May 17, 1017 
LATSID & WATER 
13 
How Much Should We Eat? 
By H. Onslow 
The wriler (ij nii\vaUta}>lc article :on food valuer ix on the 
staff of the Cambridge Bio-Chemical Laboratory, under 
Projesxhr Hopkins 
T\\\l shortaf;c of home-grown food and the activities 
of ('-boats in sinking our imports have made the 
question of food a subject of the greatest interest 
in the eyes of the public. The alarming increase 
in the number of ships attacked and sunk may render the 
solution of the problem the decisive factor in winning the war, 
and it' has already' made a system of compulsory rationing 
imminent. In spite of this there still exists an e.xtraordinary 
degree of ignorance in the minds of even educated people. 
It is the purpose of the following paragraphs to enurnerate 
as briefly as possible the food requirements of the human 
body, as well* as some of the factors which determine the 
amount of food necessary to maintain the healtli of different 
people under varying circumstances. 
Special Uses of Food 
Food is used by the body for threes main purposes. ^ifBt, 
to maintain the heat of the body, and to provide the eiiergy 
for muscular movement ; secondly, to supply material for 
the normal waste of tissue as well as for the growth of new 
tissues ; and, thirdly, to satisfy the demands of the body 
for a' series of substances whose function may be said to 
resemble the lubricant of a machine'. 
• Heat and energy as is well known are mutually con- 
vertible. The principal energy producers of the body are 
•represented by two classes of foodstuffs. These are the 
starchy foods and the sugars, known' collectively as carbo- 
hydrates ; and, secondly, the fats or oils. Sugar and starch 
are for the purpose hi the body similar, because starch is 
rapidly converted into sugar by ferments in the saliva and 
elsewhere, and is thus absorbed by the blood. The chief 
sources .of carbohydrate are bread, potatoes, and sugar. 
Much sugar is derived from sweets and the large proportion 
of milk-sugar present in milk is a very important factor in the 
food of infants. Large quantities of carbohydrate are neces- 
sary to supply muscular energy, so that bread and potatoes 
become a most essetitial constituent in the diet of labourers 
arid soldiers. 
The chief sources of fat are milk and milk products, fat 
• meat, nuts and vegetable oils so largely found in margarine. 
They serve much the same purpose as carbohydrates, but are 
digested more slowly and absorbed less easily. This accounts 
for the fact that " rich " dishes are more indigestible than 
lean meat and tend to remain longer in the stomach. The 
rhost important use of fat, however, is to supply heat, and for 
this purpose it is more than twice as effective as su'gar. 
Fats also can be stored by the body in the form of adipose 
tissue, thereby serving the purpose of a reserve material in 
periods of privation, a protective coat for many organs as 
well as a non-conducting layer to maintain warmth, a function 
which reaches its extreme development in the blubber deposits 
of the Eskimo and arctic animals. 
The class of food substances known as proteins differs greatly 
from those already described. Proteins are chiefly derived from 
lean meat, milk and cheese, as well as oatmeal, flour, and a 
number of leguminous vegetables such as beans and lentils. 
Though we are accustomed to derive most of our protein 
from meat, that from vegetables is equally good and should, 
of course, be largely drawn upon at the present time. Protein 
can serve a"? a source of heat and work just as carbo-hydrates, 
but it has certain specific uses ; such as to provide material 
to replace the normal >VJiste of tissue'. In the case of adults 
this is an insignificant quantity, but with children and con- 
valescents it is of f^r greater importance, because additional 
material is needed to build tissue and to make good the 
ravages of disease. .An equally important function is the 
supply of the " lubricants ". of the body already mentioned, 
which, though not actually proteins themselves, are derived 
from protein. These ser\'e many . purposes, all of them 
essential to life, such as the digestion of food, which is 
accomplished by a series of ferments. They include secretions 
of certain glands, which initiate and regulate growth, control 
the development of the voice, beard antl other sexjial 
characteristics, and generally regulate and co-ordinate the 
diverse organs of the t)ody. ' . , 
• Finally, protein has the effect of stimulating the body to 
deal more vigorously and effec-tively with the other food 
present. It may be said to cause the fire to burn more 
briehtly and can be compared to the effect of air in a draught 
furnace. This specific action is of extreme importance to 
soldiers or athletes wlio may be suddenlv called upon to 
perform great feats of endurance, and who require large 
amounts of energy always available in case of emergency. 
The process by which protein is transferred from the stomach, 
whtTo It exists as food, and is absorbed by tlie blood, to be 
eventually incorporated in the tissues themselves, is one of 
extreme interest. WTien protein is acted upon by digestive 
]ui((^ it is broken down into a series of simple crystalline 
nitngonons substances called amino-acids. These bodies are 
■ absd-bed from the intestine as fast as they are made, and 
arc It once carried away by the blood stream from which 
theyare picked up by the various tissues in the exact pro- 
poitnns required. So rapid is the transference that it was 
only recently, by means of an improvement in chemical 
methds devised by an American, the increase in amino acids 
• -rl^ atually been detected in the blood after a full meal. 
Ihis k-eak down of the proteins into simple chemical bodies 
has hn\ illustrated by an ingenious analogy. Suppose for a 
momet that the word albumin were broken up by digestion 
' u*'* ^'^J«'tters a, b, i, 1, m, n, u, then if these letters were 
absortxl they could be reconstructed into " albumin ' again. 
In thisvay, albumm for instance, is broken d(nvn in the blood 
to discinected letters (amino acid.s) so that the albumin in 
the tisses can select the appropriate letters, arranging them 
so as tGpell " albumin." All proteins are, in fact, reduced 
to a cmmon currency, and every protein in the body 
IS capale of making itsowTi specific rearrangement from 
the conion stock of amino-acids, so that it matters not 
one whit^hat proteins are ingested, provided all necessary 
amino-ads are present in sufficient quantities. Some 
proteins, however, lack , certain amino-acids altogether 
and in a.protems they are present in widely differing pro- 
portions. It IS obvious, therefore, that if the diet lacks one 
or more cthe essential amino-acids, as when gelatin alone is 
consumed)ris deficient, as when some plant proteins are used 
the body mnot possibly grow. A mixed diet is an obvious 
necessity all the diverse proteins of the body are to obtain 
their vane; constituents in the adequate amounts. 
Amounbf Food Required 
Since f0( is needed to enable the body to do work as well 
as to mauiin its normaL tempei^ture.'it is clear that the 
amount ne^sary will be more for men doing hea\'v muscular 
work than r professional men doing intellectual work which 
requires noechanical energy. Similariv, a man expo.sed to 
great cold suffering from a fever requires more food to 
supply the.tra heat radiated. It is trae that in a cold 
chmate the-at given off may be partly adjusted by regula- 
ting the -clung, but it is a common experience of arctic 
ex-plorers 01 to be warm at night after a full meal 
There mabe some difficulty in understanding how the 
body can dee heat from snibstances like bread and meat. 
1 hey are as natter of fact burnt by the body just as coal is 
burnt m tkfumace. The body burns or oxidises food 
with the aid axygen from the lungs and the end products 
m both casdre identical, nameiv carbon dioxide (CO.) 
which IS expd by the lungs, and" water which is excretccl 
by the skin ngs and kidneys. Further, the actual heat 
geiierated bygfam of sugar when completely burnt in the 
body IS the se as if it were burnt to the same degree in a 
closed vessel, his amount can be determined in a laborator\' 
and from thista the heat-value of any food sample can be 
easily deducechc heat.is measured in calories, a caloric bein" 
^he amount o;at required to raise one litre of water one 
t^egree Centigr. Experiment shows that • 
I gram of sugai-es 40 cals. i gram of .starch gives 4- 1 cals. 
Hit*" .■■?" ■ 93 .. 1 ,. ..protein,, 4-1 
'Now if the of the con.servation of energy is to hold 
.gOAd in the bo the total energy' of the food must exactly 
1 fft^?M^c ^um c c work done and the heat given out. This 
,,^^ repeatedly 1, proNed true by placing men in a chamber 
..a,^;^'!'-!' ti° Eradiated can be macPe to raise the tern- 
„;pera;tur9, of a «„ volume of water. Eavoisier first made 
tjif^, experiment ,lacing a guinea-i>ig within a hollow block 
, nl i^e.. Alter a I be was able to measure the water melted 
.„\)y the heat of tViimal. 
• : Tl^ 'practical .tion that arises is the number of calories' 
required under .rent conditions, fhe amount of 'heat 
radui ed, etc an,o work of the heart and lungs is shown 
in lable I Ihis-ther with the cnergN- required for bodily 
movement in a .ntary occupation amounts to 2700 
calones. With a.onal work more calories arc required 
