¢ 
DECEMBER 14, 1916] 
prises the fats and: oils. These serve to supply 
energy which may be transformed into either work 
or heat, or partly into one and partly into the 
other. ‘A certain amount of fat is necessary for 
health, but it is, to a large extent, replaceable by 
foods of the following class, namely, starches and 
sugars. Fats, like proteins, are derived from both 
animal and vegetable sources. Most foods con- 
tain fat in a form not visible to the naked eye. 
The third class of substances in our diet, neces- 
sary for healthy nutrition, includes starch and 
sugar. These are known as carbohydrate food- 
stuffs. They are used solely for the performance 
of work and the production of heat, and are not 
stored as such to any extent in the human body. 
But if taken too abundantly or by those who lead 
sedentary lives they may be converted into fat, 
and are then stored up in this form. 
A suitable diet must also supply a certain 
amount of mineral salts, such as phosphate of 
lime, and many others which enter into the com- 
position of flesh and other body-tissues. These are 
contained in most articles of diet in sufficient quan- 
tities, and hence (with one exception, namely, 
common salt) are not specially added to the food. 
Lastly, it has been found by recent investiga- 
tions that to maintain health our food must con- 
tain certain accessory substances, the nature of 
which is not fully known, but the lack of which 
brings about diseases, such as polyneuritis or 
beri-beri, scurvy, and possibly rickets. These 
substances, known as “‘vitamines,” are present in 
minute. but sufficient quantities in most natural 
foods. They are, however, liable to be removed 
in the process of manufacture or destroyed in the 
preparation of foods. Hence it is desirable that 
the daily diet should include some raw food, such 
as salads or fruit. All vitamines are not killed 
by the cooking of food; some are undoubtedly 
thermostable. 
An adequate diet must not merely include the 
foregoing classes of foodstuffs, mineral salts, and 
vitamines, but must supply them in certain pro- 
portions and in sufficient quantities to cover the 
‘daily expenditure of material and energy which 
the body sustains. 
Availability of Foods. 
Before inquiring how these proportions and 
‘quantities have been ascertained, it should be re- 
membered that all the food taken into the body 
is not utilised. The greater part is absorbed and 
used up or-oxidised in the body-tissues, but part 
is rejected as waste. The proportion of the former 
or available fraction is higher in the case of animal 
than in that of vegetable foods. Thus, for ex- 
ample, of meat protein 97 per cent. is absorbed, 
of cereal protein 85 per cent., and of leguminous 
protein 82°5 per cent. 
The average absorption of nutritive substances 
from different classes of foods is as follows :— 
Protein Fat Carbohydrates 
percent. percent. per cent. 
Animal foods (meat, cess, sire Oak OS | <0 95 
+ Vegetable foods.. z TT Se (cl eam: | 
Ordinary mixed diet DP Peta, ce OG 35°97 
NO. 2459, VOL. 98] 
NATURE 
291 
The Daily Expenditure of Energy. 
The quantities of different foodstuffs necessary 
for a day’s ration have been determined in various 
ways, such as by ascertaining the average con- 
sumption per head of different classes of the com- 
munity, or of the inmates of large public institu- 
tions, or, lastly, of the population of whole towns 
or countries. This has been supplemented and 
confirmed by “balance”’ experiments, in which the 
amount of food required to keep the body in equili- 
brium under different conditions without gain or 
loss of weight has been determined. But the most 
fruitful of all our information has come from a 
comparison .of the energy expended in different 
ways by the human body, with the food require- 
ments necessary to meet it. In this matter we 
have only to deal with three forms of energy :— 
(1) Latent or chemical energy contained and sup- 
plied in our foods; (2) mechanical energy available 
for. the performance of useful work; (3) heat 
energy furnished by the burning or oxidising of 
food within the body and used to maintain its 
normal temperature. The first-named represents 
the income side of the energy balance-sheet, the 
last two the expenditure side. 
Each foodstuff has its own energy-value. This 
has been ascertained by burning a known weight 
of the dried material in a suitable calorimeter, and 
finding out how much heat is thereby generated. 
The unit of energy employed in these measure- 
nents is the quantity of heat required to raise 
the temperature of 1 kilogram of water from 15° 
to 16° C. It is known as the Calorie (written 
with capital C), or large calorie, to distinguish 
it from the small or micro-calorie used in. physi- 
cal measurements. 
Food, when oxidised or burnt in the human 
body, generates the same amount of heat as when 
burnt to the same degree or state in the calori- 
meter. This has been abundantly proved by ex- 
periment and with surprisingly accurate results. 
Thus 10 grams of sugar yield 41 Calories of heat 
when burnt in the calorimeter. If added to the 
diet of a resting man, the additional heat gener- 
ated by his body is exactly the same, the differ- 
ence not exceeding one-tenth of 1 per cent. The 
energy-value to the human body of 1 gram of 
protein foodstuff has been found to be 4°1 Calories, 
of 1 gram of fat 9°3 Calories, and of 1 gram of 
starch or sugar 471 Calories. 
The daily ‘expenditure of energy by an average 
man, living at ease, is as follows in a temperate 
climate :— 
Calories 
Radiation of heat from the body as ordinarily 
clothed (64 Calories per hour) cf ss: 1536 
Evaporation of water at skin and lungs... oa AONE 
Heating of respired air... 80 
Heating of food and drink to body temperature 53 
Work of the heart and muscles of respiration, etc. 150 
Total. -<.. er 2430 
Less than the above—namely, about 2000 
Calories—is expended by a man at absolute rest, 
such as when lying in bed; considerably more 
