It is evident from what has already been seen that the custom which has been more or less prevalent of referring to the ash or mineral matter of a food as if it were a substance is wholly illogical and incorrect. Food ash is always a mixture of the compounds of several different elements, and each element has its own functions and significance in nutrition. Even elements so closely related chemically as are sodium and potassium, or calcium and magnesium, are not only not interchangeable, but are, in some of their functions, directly antagonistic in their action in the body. Bunge's experiment showing the effect of potassium upon sodium excretion has already been noted. Meltzer and his associates have shown that the injection of magnesium salts has a marked general inhibitory effect, and that this can be quickly overcome by the subsequent injection of calcium salt. Summarizing the results of extended series of investigations by himself and others, Meltzer stated, in the Transactions of the Association of the American Physicians for 1908:

"Calcium is capable of correcting the disturbances of the inorganic equilibrium in the animal body, whatever the directions of the deviations from the normal may be. Any abnormal effect which sodium, potassium, or magnesium may produce, whether the abnormality be in the direction of increased irritability or of decreased irritability, calcium is capable of reestablishing the normal equilibrium."

More recently Hart and Steenbock have found that the addition of magnesium salts to an otherwise well-balanced ration tends to cause a loss of calcium from the body. Several other observers have reported similar unfavorable effects of magnesium upon the metabolism of calcium, and some are inclined to regard this as a matter of much importance to the well-being of the body. On the other hand, calcium seems to exert a favor-able influence upon the economy of iron in metabolism, inasmuch as it appears to be possible to maintain equilibrium upon a smaller amount of iron when the food contains an abundance of calcium.

It would thus appear that an adequate study of the subject should take account of the relative, as well as the absolute, amounts of the different inorganic elements of the food. Tables showing these elements for the different articles of food are included in the Appendix at the back of this book. Not only do the different food materials differ greatly in the absolute and relative abundance of the different elements, but the same is also true of the total food intake of different groups of people. Studies of 150 freely chosen American dietaries each covering the food of a group of people for a week or more show the following range and average intake, per man per day and per 3000 Calories.

Inorganic Elements In 150 American Dietaries

Since these dietary records did not show the quantities of salt used, the figures for sodium and chlorine in the table cover only the amounts in the food as purchased and are greatly below the actual intake of these elements. It will be seen that the intake of any given element may be widely different in the different dietaries, even though each represents the daily average for at least a week. To some extent this is due to the variable amounts of total food consumed, but even when the data are reduced to a uniform basis of 3000 Calories the differences between minimum and maximum are still quite wide.