As an illustration of how the body behaves with a low nitrogen intake followed by a sudden increase in the income of proteid, some data from an experiment performed by Sivén[27] on himself may be cited:
| Date. | Body-weight. | Nitrogen of the Food. | Nitrogen excreted. | Nitrogen Balance. |
|---|---|---|---|---|
| kilos | grams | grams | grams | |
| Nov. 6 | 65.4 | 2.69 | 8.31 | –5.62 |
| 7 | 65.4 | 2.69 | 5.37 | –2.68 |
| 8 | 65.1 | 2.69 | 5.71 | –3.02 |
| 9 | 65.3 | 2.69 | 4.88 | –2.19 |
| 10 | 65.0 | 2.69 | 4.32 | –1.63 |
| 11 | 64.9 | 2.69 | 4.25 | –1.56 |
| 12 | 64.9 | 2.69 | 4.47 | –1.78 |
| 13 | 64.6 | 2.96 | 4.88 | –1.92 |
| 14 | 64.4 | 2.96 | 4.30 | –1.44 |
| 15 | 64.3 | 2.96 | 4.75 | –1.79 |
| 16 | 64.4 | 2.96 | 4.36 | –1.40 |
| 17 | 64.4 | 2.96 | 4.13 | –1.17 |
| 18 | 64.4 | 2.96 | 4.35 | –1.39 |
| 19 | 64.4 | 2.96 | 4.32 | –1.36 |
| 20 | 64.4 | 2.96 | 4.22 | –1.26 |
| 21 | 64.0 | 2.96 | 4.06 | –1.10 |
| –31.31 | ||||
| 22 | 64.1 | 4.02 | 4.22 | –0.20 |
| 23 | 64.4 | 4.02 | 4.35 | –0.33 |
| 24 | 64.4 | 4.02 | 4.21 | –0.19 |
| 25 | 64.4 | 4.02 | 4.40 | –0.38 |
| –1.10 | ||||
| 26 | 64.2 | 8.24 | 6.56 | +1.68 |
| 27 | 64.4 | 13.45 | 8.67 | +4.78 |
| 28 | 64.4 | 13.66 | 10.54 | +3.12 |
| 29 | 64.0 | 13.45 | 11.10 | +2.35 |
| 30 | 64.2 | 13.24 | 12.83 | +0.41 |
| Dec. 1 | 64.2 | 13.24 | 11.70 | +1.54 |
| 2 | 63.9 | 12.61 | 12.00 | +0.61 |
| +14.49 | ||||
| 3 | 64.0 | 22.93 | 16.24 | +6.69 |
| 4 | 63.9 | 22.41 | 21.47 | +0.94 |
| 5 | 63.9 | 22.41 | 23.10 | –0.69 |
| 6 | 63.6 | 23.35 | 23.12 | +0.23 |
| 7 | 63.9 | 23.04 | 22.82 | +0.22 |
| 8 | 63.8 | 22.62 | 22.86 | –0.24 |
| +6.15 |
I have ventured to give these data in some detail, because of their exceeding great interest in several directions aside from the point under discussion. Confining our attention to the nitrogen exchange, it is to be observed that for a period of two weeks Sivén lived on less than 3 grams of nitrogen per day, and without any excessive intake of carbohydrate or fat. During this time, the body naturally was in a condition of minus balance as regards nitrogen, the output being considerably larger than the income. The total amount of nitrogen lost in the period, 31 grams, corresponds to a breaking down of 193 grams of tissue proteid, or over one-third of a pound. On increasing the income of nitrogen to 4 grams per day, the nitrogen loss still continued, though at a much lower rate; indeed, the body is seen to approach very closely to a condition of nitrogen equilibrium. Still further increase of the nitrogen income to 13 grams per day was followed at once by a slight accumulation of proteid, and the body showed a decided plus balance of nitrogen, as on November 27. This, however, is seen to decrease gradually with a corresponding daily increase in the outgo of nitrogen, until on December 2 the body was once more practically in nitrogenous equilibrium. On again increasing the nitrogen income, to 23 grams per day, the same process was repeated, although in this case the body more quickly approached a condition of nitrogen balance.
We see in these data striking confirmation of the statement that the nitrogen outgo tends to keep pace with the income of nitrogen, the body always striving to maintain a condition of nitrogen equilibrium. Consequently, the fasting man having lost largely of his store of proteid can replace the latter only slowly, even though he eats abundantly of proteid food. Thus, Sivén in the week ending December 2, though taking over 13 grams of nitrogen a day, retained in his body only 14.5 grams of nitrogen during the entire seven days; while in the six days following, with a daily intake of 23 grams of nitrogen, he gained only about 8 grams additional. The human body does not readily store up proteid, and this is true no matter how greatly the tissues are in need of replenishment.
If the daily income is reinforced by the addition of carbohydrate or fat, there is observed a decided influence on the outgo of nitrogen; the rate or extent of proteid metabolism is at once modified, fat and carbohydrate both having a direct saving effect on proteid. Neither fat nor carbohydrate can prevent the katabolism of proteid, but they can and do decrease it, and thus serve as proteid-sparers. In the fasting body, or where there is only an intake of proteid, the latter material, except for the fat contained in the tissues, must serve the double purpose of meeting the specific nitrogen requirements of the body and furnishing the requisite energy. The energy requirements, however, can be met more advantageously by either of the non-nitrogenous foodstuffs, and just so far as they are oxidized, so far will there be a saving of proteid. Herein lies the philosophy of a mixed diet, with its natural intermingling of proteid, fat, and carbohydrate. For the same reason, the body of a man rich in fat will in fasting lose far less proteid per day than the lean man; or, if fed with a given amount of proteid food, the fat man may attain nitrogen equilibrium, or even store up a little proteid, while on the same diet the lean man will lose proteid. Further, if a man is in nitrogen balance with a given amount of proteid food, the addition of fat or carbohydrate to the diet will permit of a reduction in the amount of proteid necessary to maintain nitrogenous equilibrium. Fat, however, when added to food, does not always protect proteid to the extent possibly suggested by the preceding statements. The following data from oft-quoted experiments by Voit[28] on dogs will serve to illustrate:
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Fat. | Metabolized. | On the Body. |
grams | grams | grams | grams |
1500 | 0 | 1512 | –12 |
1500 | 150 | 1474 | +26 |
| |||
500 | 0 | 556 | –56 |
500 | 100 | 520 | –20 |
Food. | Flesh. | ||
|---|---|---|---|
Meat. | Fat. | Metabolized. | On the Body. |
grams | grams | grams | grams |
1500 | 0 | 1512 | –12 |
1500 | 150 | 1474 | +26 |
| |||
500 | 0 | 556 | –56 |
500 | 100 | 520 | –20 |
It is to be observed that in both of these experiments the fairly large addition of fat results in a saving of proteid, but the sparing effect in the first experiment amounts to only 38 grams of proteid for the 150 grams of fat added. In the second experiment, however, there is a saving of 36 grams of proteid, although only 100 grams of fat were fed. The radical point of difference in the two experiments is the amount of proteid ingested. Proteid food stimulates proteid metabolism; it likewise accelerates the metabolism of non-nitrogenous matter, consequently the sparing or protecting effect of fat on proteid is most conspicuous when the intake of proteid is relatively small. Only under such conditions, does fat protect in large degree the consumption of proteid in the body. In the ordinary, daily, dietary of man, with its great variety of food materials and with its proteid-content not exceeding 125 grams, fat is apt to be a conspicuous element, and under such conditions its sparing effect on proteid metabolism is most marked. Further, it must not be forgotten, as Voit originally pointed out, that the adipose tissue of the body acts like the food-fat, and consequently the proteid-sparing effect of the former may be added to that of the latter.
The addition of carbohydrate to a meat diet produces at once a saving in the decomposition of proteid, as shown in the following figures, covering an experiment of two days:
| Meat. | Sugar. | Proteid metabolized. |
| 500 grams. | 200 grams. | 502 grams. |
| 500 | 0 | 564 |