Without the sugar, there was a minus balance of 64 grams of proteid, but addition of the carbohydrate caused practically a saving of all of this, with establishment of essentially a nitrogen balance. The sparing of proteid by carbohydrate is greater than by fats, a fact of considerable dietetic importance which is well illustrated by the following experiments (on dogs) taken from Voit:

Food.

Flesh.

Meat.

Non-nitrogenous Food.

Metabolized.

Balance of the Body.

grams

grams

grams

grams

500

250 Fat

558

–58

500

300 Sugar

466

+34

500

200 Sugar

505

–5

800

250 Starch

745

+55

800

200 Fat

773

+27

2000

200–300 Starch

1792

+208

2000

250 Fat

1883

+117

Food.

Flesh.

Meat.

Non-nitrogenous Food.

Metabolized.

Balance of the Body.

grams

grams

grams

grams

500

250 Fat

558

–58

500

300 Sugar

466

+34

500

200 Sugar

505

–5

800

250 Starch

745

+55

800

200 Fat

773

+27

 2000

200–300 Starch

1792

+208

 2000

250 Fat

1883

+117

In considering the results of this experiment, it must be remembered that the calorific or fuel value of fat as compared with carbohydrate is as 9.3 : 4.1; in other words, fats have a fuel value of more than twice that of carbohydrates. In spite of this fact, it is clearly evident that carbohydrates as a class—for the different sugars and starches act alike in this respect—are far more efficient than fats in saving proteid. Thus, with an income of 500 grams of meat and 250 grams of fat, the body of the animal lost 58 grams of proteid, while with a like amount of meat and 300 grams of sugar the body not only saved the 58 grams, but in addition stored 34 grams of proteid, showing a plus balance to that extent. The sparing of proteid by carbohydrate amounts on an average, according to Voit, to 9 per cent—in the highest cases to 15 per cent—of the proteid given, while the saving produced by fat averages only 7 per cent. Further, increasing quantities of carbohydrates in the food diminish the rate of proteid metabolism much more regularly and constantly than increasing quantities of fat. We may attribute this difference in action, in a measure at least, to the greater ease in oxidation and utilization of the carbohydrate. In any event, starches and sugars are most valuable adjuncts to the daily diet, because of this marked proteid-saving power, while their fuel value adds just so much to the total energy intake.

A more striking illustration of the action of carbohydrate in sparing proteid is seen in experiments on man, where the nitrogen intake is reduced to a minimum, so as to constitute a condition of specific nitrogen-hunger. In such a case, increasing amounts of carbohydrate added to the intake reduce enormously the using up of tissue proteid. The following experiment with a young man 22 years old and 71.3 kilos body-weight, reported by Landergren,[29] affords good evidence of the extent to which this proteid sparing power may manifest itself.

We see here the nitrogen consumption fall to the exceedingly low level of 3.34 grams per day, or 0.047 gram per kilo of body-weight. To appreciate the full significance of this drop in the extent of proteid metabolism, we may recall that Succi, with a body-weight of only 62.4 kilos, on the seventh day of fasting excreted 9.4 grams of nitrogen, corresponding to a metabolism of 58.7 grams of tissue proteid. In other words, with an intake of only 5.6 grams of proteid, the addition of 908 grams of carbohydrate, with a total fuel value of 3745 calories, reduced the consumption of tissue proteid to 20.8 grams. The same individual, if fasting, would undoubtedly have used up at least 70 grams of tissue proteid.

Day.

Intake.

Output.

Proteid
metabolized.

Proteid.

Fat.

Carbo-
hydrate.

Alcohol.

Calories.

Nitrogen
of Urine.

grams

grams

grams

grams

grams

grams

1

35.2

6.1

507

26.6

2465.9

12.16

76.0

2

28.7

4.7

787

26.6

3574.3

8.37

52.3

3

28.8

4.7

841

26.6

3796.1

5.02

31.3

4

28.3

4.9

839

13.3

3690.5

4.50

28.1

5

5.4

. .

898

. . . .

3703.9

4.01

25.0

6

6.0

. .

931

. . . .

3841.7

3.36

21.0

7

5.6

. .

908

. . . .

3745.8

3.34

20.8

It is evident from what has been said that both of these non-nitrogenous foods, fat and carbohydrate, play a very important part in nutrition, because of their ability to maintain in a measure the integrity of tissue proteid. When we recall that a diet of pure proteid, such as meat or eggs, must be excessive in quantity in order to meet the energy requirements of the body, and that the stimulating action of proteid food serves to whip up body metabolism, we can appreciate at full measure the great physiological economy which results from the addition of carbohydrate and fat to the daily diet. The establishment of nitrogenous equilibrium is made possible at a much lower level by the judicious addition of these two non-nitrogenous foodstuffs. The same principle may be illustrated in another way, viz., by noting the effect on tissue proteid of withdrawal of a portion of the fat or carbohydrate of the intake, in the case of a person nearly or quite in nitrogen balance. The following experiment[30] affords a good example of what will occur under such treatment:

Income.

Output of
Nitrogen.

Balance of
Nitrogen
in Body.

Nitrogen.

Fat.

Carbo-
hydrate.

Calories.

grams

grams

grams

grams

Av. of 3 days

15.782

40.47

289.6

1955

14.927

+0.862

Nov. 30

15.782

40.34

177.3

1493

14.959

+0.830

Dec. 1

15.782

40.34

177.3

1493

17.546

–1.757

2

15.782

40.34

177.3

1493

18.452

–2.663

Average of the last two days . . . . . . . . . -2.210

Starting with the body in a condition of plus nitrogen balance, i. e., with a mixed diet more than sufficient to maintain the tissue proteid intact, the reduction of the fuel value of the food from 1955 to 1493 calories by cutting off 112 grams of carbohydrate per day was followed by a gradual, but marked, increase in the output of nitrogen; indicating thereby the extent to which the body proteid was then drawn upon to make good the loss of energy-containing income. The body showed at the close of the experiment a minus nitrogen balance averaging 2.2 grams per day, or a loss of 13.8 grams of tissue proteid, which would obviously have continued, under the above conditions, until the body was exhausted. In other words, the 112 grams of carbohydrate, if added to the diet on December 3 and the following days, would have quickly saved the daily loss of 2.4 grams of nitrogen, and thus changed the drain of tissue proteid to an actual gain, with consequent establishment of a growing plus balance.