Except for the hydrochloric acid of the gastric juice, practically all the chlorine involved in metabolism enters, exists in, and leaves the body in the form of chlorides - much the greater part as Sodium chloride. The amount of sodium chloride which is ordinarily added to food as a condiment is so large that the amounts of sodium and chlorine present in the various foods in the fresh state become of little practical consequence. Among animals the herbivora require salt while the carnivora do not, the latter obtaining sufficient salt for their needs from the flesh, and more especially from the blood, of their prey.

Sodium occurs, chiefly as chloride, abundantly in the blood and other fluids of the animal body and in much lower concentration in the tissues. Potassium, on the other hand, occurs to a greater extent as phosphate than as chloride. It is most abundant in the soft solid tissues - in the corpuscles of the blood, the protoplasm of the muscles, and other organs, and also in the highly specialized fluids which some of the glandular organs secrete, notably in milk. Since the cells are in constant contact with the circulating fluids, the abundance of potassium in the cells and of sodium in the fluids, makes it evident that the taking up of salts by the cells is an active or "selective" process. A conspicuous function of the salts in the tissues is the maintenance of the normal osmotic pressure, but solutions of different salts of equal osmotic pressure are by no means interchangeable, and it is not possible to replace successfully the potassium in the cell by an equivalent amount of sodium.

There seems to be a relation between the taking up of salt and the retention of water in the tissues. The effect of decreasing the salt in the diet is to decrease the quantity of salt in the tissues, and at the same time their water content. An explanation of this lies in the fact that, since body tissues and fluids must maintain a constant concentration of sodium chloride, a reduction in the absolute quantity of salt must result in a corresponding reduction in the quantity of water present.

Attention is frequently called to the fact that sodium chloride is the only salt which we seem to crave in greater quantities than occur naturally in our food, and that we share this appetite with the herbivorous animals. Bunge holds that this is because a high intake of potassium (as in most vegetable foods) tends to increase sodium elimination. Bunge tested this theory upon his own person by taking 18 grams of potash (as phosphate and citrate) in one day. This increased the elimination of sodium chloride by 6 grams.

In his Physiological and Pathological Chemistry (Chapter VII (Conditions Governing Energy Metabolism And Total Food Requirement. Basal Metabolism Of The Adult)), Bunge records extended and interesting observations and discussion upon the relation of diet to the craving for salt, and concludes that while one might live without the addition of salt to the food even on a diet largely vegetarian, yet without salt we should have a strong disinclination to eat much of the vegetables rich in potassium, such as potatoes. "The use of salt enables us to employ a greater variety of the earth's products as food than we could do without it." But also, according to Bunge: "We are accustomed to take far too much salt with our viands. Salt is not only an aliment, it is also a condiment, and easily lends itself, as all such things do, to abuse." While Bunge's explanations may not be entirely adequate in detail, there seems to be little doubt as to the correctness of his main deductions.

Since the sodium chloride taken with the food passes through the body and is excreted by the kidneys without undergoing any chemical change, the rate of excretion quickly adapts itself to the rate of intake within wide variations.

When no chloride is taken, the rate of excretion falls rapidly to a point where the daily loss is only a very small fraction of the amount ordinarily consumed and excreted. Thus in an experiment by Goodall and Joslin* in which a healthy man was placed upon a diet adequate in protein and energy value but practically free from salt, the excretion of chlorine on each of 13 successive days was-respectively: 4.60, 2.52, 1.88, 0.87, 0.69, 0.48, 0.46, 0.40, 0.26, 0.22, 0.22, 0.17, 0.17 grams.

Cetti in ten days of fasting excreted all together 13.13 grams, and Belli in ten days on a diet poor in salt lost 11.8 grams of sodium chloride. In Benedict's recent study of prolonged fasting * his subject lost 8.44 grams of chlorine (equivalent to 13.93 grams sodium chloride) during the first ten days, 2.13 grams chlorine during the second ten days, and 1.57 grams chlorine during the third ten days of the fast. (The detailed data may be found on a later page.) Since the body is supposed to contain about 100 grams of sodium chloride, it will be seen that even when there was complete deprivation of salt for ten to thirty days, the total losses did not exceed 10 to 20 per cent of the amount estimated as usually present in the body. The salt thus readily given off by the body has been regarded by some as a measure of the excess which the body has been forced to carry in consequence of the extravagant amounts of salt which are commonly taken with the food. Magnus-Levy, however, thinks that the reduced amount of sodium chloride left in the body after such a loss is "not a physiological optimum, but rather a physiological minimum."

* Goodall and Joslin, Transactions of the Association of American Physicians, Vol. 23, page 92 (1908).

Moderate variations in the amount of salt taken have no significant effect upon metabolism. Large amounts increase the quantity of protein catabolized, and, through overstimulating the digestive tract, may also interfere with the absorption and utilization of the food.