43. Energy of vegetable foods. Experiments show that if completely burned,

1

gram of

fat

yields

9.3

Calories

"

" "

carbohydrate

"

4.1

"

"

" "

proteid

"

4.1

"

These figures also indicate approximately the amount of energy which would be obtained from equal quantities of the same substances consumed in the human body. To estimate, therefore, the amount of energy obtainable from 100 grams of any food of which we know the chemical composition, we have only to multiply the percentage of each nutrient by the number of Calories yielded by a single gram, and add the products thus obtained. This has been done for the vegetable foods of which the composition is given in the chemical chart (Fig. 120); and the number of Calories is indicated by heavy lines having lengths proportionate to the amount of energy yielded by the foods they represent. Foods which yield much energy are commonly described as being "hearty": the lines in the chart may be said therefore to indicate the relative "heartiness" or fuel-value of common vegetable foods.

But it may be asked, Does a fat and a carbohydrate serve us in exactly the same way? Physiologists tell us that either may replace the other in our food, provided the amounts eaten represent an equivalent number of Calories; but there is this difference that, whereas carbohydrates (which, so far as they are digestible, enter the blood as sugar) are immediately after digestion available as a source of heat and muscular energy, fats require to undergo some preliminary transformation in the body, before they can be used, and are therefore less serviceable for immediate needs. Fat, however, since it contains so much more energy than glucose in proportion to its bulk, is particularly well adapted for storage in our bodies as reserve material; and what is absorbed from our food needs to undergo scarcely any change before being laid away.

These differences in usefulness between fats and carbohydrates have been well expressed by comparing the latter to ready cash, and the former to money in a savings bank. This helps us to understand the benefit which pedestrians and bicyclists derive from the use of sweet chocolate. The large proportion of sugar (about 50%) yields up its energy immediately in time of need, while the considerable proteid offers material for the repair of muscular loss, and the abundant oil remains as a more slowly available reserve.

Likewise, the special craving which young people have for sweets, receives at once its explanation and justification when we remember the extraordinary activity which belongs properly to their period of life. It needs to be pointed out, however, that the quantity of carbohydrate eaten should be strictly proportioned, to the amount of bodily activity; for otherwise there will be left in the system an excess of sugar, which may either go to produce an unhealthy accumulation of fat, or by undergoing acid decomposition, seriously disorder the digestive organs. Too much sweet food and too little exercise is one of the commonest causes of indigestion and obesity.