THE USES OF FOOD
We have had a good deal to say thus far about power development in
living animals, and have talked about food in connection with its use as
fuel for the purpose. While we are on the topic it may be as well to say
something about other uses to which food is put in animals besides that
of serving as fuel, and also something about what is done with the power
that is developed by the burning of such food as is used for fuel. To
begin with, it is evident that one use that is made of food is to build
the body itself. The new-born infant usually weighs somewhere between 5
and 12 pounds. From birth until the body gets its growth there is an
almost continuous gain in weight until a total which may range anywhere
between 90 and 250 pounds is reached. Of course, every bit of this
additional material came into the body in the form of food. The whole
mass of the body divides itself, as has been said before, into living
protoplasm and nonliving substance. We do not know accurately what
proportion of the whole weight is made up by protoplasm; it has been
estimated at about 60 per cent, but any estimate can be only very rough
because about half of the nonliving substance consists of fat deposits
which vary greatly in different people.
In any case, that part of the food which goes to make gain in weight is
passed over to the living
[Illustration: DIAGRAM SHOWING THE RATE OF GROWTH IN MAN]
cells. If we accept the rough estimate given above, about 60 per cent is
then used for the actual manufacture of new protoplasm; the remainder is
worked over by cells specially devoted for the purpose and put into
place to serve as supporting structure, or to be held in reserve as fat.
Living protoplasm is chemically a very complex mixture. In consistency
it resembles a rather thin, transparent jelly; the thickness of the
jelly depends on how much water it contains and this varies greatly in
different kinds of protoplasm. The watery part of the protoplasm has
dissolved in it several substances; among them may be mentioned ordinary
table salt; also salts of potash and lime. Only tiny amounts of these
are present, but it is a curious fact that without these tiny amounts of
salts protoplasm cannot live. The chief solid substance in protoplasm is
protein; this
[Illustration: A FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS--THE
UNDERWEIGHT EMPLOYEES ARE GIVEN A MILK LUNCH]
[Illustration: Photo, Paul Thompson
THE WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO DETERMINE HOW
NEARLY IT APPROACHES CORRECT STANDARDS OF PHYSICAL DEVELOPMENT]
material, which is one of the most complex substances known to
chemistry, has certain peculiarities which seem to fit it specially to
serve as the chemical basis of life. Evidently of all the foodstuffs
protein is the most important for the manufacture of new protoplasm, in
other words for growth. In the case of a tiny one-celled animal, whose
body is made up of protoplasm, not much else would be needed, but any
animal that has a bony skeleton has to build this up to keep pace with
the growth of the soft parts of the body. For this purpose mineral
substances are needed, chiefly lime salts.
In addition to these foods which are actually used for making new body
substance it has recently been discovered that proper growth in the
higher animals, including man, depends on the presence in the diet of
certain dietary accessories, whose use is not at all understood,
although there is no doubt of their importance. These materials, to
which has been given the rather cumbersome name of “growth-promoting
vitamines,” are found dissolved in certain food fats. Apparently they
are insoluble in water and soluble in oil. Most animal fats appear to
contain them in small amounts, while most vegetable fats do not. Milk
and eggs, which are growth foods in an especial sense, are richer in
these accessories than any other articles of the diet. The discovery of
these facts has emphasized the importance of including animal fats in
the diet of growing children, milk and eggs particularly. Since milk is
also rich in the lime salts which are necessary for bone formation it
forms the best single foodstuff for children that there is. When very
young children have to be fed on cow’s milk, which differs somewhat in
proportion from mother’s milk, it is often found necessary to feed the
top milk diluted with water, instead of the whole milk. When this is
done, lime water is usually used in part for diluting the milk, instead
of all ordinary water. In this way the proportion of lime is brought up
enough to insure that the child will get plenty of it.
In addition to the use of protein as a growth food it has another use
which no other kind of foodstuff can share. This is also because protein
is the foundation material of living protoplasm. We do not know a great
deal about what goes on in living protoplasm to make up what we call the
life processes, but we do know that these processes are of a chemical
nature, and that in connection with them there is a steady wastage of
protein. The protein that thus goes to waste is broken down into simpler
chemical compounds which are expelled from the cells. Why this occurs we
do not know, but since it does it is evident that unless the wastage is
made good the time will presently come when so much protein will have
been lost from the protoplasm that it can no longer exist as such and
must die. As a matter of fact, one might go on a diet excessively rich
in starchy foods and fats and still starve to death if there were no
protein present. This use of protein is called cell maintenance to
distinguish it from the other special use of protein in cell growth.
Evidently, whatever may be missing from the diet, protein must not be
left out. Fortunately most of our common foods contain protein. It is
especially abundant in lean meat, in dried beans and peas, and in grain.
Potatoes and most garden vegetables are deficient in protein, as are
almost all common fruits. Bread and meat are our chief stand-bys as
furnishers of protein.
Just as there are vitamines that are important for growth, so are there
vitamines that are necessary for cell maintenance. Many years ago Dr.
Sylvester Graham made himself prominent by arguing that the outer coats
of wheat grains contain something that is needed in the diet, which is
removed in the process of manufacturing white flour. He accordingly
invented a form of flour, familiar to us all under his name, which
includes some of the bran from the outer layers of the wheat. This idea,
which originated with Dr. Graham, has since been substantiated, although
not precisely as Graham had it. We know that there are necessary
accessories to the diet, but we know, also, that they are much more
widespread than Graham thought. They occur in so many kinds of
foodstuffs that anyone who eats a mixed diet usually gets enough of them
for his needs. The ill effects of their lack are most evident when the
diet is restricted to a few kinds of food which happen not to contain
them. A striking example of bodily injury directly due to the absence of
these vitamines from the food is seen among Orientals whose diet is apt
to be made up of rice plus small amounts of other substances. Of recent
years the natives of Japan and China and the Philippines have suffered
much from a disease of the nerves known as beriberi. Investigation has
shown that this disease is due to the absence from the diet of needed
vitamines, and dates from the time when rice-milling machinery was
introduced. The old hand methods of milling rice were so imperfect that
much of the hull was left clinging to the grains, but machinery
polishes the rice clean of every trace of hull. The hulls of rice
contain the accessory that is wanting from the polished grains. Wherever
it has been possible to bring about the use of unpolished (brown) rice
instead of the usual polished kind, beriberi has disappeared. Or the
same result can be secured by adding small amounts of beans to the diet.
It is probable, also, that the hulls of most grains, including wheat,
contain some of the same, or a similar accessory, so to that extent Dr.
Graham was right in emphasizing the importance of adding hulls to the
flour. Quite recently it has been shown that raw foods are richer in
these accessories than cooked, and that ordinary compressed yeast
contains more of them than any other easily obtainable material. Many
people are being benefited by taking part or all of a yeast cake daily
in a glass of milk.
For growth, or the making of new protoplasm, and for maintenance, or the
repair of protoplasmic wastage, then, we must eat protein-containing
foods, also foods containing various kinds of salts, and foods
containing the necessary vitamines. All these are to provide required
materials; the actual substances built into the protoplasm. There
remains the requirement of power, for both growth and maintenance
represent chemical activity on the part of the cell, and this activity
depends on power just as does any other activity. In saying this we are
merely saying over again in different words what was set down at the
very beginning of the book as the chief sign of life, namely, the
necessity on the part of living cells of continuous power development.
The use of food as a source of energy or power has been talked about
already, but it is necessary to say something about the different sorts
of power development that may go on in cells, and since we shall have
to talk about this a good deal, right here is a good place to bring in
for the first time a word that has come to be used whenever the matter
of the chemical activities of living cells is being mentioned. The word
is _metabolism_; when we speak of cell metabolism we mean the chemical
processes that are going on in the cells. Hereafter, instead of saying
power development, the word metabolism will be used as meaning
practically the same thing.
First of all, in describing the various kinds of metabolism that cells
may show, we have the metabolism of rest. By this we mean the power
development that is going on when the cell is doing nothing more than
keeping alive; neither growing nor showing any special activity. This is
evidently the minimum amount that any cell can show, so it is often
referred to as the _basic metabolism_. We know of at least two things
that may change the amount of basic metabolism; the first of these is a
change in temperature; when a cell is cold, its basic metabolism is less
than when it is warm. There is a very simple chemical reason for this,
namely, that chemical processes as a rule go on more slowly the lower
the temperature. Since all metabolism consists of chemical processes,
this rule applies not only to basic metabolism, but to all other kinds
as well, and, as we shall see, explains why the lower animals show such
marked differences in behavior in cold and warm weather. The second
thing that influences the amount of basic metabolism is the percentage
of water in the protoplasm of the cell. Highly organized animals, like
ourselves, are destroyed if the cells lose more than a small fraction
of their water, but there are many of the lower animals that can be
dried until their bodies contain only a very little water and still
live. This applies to microscopic forms that live in puddles and similar
places; when the puddle dries up the animal dries up too, until all that
is left of it is a tiny particle of highly concentrated protoplasm. But
this tiny particle preserves all the original cells, or at least enough
of them to make a fresh start, and a very sluggish metabolism goes on in
each cell. Of course, the advantage of this is that the stored food
materials will not be used up as rapidly as they would if metabolism
went on at the usual rate, and so there is a better chance that the
animal may survive until more water falls or drains into the puddle, or
until the particle of dust which the animal has become may be blown by
the wind where it will fall into another one. Whenever either of these
things happens the protoplasm takes up water again and the former rate
of metabolism is resumed. It is only by means of this reduction in rate
of metabolism that many kinds of animals are able to persist, for in
large parts of the globe there is a period of each year when conditions
become so unfavorable that the usual rate of metabolism could not
possibly be maintained.
Next in order to basic metabolism comes the metabolism of growth, by
which we mean the energy necessary for the making of new protoplasm. Not
a great deal is known about growth metabolism; in fact, about the only
reason for believing that it requires any energy at all is that the
metabolism of young animals, whenever it has been studied, has been
found to be greater in proportion than that of animals that are fully
grown. It is hard to account for this, unless the growth process
itself, namely, the making of new protoplasm, requires energy. When we
think of the extreme complexity of living protoplasm, we can easily
believe that its formation involves the expenditure of energy, perhaps
in considerable amounts.
The last kind of power development to be considered is the metabolism of
special activity. Most kinds of cells, particularly in highly organized
animals, have some special kind of work to do. For example, the muscle
cells have the task of making the motions; the gland cells of
manufacturing the secretions, and so on. These we speak of as the
particular functions of the cells, and the metabolism by which they are
performed as _functional_ metabolism. In some of the lower animals one
can scarcely tell where basic metabolism leaves off and functional
begins. There is a small shrimp, about a half inch long, that is found
quite commonly in small ponds. This little animal has several pairs of
legs by which he swims about, and the strokes of these legs go on
continuously, day and night, with almost no interruptions, at the rate
of a hundred or more a minute, for days or even weeks. It looks as
though this, and other animals, that are continuously on the move, were
organized without any sharp line between basic and functional
metabolism; their protoplasm liberates energy by the oxidation of food,
and various things happen as the result; among them are the maintenance
of the protoplasm and the making of motions. In the higher animals the
distinction between basic and functional metabolism is sharp, and,
necessarily so, for the well-being of any of the higher animals requires
that he shall have pretty complete control over the activities of his
protoplasm, and this he could not have if the functional metabolism were
blended in with the basic. In other words, it is as important for bodily
well-being that the cells be able to become inactive as that they be
capable of activity.