We must now pass on to consider the means at our disposal for determining the amount of food necessary to satisfy the nutritive requirements of the body. These may shortly be classified as "scientific" and "experimental," and it may be admitted at once that no method is altogether satisfactory.

Scientific Determination Of The Quantity Required

It is easy to understand that if we could estimate the amount of nitrogen, carbon, and oxygen contained in the excretory waste matters of the body, we could form a very fair estimate of the amount and character of the food required to provide it with pabulum sufficient for repair and the production of energy. For practical purposes the nitrogen and carbon alone are computed, the former in the urine and faeces and the latter in the expired air. When the amount of nitrogen excreted is as nearly as possible balanced by the amount ingested in the food, then the body is said to be in a condition of "nitrogen equilibrium" - a pretty fair index of health. Assuming that an individual were to excrete 300 grains of nitrogen and 4800 grains of carbon, it would be possible to estimate the amount of meat and bread which would supply these quantities. Meat contains about 11 per cent. of carbon and 3 per cent, of nitrogen, hence 6 lbs. of it would give us 4800 grains carbon and 1309 grains of nitrogen, or 1009 grains too much. Bread contains 30 per cent. of carbon and 1 per cent. of nitrogen, hence 4 lbs. would give 9000 grains of carbon and 300 grains of nitrogen, or 4200 grains too much carbon. By eating meat alone we should require to consume 4 3/4 lbs. too much to get enough carbon. By confining our attention to bread alone we should require to consume 2 lbs. too much to get enough nitrogen. By combining the two substances we could get a reasonable amount of daily nutriment as follows:

This is, however, a very clumsy method of computation, and is never employed nowadays.

The scientific method in use is founded upon the conception of food as fuel. When fuel is burned, it is consumed, giving up its heat, which may be transformed into energy, and its constituent parts are disassociated from their organic union of carbon, hydrogen, and oxygen, reappearing as carbonic acid and water when the combustion is complete. A precisely similar process takes place in the human body when fats, carbohydrates, and proteins slowly undergo the series of changes which consume them with the production of heat and energy, and the excretion from the first two of carbonic acid and water, and, from the last, of these same products with urea, uric acid, creatinin, and sulphates in addition.

It is quite easy to ascertain the amount of heat emitted by employing a "bomb" calorimeter, an instrument so constructed that a measured quantity of water absorbs the heat, the amount of which is clearly shown by a thermometer. It is found that when a gram of carbohydrate or protein is enclosed in the little central chamber of the calorimeter and ignited by an electric current it can raise the temperature of a kilogram of water to 4.l° C., whilst 1 gram of fat completely consumed imparts a temperature of 9.3° C. to the water. The unit employed is called a (large) calorie, and hence:

With these facts in our possession it is a simple calculation to estimate the number of calories in any food whose percentage composition is known. Medium fat beef, for instance, contains 76.5 per cent. of water, 20 per cent. of protein, 1.5 per cent. of fat, and 13 per cent. of ash or mineral matter. Water is a most valuable medium for assisting the oxidation and other processes which are necessary for the production of energy, but does not contribute in any direct fashion to its amount; the role played by mineral matter we can only surmise, but in all probability it is likewise only adjuvant. Hence we have only to take into account the protein and fat.

One ounce of dry protein or dry carbohydrate produces 116 calories of heat on combustion so far as it is complete in the body, and 1 ounce of pure fat yields 263 calories; 1 ounce of fat being therefore equal in energy value to 2 1/4 ounces of protein or carbohydrate. Therefore, 1 ounce of beef with the above composition will contain 20 x 1.16 calories of protein = 23.2 calories, and 1.5 x 2.63 calories of fat = 3.8 calories.

In a similar manner 1 ounce of Cheddar cheese, which like most cheeses contains about one-third water, one-third protein, and the other third fat, will contain 33.3 x 1.16 = 38.6 calories of protein, and 33.3 x 2.63 = 87.5 calories of fat, more than twenty times as much as medium fat beef. Of course all cheeses are not so rich in fat, and most good beef contains a little more than 1.5 per cent. of fat, but the calculation is not far from the truth. It has indeed been asserted that a cheese of 20 lbs. weight contains more nutriment than a sheep's carcase of 60 lbs., and at about one-sixth of the cost, proving that cheese is a substitute for meat of quite transcendent importance in a poor household.

Given a knowledge of the percentage composition of any food it is not difficult in this way to estimate its caloric value, and I append the following short table complete from the Battle Creek Sanitarium diet-list, which may prove useful for reference. (See pp. 40 and 41.)

The Work Of The Body

It must not, however, be supposed that the energy in an ounce of these foods is as available in the human body as it is when completely burned up in a calorimeter. Losses are sustained during digestion, for, as we have already seen, few if any foods are digested absolutely and fewer still are completely absorbed. In addition to this, proteins are never totally consumed in the body, quite one-fifth of their value escaping in the urine unoxidised. It is also important to remember that a great deal of energy is utilised in the processes of mastication and digestion, and Zuntz has calculated that as much as 48 per cent. of the digested material from hay used in feeding a horse is dissipated in this fashion. It is estimated that in a human being no less than 2800 foot-tons of energy are expended each day in keeping the circulation, respiration, and digestion in working order and maintaining, by evaporation and radiation from the skin, the temperature at the normal amount of 98.4° F. In other words, if employed to work an elevator this amount of energy would raise a weight of 2800 tons one foot high.

It is incredible to think that this amount of energy must be expended before one stroke of external work is done, and even more amazing to know that to produce 300 foot-tons of labour each day the body must actually provide another 1500 foot-tons which is thrown off in heat. This is, however, a better result than that shown by the best steam-engine in existence, in addition to which the body utilises the heat for improving the value of its functions.

It has been found possible, by using a much larger instrument called a respiration calorimeter in which a human being may be enclosed for as long as two weeks, to estimate not only the output of work but also the total heat, carbonic acid, and water output of an individual, as well as to measure the oxygen which he inhales. The amount of course varies with the size, age, sex, and degree of bodily activity; but a man of average size, weighing 66 kilograms and at rest within the calorimeter, has been ascertained to have an energy requirement for 24 hours of close upon 2300 calories. This might be looked upon, therefore, as the minimum energy value of the food necessary to keep such an individual

Caloric Value Of Common Foods

from losing body substance and either eating in upon his reserves or living upon his own flesh. Expenditure of energy in any form, whether muscular, mental, or otherwise, would of course require a corresponding allowance of food, and it is calculated that an average man in the ordinary pursuit of his daily vocation requires something between 2600 and 3200 calories of food value.