals 
that have relatively long gestation peri- 
ods and usually produce only one or 
two offspring at each birth. Survival of 
these newborns is critical to the con- 
tinuance of each species, and for almost 
all mammals, milk is the material that 
provides total nourishment during the 
initial stages of life. The exception is the 
human species, which in the last several 
decades has in large numbers shifted 
from milk to substitutes. 
At the turn of the century approxi- 
mately 50 percent of all newborn babies 
in the United States were breast-fed for 
at least the first twelve months of life. 
Recent surveys indicate that during the 
1970s about one in three babies was 
nursed during the first month of life. 
Concurrently, the duration of breast- 
feeding declined rapidly, to the point 
that about 5 percent of all infants were 
nursed until six months of age and 1 
percent until twelve months of age. At 
present there is a shift among new 
mothers to return to breast-feeding, but 
this group is small in relation to the to- 
tal population. In today’s era, when in- 
formation is prized and decision 
making based on biological instinct is, 
perhaps unfortunately, frowned upon, 
it would «eem that data about such an 
important commodity as milk (and its 
commercial formula substitutes) should 
be of concern to both the public and the 
medical profession. 
Too frequently the assumption is 
made that “milk is milk,” and whether 
a newborn human drinks mother’s milk 
or cow’s milk is of no large conse- 
quence. A second assumption is that to- 
day’s technology is so advanced that 
artificial formulas can duplicate the 
quality of natural human milk. Both of 
these assumptions are incorrect. Com- 
parison of the composition of milk from 
various species shows that there exist 
significant qualitative and quantitative 
differences, which seem to reflect the 
varying needs of each species for proper 
development in early life. For example, 
compared to human milk, the milk of 
the bovine species has twice as much 
protein, which after digestion provides 
the newborn with a source of raw mate- 
rials, that is, amino acids, for muscle 
growth. This increased protein content 
reflects the needs of almost all newborn 
animals (excluding humans) to grow 
rapidly in order to avoid predators, be- 
come independent quickly, and survive. 
A newborn cow, for instance, will dou- 
ble its birth weight in about 50 days, 
whereas a newborn human requires 1 80 
days. Because human milk is relatively 
low in protein does not mean that it is 
insufficient in that respect. Human in- 
fants do not need to add bulk as rapidly 
as other newborn animals, but have dif- 
ferent needs, including maintaining a 
higher rate of brain growth. 
Myelination — the surrounding of 
nerve axons with a lipoprotein mem- 
brane, or sheath, necessary for the 
proper conduction of nerve impulses— 
is an important process of human brain 
growth in the first year of life. Myelina- 
tion requires substantial quantities of 
lipid, and human milk has a relatively 
high lipid content. Lipid intake is also 
important for the baby of lower weight 
who, to maintain a proper body tem- 
perature, must produce fat for insula- 
tion. 
Comparative analyses of milk com- 
positions provide many examples such 
as these and quite strongly demonstrate 
that each species has during its evolu- 
tion devised a milk composition that is 
optimal for its specific needs. The high 
degree of sophistication of the biologi- 
cal mechanisms that control the com- 
position of milk is demonstrated by the 
capacity of mothers of preterm babies 
to produce a milk with a higher content 
of protein and lipid than is found in the 
milk of mothers of full-term babies. Na- 
ture apparently provides the preterm, 
low-weight infant with extra protein to 
increase its body mass, or weight, and 
with extra lipid to stabilize body tem- 
perature and to provide more fuel for 
energy production. 
Most of our present knowledge of 
milk composition concerns those com- 
ponents that are present in milk in the 
largest amounts — macronutrients such 
as protein, carbohydrate, and lipid. 
There is also a reasonable body of data 
describing the levels and different types 
of minerals and vitamins. Milk, howev- « 
er, is more complex than is currently 
understood, and there are increasing 
feelings in the scientific community 
that, in addition to satisfying nutrition- 
al needs, milk may provide other, subtle 
but important factors for growth and 
development in the newborn. For ex- 
ample, the antibodies passed through 
milk from mother to child help the 
newborn, whose ability to produce anti- 
bodies has not fully developed, resist 
microbial infections. Breast-fed babies 
do, in fact, suffer fewer infections, par- 
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