Popular Science Monthly/Volume 1/August 1872/The Nutritive Salts of Food
|THE NUTRITIVE SALTS OF FOOD.|
IN order to understand the importance of the nutritive salts in food, we must first ascertain how far its mineral elements are nutritious, how far they are indispensable, and when they may be considered as in excess.
The investigations of Liebig, who first studied this question, and those of his school, demonstrate that certain salts are closely combined with the other elements of living bodies, forming integral parts of them. In all the tissues of the organism is found potash in combination usually with phosphoric acid. In the. blood the salts of soda predominate. When we reduce the organs to ashes, these vary but very little in quality or in quantity. It is unquestionable that without these salts no organ is developed, and that there is no secretion by the glandular cellules. All the secretions contain certain salts, most of them characteristic. These salts have definite functions to discharge in the animal economy. Still, we do not know the quantity of these salts which must enter into the food, to support the body, though this is a highly-important question in the hygiene of alimentation. Direct experiments alone could decide, for Voit shows that those made by Magendie, and which are so frequently referred to, do not establish the points they are supposed to establish. Recent studies upon the nutritive value of gelatine have shown that Magendie failed to take into account some of the principal points of the question he was considering.
"For a long time," says Voit, "it had been my purpose to ascertain by thorough experimentation the value of salts in nutrition, with a view to examining how long an animal could live without them, and what symptoms it would manifest. For this purpose I had accumulated a quantity of the residuum of meat-extract. In the mean time appeared a remarkable work by Kemmerich, in which were proposed other questions, having a bearing, in many respects, upon those I had proposed to myself. Kemmerich starts out with the supposition, which Liebig, too, admits, that the residuum of the meat, without the extract, has no value as nutriment. According to his investigations, the action of meat-extract is attributable to the potash it contains, whence he concludes that the residuum is of no value for nutrition, owing to the absence of the potash. He therefore attempted to utilize it by adding to it the salts contained in meat. To this moist residuum, three times exhausted by boiling, he adds an artificial mixture of the salts of meat and common table-salt; and on this food exclusively he fed two dogs six weeks old, they devouring it ravenously. The experiment was continued for three months, and at the end of that period the dogs had gained considerably in weight."
Kemmerich repeated his experiments on the same animals, giving them now the residuum without the mixture of salts. He now observed that the animals consumed less and less from day to day, even when suffering from hunger. Again, experimenting on two dogs six weeks old, he fed to the one the residuum mixed with the salts; to the other the residuum mixed only with common table-salt. After twenty-one days a wide difference was observable between the two animals. The first one weighed much more than the second, was stronger and more intelligent. The second one had gained a little in weight, it is true, but his condition was pitiable. He could hardly walk; his eyes were dull and expressionless; his body appeared emaciated, and he took his food reluctantly and without relish.
Voit here makes an objection to the conclusions drawn by Kemmerich; but his objection is not valid, because he does not advert to the fact that the dogs in question, owing to their age, must necessarily gain considerably in weight. His objection would hold good only in the case of full-grown animals. However, to remove all doubt as to the increase of weight when the salts of meat are added to the residuum, Kemmerich gave the second dog the residuum with these salts; and to the first dog the same, with table-salt only; the quantity of residuum being in both cases equal. The result was, that the dog which had before got only common salt, now gained much faster than the other, which latter, however, gained 530 grammes in thirty-two days.
There is no need to give in full Voit's argument on the results of Kemmerich's experiments. It is enough for us to state, with Voit, that these experiments demonstrate the fact that a carnivorous animal can live on meat-residuum, provided that there be added to it the salts of meat and common salt. Still, these experiments do not answer the principal questions we have put, viz., How long can an adult organism live without salts? and, What are the symptoms they manifest, when thus deprived? So the attempt was made to study, in adult animals, the changes that occur in albuminoid substances, when the salts are extracted.
Dr. J. Zörster made the experiments. The purpose was to sustain animal life as long as it was possible, on food very poor in salts. The animals experimented upon were never given albuminous substances alone, but always mixed with a sufficient quantity of non-nitrogenous elements, such as fat, starch, or sugar. The albuminate employed was the residuum of meat dried in the oven, pulverized, and then boiled three times in water. Cheese, also, deprived of the salts, was given. Pigeons and mice took this food, in small morsels, for some time. Dogs also took it for a short time, but then refused it. Then they were compelled to take it. The mice lived from twenty-one to thirty days; the pigeons from thirteen to twenty-nine days; the dogs from twenty-six to thirty-six days.
Digestion went on regularly for the greater part of the time. The excreta, whether in quantity or in quality, were as usual. Consequently neither the process of digestion, nor the absorption of the materials digested, is influenced by the absence of salts from the food. In the case of a dog with a gastric fistula, fed on food deprived of the salts, direct evidence was obtained of the fact that the secretion of acids in the stomach goes on. After having been kept for a considerable period on this diet, the dog commenced to reject it from his stomach. What was thus rejected, though it had been in the stomach several hours, was not soured, nor had it the least unpleasant odor. It looked like food that had suffered no change. It is, therefore, only at a late period in such dieting that the absence of the salts makes itself felt.
This was altogether unexpected; but the examination of the excreta, more especially of the urine, led to a conclusion of still greater moment. This urine contains traces only of the chloride of sodium, but little phosphoric acid, etc., although in amount the urine was the same as before the experiment. The residuum of meat which was given was not entirely deprived of its salts. There will always remain a small amount of phosphates. When, therefore, the organism is stinted in these salts, it daily parts with a small quantity of them through the excrement and the urine, but retains the greater part. The organs are tenacious of these salts. When we add salt to the food, it goes first into the blood, but it is not then immediately thrown off by the kidneys, etc. It is distributed through the system, and each one of the organs takes according to its wants. In the blood are also to be found the salts evolved in the destruction of the animal's substance, under the influence of abstinence. All these salts are then distributed, as we have already said, and thus that portion of them which is not thrown off serves again and again for the needs of the tissues. On this account their quantity varies but little in the system. We will not follow our author in his attempt to account theoretically for these phenomena, inasmuch as it belongs to the province of pure reasoning. We must be content to continue in the domain of experiment.
After a period of complete abstinence from salts, certain remarkable phenomena are to be observed in dogs. Though they may not decrease in weight, nor lose flesh or fat, they still become weak, dull, and lie down in a corner, languid and indifferent. On one occasion, even, as in the case of a dog of Dr. Bischoff's that was for a long time fed on bread alone, the animal apparently went .mad. He fell into a rage, and ran round and round, heeding neither the voice nor the whip. The attack did not return, but the nervous system became more and more unstrung; paralytic symptoms were manifested in the hinder extremities; the animal's hind-legs failed him at every step, and he fell on his side: in the head were observable oscillatory movements, particularly when he ate or drank. On pushing the experiment still further, the animals invariably succumbed. If, however, instead of continuing the experiment, we give the dog the ordinary mixed food, he will commence gradually to recover his strength, and finally will be quite restored to health. During the period of recovery he will consume an unusual amount of food.
Thus is it demonstrated, in accordance with the views which Liebig was the first to maintain, that the salts are absolutely necessary. Without them the organism fails, even though it were to receive all the other elements. It will not, however, succumb instantaneously, but only after the lapse of a certain period of time. The salts differ in many respects from organic substances. When the latter are decomposed, the residuum cannot be utilized by animals, and must consequently be evacuated. Not so with the salts, which, according to Voit, are not transformed. This view will not pass unchallenged, when we call to mind the later researches of W. Marcet with regard to the constitution of muscular tissue, wherein the English physiologist has found that the phosphates enter into the nutritive process in the colloid state, but quit it in the crystalloid state.
Klein and Verson, according to Prof. Voit, have claimed that common salt is not a nutritive salt, but a mere condiment, and that it may be altogether wanting in the body without inconvenience. These two gentlemen lived for eight days, consuming the while only 1.4 gramme of salt per day, without experiencing any indisposition. They, in fact, threw off, in the eight days, 46.9 grammes of chloride of sodium in the liquid and solid excreta; but we have to observe that they had been in the habit of consuming salt very freely (about 27 grammes per day). Still the common salt in the system was not exhausted.
Voit states, in a note, that in the case of dogs which have the normal supply of nitrogen, the addition of common salt to their food increases the amount of urine, as also the proportion of water in the same. Klein and Verson thought that, when they are deprived of this salt, the urine must decrease; whereas they found, on the contrary, that it increased. The salt only excites indirectly the increase of urine, by promoting the transformation of the albumen; and it may well be that, when the supply of salt is deficient, this transformation is similarly accelerated by other influences. Klein and Verson have not determined what are the necessary conditions for recognizing the action of any substance in this respect. We cannot tell whether or not the nitrogenous elements of the food, during their experiments, were a constant quantity, for they say "taking about 420 grammes of beef, etc." Nor could they say whether the nitrogen in their system was in normal quantity. But, aside from this, I do not find that an increased quantity of urine is by their figures shown to follow the privation of salt.
After reciting the facts which are ascertained with regard to the use of salt by domestic animals, the author adds that the constituent salts of the organs, of which we spoke above, are as necessary for the support of the organism as albumen, water, or the organic non-nitrogenous elements; but that, notwithstanding, no symptoms of scurvy or of disease in the bones is observable, even where animals are for a long period deprived of salt. He calls particular attention to this fact: Kemmerich, on one occasion, gave a dog during seventeen days the residuum of meat, with the salts of potash only, that is, the phosphate of potash and the chloride of potassium. He had deprived the food of its salts of soda. And yet the serum of the blood was found to contain these soda-salts almost exclusively, while in the urine were found only the salts of potash. The salts of soda thus were altogether retained, as in the case where the salts were withheld.
It is not the functions of the nutritive salts that have been exaggerated hitherto, but rather the proportion in which they must enter into the food. They might he withheld for as many as forty days, for they are found in sufficient quantity in all substances which contain the other elements of nutrition. Haubner has stated that pigeons fed on grain, without lime, quickly die; but Voit has kept them on such food for a whole year. On food deprived of salts a pigeon can live for about thirty days.
When it is said that, without the nutritive salts, the residuum of meat, or any other kind of food, possesses no nutritive value whatsoever, the statement is true only in a certain sense, and as far as the duration of complete nutrition is concerned. Within a certain period of time, in the absence of the salts, the albuminates will cease to be assimilated, as also the fats and the hydrates of carbon. It is the salts that render the organic elements nutritive. The author hence concludes that none of these elements, whether organic or mineral, have any absolute nutritive value, and that they cannot be considered apart by themselves. They coöperate mutually in nutrition, and so are all equally indispensable to constitute proper food, such as may support life and strength. This is the most important datum of the numerous and varied physiological experiments made in Germany during the past few years, and it is a new discovery for us. Our physiologists and hygienists had no suspicion of such facts.
As we consume, with our food, considerably more of these salts than is needed to support the body, the question arises, Is this simply surplus, or are we benefited by it, as being a flavoring for the food? Much has been said about the extractive elements of meat, and it has been supposed that these elements form the true distinction between animal and vegetable food. According to this view they constitute the peculiar action of meat and of meat-extract. Here we must make a distinction between the nutritive element and the condiment.
The extractive elements of flesh-meat are the products of regressive change, and are not necessary for the constitution and formation of the organs; nor can they, when taken with food, add to the substance of those organs. The elements of this extract have been got in isolated forms, as creatine, sarkine, taurine, urea, uric acid, tyrosine, lactic acid, acetic acid, etc.; each organ has its own characteristic extractive principles, or its own products of decomposition, the conditions of this decomposition varying for the various organs.