Heat and energy are mutually convertible and are measured in units termed calories, each calorie representing the heat necessary to raise one kilogram of pure water, one degree centigrade, in temperature.

The human body, ,in a state of physical rest, but functionally active, requires daily about 30 calories per kilogram, or 2100 calories for the standard man of 70 kilograms or 154 pounds. As differences in weight depend largely on nearly quiescent tissues, such as bone, connective tissue and fat, it is practically unnecessary to make allowance for moderate variations from the standard. Moderate physical activity, as for a healthy business or professional man, requires about 35 calories per kilogram or a total of about 2500; hard labor requires 40 or more calories per kilogram, or a total of 3000 up to 4000.

The heat and energy required by the body are furnished by the oxidation of organic foods:

As a matter of scientific empiricism, it is known that the body requires daily about 1/1000 of its weight in protein or 1 gram per kilogram, to make good the inevitable waste of tissue. The extreme lower limit of protein requirement has been set by Chittenden at a trifle less than 50 grams. Most authorities agree that 100 grams may be taken without harm and many believe that this amount is advisable to afford a margin of safety. Any considerable series of active adults, allowed to select food according to their natural appetite, will take 100 - 125 grams of protein a day.

* Stated at 4.1 calories per gram, for practical purposes, by most recent authorities.

While in diabetes and various other conditions, it is often advisable to give relatively large amounts of protein, the normal upper limit may be set at about 150 grams.

Fats and carbohydrates are, to a certain degree mutually vicarious. Fatty food is apparently not at all necessary to maintain a state of health or even to allow the deposition of fat. It is, however, difficult to select a diet list that shall contain less than 10 grams of fat for the day's ration. On the other hand, the physiologic provisions for the digestion of fat are not so perfect as for proteins and carbohydrates, and it is practically impossible to give more than 150 grams a day without marked loss of fat in the bowel, diarrhoea, disturbance of digestion and even toxic action from incompletely catabolized fat.

The lower limit for carbohydrates may be set at 80 grams for, if less is taken, acid intoxication is imminent. The extreme upper limit may be set at 920 grams, determined arithmetically as follows: Suppose the maximum requirement of calories, 4000, the minimum of fat, zero, and the minimum of protein, 50 grams. This amount of protein at 4.9 calories per gram, yields 245 calories, leaving 3755 calories to be produced by carbohydrates, at 4.1 calories per gram. It is, of course, undesirable - that a man at heavy labor should take the minimum either of protein or fat. Under ordinary conditions, the carbohydrates should lie between 250 and 500 grams.

While the number of calories required varies according to the amount of exercise, temperature, etc., the waste of tissues is practically constant, unless affected by pathologic conditions, such as starvation, the perverted catabolism of diabetes, etc. Hence, the laborer requires little more protein than the brain worker, or the idler. 2

The standard diet for a man at light exercise may be stated as follows:

The protein may, however, vary between 50 and 125 grams, and the fat between 30 and 100 grams, with corresponding variation in the carbohydrates to make up the required total of calories.

On a low protein basis, the standard diet may be stated as follows:

According to Rubner, the caloric value of proteins and carbohydrates are equal, 4.1 calories per gram. Thus, so far as the maintenance of heat and energy are conoerned, they may be used interchangeably, and 610 grams of both will support the body. Since the caloric value of fats is 9.3 calories per gram every ten grams of fat used replace not quite 23 grams of protein and carbohydrate.

The rectangle 426-396 to 312-282 indicates the limits of optimum dietetic ranges and the polygon with zig-zag lines, that appropriate for diabetes, except the severe grades in which carbohydrates must be kept at or even below the theoretic minimum of 80 grams, below which fatty acid intoxication is liable to develop. In such cases, the table may be continued to the right, increasing the protein by ten in a column and decreasing the carbohydrate correspondingly. Such an extension is not only theoretic but practical since, while nitrogenous elimination is unduly increased, protein may be utilized with comparatively slight waste, up to a maximum of over 200 grams. On the other hand, if much more than 150 grams of fat is given, there is both physiologic waste of this substance and disturbance of digestion causing waste of other ingredients. The first three vertical columns at the left down to about 120 for fat, represent the optimum diet according to Chittenden, although even he would scarcely claim that a man of average size would maintain nitrogen equilibrium on the 50 gram ration of protein or invariably on the 60 gram ration.

Comparing the top and bottom lines, we get an idea of the possible increase in portability of food, by increasing the proportion of fat. While the top line probably represents a possible ration, the second or third lines represent a ration about as nearly fat-free as can be obtained from the use of ordinary food-stuffs.

It will be noted that every 100 calories correspond to about 25 grams of protein or carbohydrate or both and to about 11 of fat. Similar tables could easily be constructed for any given number of calories, but, as for persons under medical observation, it rarely happens that we need to increase the calories by more than a very few hundreds or should be satisfied with a number similarly below the standard, unless we can safely increase or reduce any one food ingredient without regard to the reciprocal relations shown in the table, additional tables are scarcely needed.