If the composition of a food is known, its approximate fuel value is easily computed by means of the above factors. Thus milk of about average composition contains:

Protein, 3.3 per cent; fat, 4.0 per cent; carbohydrate, 5.0 per cent. One hundred grams of such milk will furnish in the form of protein (3.3 X 4. =) 13.2 Calories; of fat (4.0 X 9. =) 36.0 Calories; of carbohydrate (5.0 X 4. = ) 20.0 Calories; total for 100 grams of milk, 69.2 Calories.

Eggs contain * on the average, in the edible portion, 13.4 per cent protein, 10.5 per cent fat, and no appreciable amount of carbohydrate. They would then furnish per 100 grams (13.4 X 4) + (10.5 X 9) = 148.1 Calories.

Milk and eggs are sufficiently similar to be used interchangeably in the adult dietary within reasonable limits, but evidently they furnish, weight for weight, very different amounts of nutrients and energy. Ordinarily the quantities to be taken as equivalent or mutually replaceable are those which furnish equal fuel value, e.g. 100-Calorie portions, the weights of which may be calculated directly from the fuel values of 100 grams.

Thus, for milk - 100 grams furnish 69.2 Calories; then, if x be the number of grams which furnish 100 Calories:

100: 69.2:: x: 100; x = 145-† Similarly for eggs:

100:148:: x: 100; x = 68.

* These and all similar statements of average composition are based on Bull. 28, Office of Experiment Stations, U. S. Dept. Agriculture.

† It is considered sufficiently accurate to state these quantities to the nearest whole number of grams.

And since the two extremes in the proportion are always the same, the weight in grams of the 100-Calorie portion may always be found by dividing 10,000 (the product of the extremes) by the number of Calories per 100 grams.

The fuel value of foods is often stated in Calories per pound. Thus in the same table (Bull. 28) from which the above figures for composition are taken, the fuel value of milk is given as 325 Calories per pound. Since 453.6 grams furnish 325 Calories, - 453.6: 325::x:100; =139.6, the number of grams required to furnish 100 Calories. This figure is about 3 per cent less than the one found above because it is based on a fuel value computed by Rubner's factors, which are 2.5 to 3.3 per cent higher than the factors based on more recent work. (See above.)

The following figures for a few common food materials* are based upon the more recent factors, and show the weight of the 100-Calorie portion in grams and ounces, and the distribution of the calories between proteins, fats, and carbohydrates:

Table Op 100 -Calorie Portions Of Food Material Based On The Factors - Protein, 4; Fat, 9; Carbohydrate, 4

* Arranged according to the classification used in the bulletins of the U. S. Department of Agriculture and in Konig's well-known reference work Die Chemie der Menschlichen Nahrungs- und Genussmittel, viz. meats, fish, eggs, dairy products, grain products, sugars and starches, vegetables, fruits, nuts, oils.

† Table 1 of Appendix B shows 100-Calorie portions of a much larger number of food materials.

Since proteins and carbohydrates have the same average fuel value and the ash of food does not as a rule constitute a large percentage, the striking differences in the weights of the various foods required to furnish 100 Calories are usually referable to differences in water content or fat content or both. That beans have nearly 20 times the fuel value of celery is essentially due to the difference in moisture, while the difference in fuel value between lean beef and bacon, or between codfish and salmon, is chiefly a matter of fat content. Meat free from fat is about three fourths water and one fourth protein, and so has a fuel value of about one Calorie per gram, while clear fat has a fuel value about nine times as great.

Fuel values of meats as given in the standard tables are apt to be somewhat misleading, inasmuch as they allow for all the fat ordinarily found on the various cuts as taken from the animal, whereas in many cases a considerable part of this fat is trimmed off by the butcher and treated as a by-product; and often much of the remaining fat is removed either in the kitchen or at the table. If a pound of steak consists of 14 ounces of clear lean, and 2 ounces of clear fat, and the fat is not eaten, at least half of the total fuel value of the pound of steak is lost.

Many vegetables are more watery than lean meats and so contrast even more strikingly with the fats. An ounce of clear fat pork is equal in fuel value to about two pounds of cabbage; an ounce of olive oil to over three pounds of lettuce.

In connection with such comparisons of fuel value, however, it should be emphasized that the fuel value of a food, while of primary importance, is not alone a complete measure of its nutritive value, which will depend in part also upon the amounts and forms of nitrogen, phosphorus, iron, and various other essential elements furnished by the food.

In order to indicate relative richness in nitrogenous constituents (protein), it is not uncommon to state the "nutritive ratio" along with the fuel value of a food. The "nutritive ratio" or "nutrient ratio" is the ratio of non-nitrogenous to nitrogenous nutrients, compared on the basis of fuel values. Since the fuel values of carbohydrates and protein are taken as equal (4 Calories per gram), and that of fats as 2¼ times as great (9 Calories per gram), the nutritive or nutrient ratio may be shown as follows:

Carbohydrate + 2¼Fat : Protein :: x : 1; or the ratio may be expressed in the form of a fraction:

Carbohydrate + 2¼ Fat Protein.

These expressions can, of course, be applied equally well to percentages or to weights of nutrients.

The same information as is given by the statement of fuel value per pound and nutritive ratio may be obtained by comparing the weight of 100-Calorie portions and the percentages of calories supplied by protein as shown in the above table. The statement that 19 per cent of the calories of milk are furnished by protein is equivalent to giving the nutritive ratio of milk as 4.3.