It is well known from common observation that
milk, when it serves as the sole food of the infant,
serves to keep it growing normally and in good
health over a long period. There has occasionally-
arisen a discussion as to whether milk is a suitable
food for the adult, and as to whether it is the " ideal"
food. Milk, like the cereal grains and most other
natural foods, contains all the essential food elements,
and human experience teaches us that the proportions
in which they occur in this product are much more
satisfactory than in many other natural foods. An-
imals grow well on milk, but it is not easy to find
even complex food mixtures of vegetable foods which
will support optimum nutrition in the omnivora
during growth.

Milk is deficient in iron, as is shown by chemical
analysis. It has long been known that there is de-
posited in the spleen of the new-born animal a reserve
supply of iron, which ordinarily suffices to tide it
over the suckling period. Ordinary drinking water
almost always contains small amounts of iron, and
this doubtless aids in some degree in preventing iron
starvation in the infant.



That milk is a complete food, capable of supplying
all the nutrients necessary for the prolonged main-
tenance of growth, health and the ability to produce
and rear young, was shown by an experiment con-
ducted by the author at the Wisconsin Experiment
Station. A female pig was removed from its mother,
which was still nursing it at the weight of 17 pounds.
She had doubtless eaten of the mother's ration to
some extent but her principle food had been her
mother's milk. After removal from the mother,
this pig was confined in a pen having a board floor,
and was fed nothing but milk during a period of 17
months. During the first few months only whole
milk was fed, but later it was necessary to replace
this in part by skim milk. The animal weighed 406
pounds at the age of thirteen months. At this age
she produced eight living and two dead pigs, and
successfully brought the young to an average weight
of seventeen pounds. She had access to wood shav-
ings, and ate some of them. There can be no doubt
that the milk which she consumed was enriched to
some extent with iron by being in contact with cans
having part of the surface free from tin. City drink-
ing water was also furnished and this contained
appreciable amounts of iron. The animal must have
been able to conserve its limited iron supply in a
very efficient manner.

Milk is, therefore, capable of nourishing the pig
during many months, with no other modification
or additions than small amounts of iron. That it is


best to select milk as a monotonous and restricted
diet during adult life, no one familiar with the prin-
ciples of nutrition would maintain. Milk is, however,
without doubt our most important food-stuff. This
U true, because the composition of milk is such that
when used in combination with other food-stuffs of
either animal or vegetable origin, it corrects their dietary
deficiencies. Combinations of equal weights of milk
and one of the cereal grains give excellent results
in the nutrition of animals during growth, and grain
mixtures supplemented with milk support well in
adult life the function of reproduction and rearing
of young. This is because of the excellent quality
of its proteins, the peculiar composition of its in-
organic content and the remarkable content of the
dietary essential, fat-soluble A, in the fats of milk.
Milk, like nearly all of the other natural foods, con-
tains a great abundance of the second dietary essen-
tial of unknown chemical nature, watei^soluble B.

The extraordinary value of the proteins of milk
has been abundantly demonstrated by experiment.
McCollum * conducted a series of experiments with
growing pigs to determine the extent to which they
could retain the protein of the food for the construc-
tion of new body protein. The pig was selected be-
cause it is necessary in such studies to work with
an animal whose growth impulse is as great as pos-
sible. Only with such species is it to be expected
that the animal will utilize the proteins for growth
to the maximum extent made possible by the chem-


ical character of the food protein. The human
infant has but little growth impulse because its
period of infancy is long and the adult size not great
as compared with the size at birth. A comparison
of the human infant with the rat and the young pig
(swine) in their capacity to grow in early life is of
interest. The human infant weighs not far from
seven pounds at birth, and during the first year of
life is ordinarily able to multiply its initial body
weight by three, for the average weight at one year
is about twenty-one pounds. We may feed it human
milk the entire time, or unmodified cows' milk during
the greater part of the year, without in any important
degree modifying its rate of growth. In the latter
case, we should be supplying it with perhaps double
the amount of protein that it would receive were it
fed human milk, since the latter contains on an aver-
age about 1.6 per cent and the former about 3.5 per
cent of protein.

In marked contrast to the feeble capacity of the
human infant to store new tissue and increase in
size, stand the rat and the pig. The rat at birth
weighs about 4.83 grams, and contains about 0.064
grams of nitrogen. At 280 days of age the male
should weigh about 280 grams, and if moderately
fat will contain about 8.5 grams of nitrogen. The
rat is able, therefore, in a period of 280 days to
multiply its initial body weight by about 55,
and its initial body nitrogen content (protein) by


The newborn pig weighing two pounds will con-
tain about 134 grams of dry matter and 11.9 grams
of nitrogen. In 280 days it may, if properly fed,
reach a weight of 300 pounds. It would then have
a nitrogen content of not less than 2407 grams.
These changes in size entail a multiplication of the
initial body weight by 150 and of the initial body
nitrogen content by 202. The farm pig is ap-
parently the most rapidly growing species of land

Such considerations determined the selection of the
pig as a subject for the test of the biological value
of the proteins of the various natural food-stuffs.
The plan involved keeping the animal for a period
of several days on a diet free from protein, but con-
taining sufficient starch to cover the energy require-
ments. When the nitrogen elimination in the urine
reached a constant low level which represented the
irreducible minimum, resulting from the "wear and
tear" of the tissues, the animal was fed a diet con-
taining protein derived solely from a single grain,
or other single food-stuff. A record was kept of the
intake of the element nitrogen, and of the daily loss
of this element through the excreta, and from these
records the percentage retained for growth was ob-
tained. Similar experiments were carried out using
milk as the sole source of protein. The following
table summarizes the results obtained.


Per Cent op Ingested Protein Retained for Growth by

the Pig

Per cent of ingested
Source of protein protein retained

Corn 20.0

Wheat 23.0

Oats (rolled) 26.0

Milk 63.0

The figures in the table are averages of a consider-
able number of results and represent the general
trend of the data. The experimental periods varied
from 30 to 60 days. There can be no doubt that the
proteins of milk are far superior to those of any foods
derived from vegetable sources.

The problem of determining the relative values of
the proteins of the different foods when fed singly, sup-
plemented with purified food additions that their diet-
ary deficiencies were made good, was approached in
a different way by McCollum and Simmonds. 2 Rats
were fed diets in which the protein was all furnished
by a single natural food-stuff, but the plane of protein
intake was varied from very low to higher intakes, in
older to determine what was the lowest per cent of
protein in the food mixture which would just suffice
to maintain an animal without loss of body weight.
The rations consisted of the following substances :

Seed Amount to give the protein intake desired

Growth-promoting fat (butter-fat) 5.0 per cent

Suitably constituted salt mixtures 3to5 per cent

Agar-agar (to furnish indigestible matter) 2.0 per cent

Dextrin To make 100 per eent.


The results showed that there are indeed very great
differences in the amounts of protein from different
seeds, which are necessary to maintain an animal
without loss of body weight. The results for the
more important seeds used as human foods are
summarized in the following table.

Table Showing the Lowest Plane op Protein Intake De-
rived from a Single Seed Which Just Suffices to Main-
tain an Animal in Body Weight, when the Factors
Other than Protein are Properly Constituted

Source of Protein Plane of Protein Necessary for Maintenance

Milk 3.0 per cent of food mixture 3

Oat (rolled) 4.5 " " " "

Millet seed 4.5 " " " "

Maize 6.0 " " " "

Wheat 6.0 " " " "

Polished rice 6.0 " " " "

Flaxseed 8.0 " " " "

Navy bean 12.0 " " " "

Pea 12.0 " " " "

These maintenance experiments were of three to six months'

The data obtained with the pig is seen to be in
harmony in a general way with those obtained with
the rat, and help to substantiate the view which is
supported by all the evidence available, viz: that
from the chemical standpoint, the dietaxy require-
ments of one species of animal are the same as those
of another. That there are great differences in the
physical characters of the diet which suffice for,


or are required by certain species as contrasted with
others, is a matter of common observation. The
ruminants actually require coarse herbage as a part
of their food, in order that the alimentary tract may
function properly, whereas such physical properties
in the diet of the omnivora are wholly out of place
beyond very limited amounts.

In considering the value of milk as a constituent of
the diet it should be borne in mind that with respect
to the protein factor it may enhance the value of the
proteins of the remainder of the food. It may supply
in relative abundance those amino-acids which are
present in such small amounts that they form the
first, second, etc., limiting factors in determining the
value of the protein for growth or maintenance, as
well as by the direct addition to the food mixture, of
the intrinsically good proteins of the milk.

When taken as the sole food supply by the adult,
milk is very liable to produce constipation and be-
cause of its high protein content, may lead to the ex-
cessive development of putrefactive bacteria in the
intestine. The cages of rats fed solely on milk de-
velop an offensive odor. The addition of carbohy-
drate, such as starch or certain of the sugars, tend
to cause the disappearance of the obnoxious flora from
the alimentary tract, and the development, instead,
of types which do not produce injurious decomposi-
tion products in their action on proteins.

Meats. — The muscle tissue of an animal con-
sists of highly specialized tissue whose chief function


is to produce mechanical work through contraction.
It is in addition a storage organ in which glycogen,
a form of starch, and also fats are stored as reserve
foods. It contains but little of cellular structures in
the sense that the glandular organs, such as the
liver, kidney, pancreas, etc, do. Chemical analysis
shows the muscle to consist, aside from the reserve
food-stuffs, principally of water, protein and salts.
The glandular organs yield a high content of nucleic
acid, while the muscle tissue yields but little in pro-
portion to its weight. The inorganic content of the
muscle tissue resembles that of the seed of the plant,
rather than the leaf both in amount and in the rel-
ative proportions among the elements.

Corresponding with the specialized function, and
the peculiarities in composition just mentioned, we
find that its dietary properties are comparable
with the seed rather than the leaf. In fact, muscle
tissue differs markedly from the seed in only one
respect, when considered as a food-stuff, viz., in the
quality of its proteins. These are distinctly better
than those of the seeds with which investigations
have been conducted. The inorganic content must
be supplemented by the same inorganic additions as
the seed, and the muscle proves to be relatively poor
in its content of the unidentified dietary essential
fat-soluble A, as compared with such foods as milk,
egg yolk and the leaves of plants.

Since the inorganic part of muscle resembles that
of the seed, except that the latter is poorer in iron


and it is low in its content of fat-soluble A, it does
not supplement the seeds in an appreciable degree
other than with respect to the protein factor. It
follows, therefore, that we should not expect to se-
cure growth and normal nutrition with mixtures of
seeds, and meat and experimental trials demonstrate
that this is the case. Mixtures of meat (muscle)
and seeds require to be supplemented with respect
to sodium, calcium and chlorine, just as do seed mix-
tures alone 4 . The fat-soluble A content of such
mixtures, unless millet seed is one of the seeds pres-
ent to the extent of 25 per cent, must be increased
by suitable additions before the optimum nutrition
can be attained, and the animals can successfully
bear the strain of reproduction and lactation. Meats
are, therefore, but partial supplementary foods when
employed with the seeds or the products prepared
from seeds, such as wheat flour, corn meal, polished
rice, etc. Such diets can be partially corrected by
the liberal use of leafy vegetables, but better by the
use of the latter along with milk.

The pronounced deficiencies of muscle tissue as a
food-stuff, naturally suggests the question of the
reason for the success of the nutrition of the strictly
carnivorous animals. The explanation is found in
the order in which such creatures select the parts of
the carcasses of their prey. The larger carnivorse,
after striking down an animal, immediately open the
large veins of the neck and suck blood as long as it
flows. Their second choice of tissues is the liver, and


following this the other glandular organs. Muscle
tissue is only eaten after these have been consumed.
With such a selection the animal secures eveiything
which it needs for its nutrition except a sufficient
amount of calcium, and this is obtained through
gnawing off the softer parts of the extremities of the
bones. The failure of many carnivora to thrive when
confined in zoos, it probably the result of their being
fed too largely upon muscle tissue and bones. They
should be supplied with an abundance of the gland-
ular organs and with blood to make their diet com-
plete. With rats McCollum, Simmonds and Parsons
have observed fairly satisfactory growth on equal
parts of muscle tissue (round steak) and dried
blood, whereas either of these alone cannot induce
growth. 4

The Glandular Organs. — The liver and kidney
may serve as typical examples of the glandular or-
gans which are employed as foods. There are cer-
tain organs of internal secretion, such as the thy-
roid, and suprarenal glands which elaborate products
which are highly active pharmacological agents, and
the liberal use of these glands as food would lead to
disastrous consequences. The glands contain but lit-
tle of the inorganic elements in which the seeds are
deficient. Their proteins are probably of excellent
quality, but have not yet been carefully investigated.
The glands consist largely of actively functioning
cells, having specialized functions, and accordingly
they prove to contain a more liberal amount of both


the fat-soluble A and water-soluble B than does the
muscle. In respect to the former of the unidentified
dietary essentials the glandular organs surpass the
seeds in value. 4 From this description it will be
seen that the glandular organs approximate more
closely complete foods than does the muscle, but
it is likewise apparent that these tissues do not
form efficient supplements for the seeds and their

Eggs. — The egg contains all the chemical com-
plexes necessary for the formation of the chick dur-
ing incubation. The egg is therefore to be expected
to furnish everything which is needed for the nutri-
tion of a mammal, for as has been already stated,
the evidence all supports the belief that the chemical
requirements of one species are the same as another.
The egg is indeed a complete food, but not one which
produces the optimum results when employed as
the sole source of nutriment. Aside from the calcium
content of the white and yolk of the egg, which is
much lower than that of milk, the contents of the egg
resemble milk in a general way in nutritional value.
The high content of milk sugar in the latter, and the
almost complete absence of carbohydrate from the
egg, cause them to differ considerably in the physi-
ological results which they produce on animals when
each is fed as the sole source of nutriment. Egg,
when fed alone, encourages much more than milk
the development of putrefactive organisms in the
alimentary tract. The shell of the egg consists


principally of calcium carbonate, and during in-
cubation this is to some extent dissolved and ab-
sorbed for the formation of the chick. When eggs
serve as human food the shells are discarded. There
are distinct differences in the chemical natures of
the constituents of eggs as contrasted with milk.
The principal protein of egg yolk, like that of milk,
contains phosphorus, but the fats of milk are phos-
phorus free, whereas phosphorized fats (e. g., leci-
thins) are very abundant in egg fats. There is an
abundance of lactose in milk, whereas the egg con-
tains but a trace of sugar. These differences have
little, if any, dietary significance. The yolk is es-
pecially rich in both the fat-soluble A and water-
soluble B. With the exception of milk the foods of
animal origin do not supplement completely the diet-
ary deficiencies of the seeds and their products.

We are now able to make certain generalizations
of fundamental importance regarding the types of
combinations of the natural food-stuffs which may
be expected to give good results in the nutrition of
an animal.

(1) Seed mixtures, no matter how complex, or
from what seeds they are derived, will never induce
optimum nutrition.

Seeds with tubers, or seeds with tubers, roots and
meat (muscle) will in all cases fail to even approxi-
mate the optimum in the nutrition of an animal
during growth.

(2) The only successful combinations of


foods or milled products for the nutrition of an ani-
mal are:

(a) Combinations of seeds, or other milled
products, tubers and roots, either
singly or collectively taken with suf-
ficient amounts of the leaves of plants.

(b) Combinations of the food-stuffs enu-
merated under (a) taken along with a
sufficient amount of milk to make good
their deficiencies.

Milk and the leaves of plant are to be regarded as
protective foods and should never be omitted from the
diet. Milk is a better protective food than are the leaves,
when used in appropriate amounts.

It should be appreciated that not all diets which
conform to the requirements laid down in the above
generalizations, will give equally good results. This
is especially true of diets of the type under (2).
Chart 6 shows the great differences in the food
values of a few mixtures of seeds and leaves. It can
be stated definitely, however, that diets which are
not made up according to the second plan, will
never be satisfactory.

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