A great deal of attention has been paid by experimentalists to the metabolism of inorganic salts. With the exception of the part played by sodium chloride in the causation of some forms of oedema, it cannot be said that clinical medicine has been enriched to any great extent by the enormous number of researches made on this subject. The effect of added salts upon the metabolism of the body as a whole is negligible. The foods commonly taken in this country contain a sufficiency of salts. The beneficial effects of the various and much vaunted mineral waters upon metabolism, effects which have often been supported by imperfect metabolic experiments, are mainly due to the regular life, judicious diet, and graduated exercise enforced at the establishments where these water cures are carried out, and to the relief of constipation.
The body contains 100-140 grammes of sodium chloride, the proportion being greater in the blood than in the tissues; this amount varies little in most diseases, and about half of the osmotic pressure of the blood is due to it. Probably not more than 3 or 4 grammes a day are needed, but the quantity taken in the food is generally much greater than this. If it be reduced, the secretion of salt in the urine will fall until equilibrium is established at a lower level. If no salt be taken, chlorides will still be excreted in the urine, so that there will be a loss from the tissues, which may amount to 15 per cent of the total amount in the body. When salt is again added to the food, this quantity is quickly regained. Constant vomiting or repeated washing out of the stomach may lead to a considerable loss of chloride in the form of the hydrochloric acid of the gastric juice. The restriction of salt in man leads to a loss of weight because the chlorides which continue to be passed out are accompanied by water. In this way the deprivation of salt may be said to cause diuresis. Large doses of salt produce diuresis directly, in health, because the flow of blood through the kidney is increased and salt is excreted together with water; the tissues also contain more water, for the salt in them holds water by osmotic attraction. The quantity of water excreted in the urine is, however, greater than that retained by the tissues.
In many cases of nephritis, the kidney does not excrete salt so easily as in health. A normal man will pass out any excess of salt in a day or two, but a nephritic may not do so for several days. This behaviour of the kidney is, however, not constant: the excretory powers for salt, as for water and nitrogen, vary from time to time, and for this reason observations only extending over a few days may give unreliable results. Failure to excrete salt is most marked in acute nephritis and in severe cases of parenchymatous nephritis. In granular kidney the excretion may be normal, except during an exacerbation or when the heart is failing.
Although we usually speak of the excretion of the chlorides, it is the base, sodium, which is retained, at all events in many cases, whilst potassium may be passed out. Herringham made observations upon cases of nephritis in which the sodium and the potassium were estimated in the food, the urine and the faeces. In a girl aged 17, suffering from severe parenchymatous nephritis, the following total figures were obtained in a nine days' experiment:
In urine and faeces
On three days no sodium at all was excreted. The patient died about a month later. Similar analyses carried out in a man aged 47 in the last stages of interstitial nephritis with a dilated hypertrophied heart gave in four days:
In urine and faeces
This retention of sodium, as several workers have noted, is commonly, but not necessarily, associated with oedema. On the other hand, when oedema is present there is frequently a retention of salt. Widal and Javal and others have found that diminishing the salt in the food in nephritis lessens the oedema, and adding salt increases it. Herringham found this to be the case also in ascites, and demonstrated the presence of a considerable quantity of sodium in the fluid. Cantineau found in a case of tuberculous peritonitis with effusion that a salt free diet was followed by the disappearance of the fluid, whilst at a later period, when at the patient's wish salt was added to the diet, the oedema and ascites recurred. Achard has used the diet in the ascites of cirrhosis. Whether the retention of salt is secondary to that of water or the converse is a disputed point. According to Bainbridge, in some cases at least, the retention of salt is primary. But, whether or no either view is correct to the exclusion of the other, experiment shows that in dropsy from renal disease, and from many other causes, and even in severe cases of nephritis without dropsy, the salt in the diet should be restricted. (Edema is lessened because salt continues to pass out in the urine and draws water with it. A limitation of salt is safer and less open to objection than a restriction of water. The amount may be reduced to 2 or 3 grammes. Milk contains about a gramme to a pint. Ordinary bread contains 5-7 grammes to the pound, and, on this account, bread must be specially baked without salt. Meat, fresh-water fish, most vegetables, eggs, butter and sweets are allowed, with not more than 3 pints of fluid.
Carducci does not recommend a salt free diet in cardiac dropsy, though others report good results. In mild cases of chronic granular kidney without oedema it is not necessary to reduce the salt in the food.