Chapter II. The Relationship Between Food, Heat, And Motion

(See Essentials of a Normal Diet.)

A Foot-pound, a Foot-ton, a British Unit of Heat. - Mechanical-equivalent of a British Unit of Heat. - Combustion of Carbon. - Consumption of Oxygen, Evolution of Heat. - Mode in which the Heat Evolved from Food is Disposed of. - Composition of some of the Principal Articles of Food and their Heat and Mechanical Equivalents. - Weight and Cost of Food compared with Work Produced. - Uses of Nitrogenous and Non-nitrogenous Foods.

The relation between food, heat, and mechanical force has been the subject of some of the most important investigations of late years, and the light which has been thrown upon it is one of the greatest steps in the progress of medical science. But so far as medical art and practice are concerned, comparatively little application has yet been made of this advance in scientific knowledge. This is only what might be expected, considering the occult nature of the subject, and the scarcity of time among the great body of medical practitioners to master and familiarise themselves with the details of such departments of medical progress.

Nevertheless, it is high time that such facts as are at present known, and such applications of them as are at present possible, should be added to the common stock of knowledge possessed by practical medical men; for it is only in this way that the public can derive benefit from our advances in science.

For the purpose, therefore, of enabling practitioners easily to familiarise themselves with these matters, the following concise statement has been prepared: -

1. A Foot-pound is the amount of mechanical force required to raise a pound weight one foot.

2. A British Unit of Heat is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.

3. This amount of heat (a British unit) may be converted into mechanical force sufficient to raise a pound weight 772 feet.

4. This amount of heat (a British unit) is generated by a pound weight falling through a space of 772 feet. Hence 772 foot-pounds is called the Mechanical Equivalent of a British unit of heat.

5. One ton weight is 2,240 lbs.; 772/2240 = 0.34465; therefore, the heat units multiplied by 0.34465, will always give the mechanical equivalent in Foot-tons.

6. The heat of combustion of carbon and hydrogen fully oxidised (to C0₂ and H₂0) is as follows: - 1 lb. = C evolves 14,500 - H evolves 62,032 = British Units of Heat.

7. The total heat of combustion of a compound containing H and C is the sum of the quantities of heat which the C and H contained in it would produce separately by their combustion.

8. It has been found that the presence of Hydrogen with Oxygen in the proportion to form water does not affect the total heat of combustion. It is only the excess of Hydrogen that can be made serviceable as a source of available heat.

9. The mean consumption of oxygen by an adult man of average stature (weight 150 lbs.), taking ordinary exercise, is about 30 ozs. avoirdupois in twenty-four hours, and the heat evolved by each 1 oz. of oxygen in combining with carbon, hydrogen, etc., is about 340 British units. Hence, 10,000 British units of heat will be evolved every twenty-four hours by the combination of 30 ozs. of oxygen with carbon, hydrogen, etc.; therefore the food of an adult man, under ordinary circumstances, should be such as may, in addition to other purposes, evolve at least 10,000 British units of heat.

10. Practical experience in the dieting of large numbers of men, and other means, have enabled us to establish the fact, that such an average man, as above described, requires, for the maintenance of health, a diet which shall contain about 4 ozs. of plastic materials, 3 ozs. of fat, and 10 ozs. of carbo-hydrates; and on careful analysis of this diet, we find that it can supply the required 10,000 British units of heat - viz., 2,516 from the plastic, 3,357 from the fat, and 4,150 from the carbohydrates; total, 10,023. (See "Essentials of a Normal Diet," Chap. III.)

11. The mode in which these 10,000 British units of heat are disposed of, and the purposes which they serve, may be seen in the following calculation, which has been made as nearly correct as possible. 8,000 British units are required as sensible heat, - to raise the temperature of the inspired air to the temperature of the body, to vaporise the pulmonary halitus, and to maintain animal heat. The mechanical equivalent of 2,000 British units (equal to 690 foot-tons) is expended in actual work, more than half of which is employed in internal vital work (the mechanical work of the heart alone is equivalent to 200 foot-tons; respiratory and other vital movements may be estimated as equivalent to nearly 200 foot-tons more), leaving about 290 foot-tons available for external work, which may be represented by the labour of walking 16 miles; but of course only so much is available for actual walking as is not used in the other external movements of the body which we daily perform.

In the following table the conditions laid down in Nos. 6, 7, 8 of the foregoing paragraphs have been observed in calculating the British units of heat given in the last column but one, and the mechanical equivalent of the Heat-units given in the last column has been calculated by the rule stated in paragraph 5.

Table I. Food broken down by element

Table II. Professor Frankland has determined the weight and cost of various alimentary articles that would be required to raise the body-weight of a person of 10 stone or 140 lbs. to a height of 10,000 feet, reckoning in accordance with Helmholtz's calculation, that the animal system is capable of turning one-fifth of the actual energy developed by the oxidation of the food to account as external work.