The nutrition of man

by R. H. Chittenden

With these facts before us, it is difficult to avoid the conclusion that some significance may attach to the specific nature of the proteid. Of course, we must not overlook the radical difference in dietary habits of man and dog. Man as an omnivorous creature has for generations been accustomed to partake largely of vegetable foods, and as a result his digestive tract and his system as a whole has become acclimated, as it were, to the nutritive effects of vegetable matter. Dogs, on the other hand, are typical carnivores, and their habits for generations have led in an opposite direction, so that their gastro-intestinal tracts and their systems have become accustomed to the effects of a diet in which animal food largely predominates. Whether these deeply ingrained characteristics are responsible in any large measure for the difference in behavior of man, on a purely vegetable diet, and dogs is open to question. It would certainly not be strange if such were the case, but as we look at the facts collected in our study of this subject, it is somewhat impressive to note how well dogs thrive on a relatively large amount of vegetable food, provided there is a modicum of animal food added thereto. In other words, these high proteid consumers are apparently quite able to utilize the vegetable foods, but there is something lacking in such a dietary which the body has great need of. Is it not quite possible, as already suggested, that the specific nature of the proteid counts for something in nutrition? The question cannot be answered definitely at present, but there are certain facts slowly accumulating which make the question a pertinent one in this connection.

Thus, it is becoming evident, as was pointed out in an earlier chapter, that the many proteid substances occurring in the animal and vegetable kingdoms are more or less unlike each other in their chemical make-up. They yield different decomposition products, or the same products in widely different proportion, when broken down by the action of hydrolyzing agents; and when we recall that the digestive enzymes of the body convert the proteids of the food into these same end-products, it is plain that in the assimilation and utilization of the proteid foodstuffs the body has to deal with these various chemical units. Hence, an animal suddenly restricted to a dietary in which all of the proteid is furnished by bread might be seriously incommoded, either by the excess of certain amino-acids resulting therefrom, or by a lack of certain other end-products to which its body is accustomed. As an example, we may take the three typical proteids of the wheat kernel, gliadin, glutenin, and leucosin, and note the very striking difference in the proportion of certain of the decomposition products of each, as reported by Osborne and Clapp.[73]

	Gliadin.	Glutenin.	Leucosin.
	per cent	per cent	per cent
Leucin	5.61	5.95	11.34
Lysin	0	1.92	2.75
Arginin	3.16	4.72	5.94
Glutaminic acid	37.33	23.42	6.73
Ammonia	5.11	4.01	1.41
Aspartic acid	0.58	0.91	3.35
Tyrosin	1.20	4.25	3.34

It is obvious from these figures that the three proteids of the wheat kernel are radically different from each other. Contrast, for example, the content of glutaminic acid in gliadin with the amount in leucosin. With such striking differences in chemical make-up, it is reasonable to assume that corresponding differences in physiological action or food values may exist. Further, “in respect to the amount of these amino-acids, leucosin more nearly resembles the animal proteins than the seed proteins thus far examined, and in this connection it is interesting to note that leucosin occurs chiefly if not wholly in the embryo of this seed and is probably one of its ‘tissue’ proteins, in contrast to the ‘reserve’ proteins of the endosperm of which gliadin and glutenin form the chief part” (Osborne and Clapp). In other words, animal proteids, such as those of meat, are characterized like leucosin by a small content of glutaminic acid and ammonia; while leucin, lysin, aspartic acid, and arginin are relatively more abundant. Until we know more on this subject, however, any broad generalization would be out of place, but certainly there is justification for the supposition that in these differences in chemical constitution are to be found explanation of some of the peculiarities common to certain varieties of proteid food. Wheat flour, aside from its starch, is composed mainly of glutenin and gliadin with their large content of glutaminic acid. Meat proteids, on the other hand, like leucosin, contain only a small fraction of this acid, and, with the other differences indicated, meat proteid and wheat proteid as food for dogs or other high proteid consumers may reasonably be expected to have at the least very unequal values. And if we go a step beyond this and suppose that in the formation of true tissue proteid or the living protoplasm of the cell, certain of these end-products of proteid decomposition are absolutely indispensable, we can easily picture for ourselves a dearth of such building stones in the long-continued use of a diet which lacks that particular proteid from which the necessary building stones can be split off in adequate number.

It has been said, notably by Munk, that in dogs fed for some time on a low proteid diet there is a diminished power of absorption from the intestinal tract, associated with weakened digestion. If it is true that a lowered proteid intake results in a diminished utilization of the ingested food, that efficiency in the digestion and absorption of foodstuffs is impaired, it can only be interpreted as meaning that some injurious influence has been exerted on the epithelial cells of the intestine or the adjacent gland cells. We have, however, failed to find any evidence of deleterious action in the dogs that we have experimented with, where due regard was paid to maintaining a diet suitable for the physiological needs of the body. In the experiments that we have cited, both nitrogen intake and the fuel value of the food per day were lower than in Munk’s experiments, but the utilization of fat and proteid was not sensibly affected. The following tables give the results with ten dogs (including the six dogs already described) for lengths of time ranging from seven to twelve months, the periods indicated being each of ten days’ duration and occurring once each month. In the first table, the utilization of fat is shown, the figures given being based on determinations of the amount of fat contained in the excrement. Knowing the amount of fat in the daily food and the amount which passed through the intestine, it is easy to calculate the percentage of fat utilized.

UTILIZATION OF FAT IN PERCENTAGES.

Periods.	Dogs.
	1	2	3	4	5	12	13	15	17	20
1	97	96	93	97	97	96	96	98	98	95
2	96	96	98	98	98	94	95	97	98	95
3	98	97	97	99	96	97	97	98	94	98
4	98	96	97	97	96	94	95	98	97	97
5	96	. .	94	98	97	95	95	98	97	96
6	97	98	94	98	97	96	94	97	96	97
7	97	98	98	97	96	93	95	97	98	96
8	. .	. .	98	96	96	96	93	97	. .	. .
9	. .	. .	98	97	98	. .	97	98	. .	. .
10	. .	. .	98	97	98	. .	. .	. .	. .	. .
11	. .	. .	97	92	97	. .	. .	. .	. .	. .
12	. .	. .	97	97	. .	. .	. .	. .	. .	. .

It is perfectly plain from these results that there was no falling off in the utilization of fat; the percentage amount digested and absorbed, as in dogs 3 and 4, was just as large at the end of the twelve months’ experiment as at the beginning. Clearly, a so-called low nitrogen intake with dogs does not lead to any loss of power in the utilization of the fat of the food. This being so, it is equally clear that the arguments based on Munk’s results in this direction, and applied to man, are without adequate foundation.

UTILIZATION OF NITROGEN IN PERCENTAGES.

Periods.	Dogs.
	1	2	3	4	5	12	13	15	17	20
1	95	91	92	94	91	91	90	93	92	91
2	92	94	94	95	93	90	92	96	92	87
3	91	92	90	91	88	89	86	95	89	91
4	90	85	90	92	91	82	83	91	83	93
5	90	82	88	92	86	85	84	96	91	90
6	86	87	89	83	86	89	87	94	91	86
7	87	87	90	83	87	83	88	90	93	91
8	. .	. .	90	83	84	81	89	89	. .	. .
9	. .	. .	89	87	92	. .	87	89	. .	. .
10	. .	. .	93	85	94	. .	. .	. .	. .	. .
11	. .	. .	93	81	86	. .	. .	. .	. .	. .
12	. .	. .	89	92	. .	. .	. .	. .	. .	. .