"tri-peptid"; etc. Peptids result from the further hydrolytic cleavage of the peptones. Many peptids have also been made in the laboratory by the linking together of amino acids.

Substances simpler than the peptones but containing several amino acid radicles are often called "polypeptids."

Proteins in nutrition. The digestion products of protein absorbed from the digestive tract into the blood stream are rapidly distributed through the body and taken up by the muscles and other tissues. A part of the nitrogenous material thus received may be utilized for the growth or "repair ("upkeep" is perhaps a preferable term) of tissue material; the remainder is split, the nitrogen being eliminated chiefly as urea and the non-nitrogenous residue being either burned as fuel or converted into carbohydrate or fat.

It should be kept in mind that in the full-grown, well-nourished organism, no increase of protein tissue ordinarily occurs; hence all the protein received from the food is burned as fuel, whether it first serves for the repair or upkeep of the body tissue or not. The exact nature of the "repair" process in the tissues is not fully known. It is also uncertain to what extent the food must supply the exact amount of each individual amino acid which is to enter into the constitution of the body proteins. It is certain that the body can make glycine (glycocoll), while it cannot make cystine or tryptophane (certainly at least not at a sufficient rate to meet its needs). Hence the protein of the food need not contain glycine radicles but must contain cystine and tryptophane radicles if it is to serve fully the nutritive requirements of the body. It is now believed that lysine cannot be made in the animal body, though the full-grown animal can maintain itself for a long time with only very small amounts of lysine. The evidence in regard to the ability of the body to make certain other amino acids is less clear. Probably either

1 For fuller discussion see Chemistry of Food and Nutrition, Revised Edition, Chapters III, V, VIII.

arginine or histidine or both and either tyrosine or phenylalanine must be supplied by the food protein.

The energy value to the body of average food protein is 4.0 Calories per gram, or 1814 Calories per pound.

Ash Constituents

Sulphur compounds. Sulphur occurs in the food, as it does in the body, chiefly as a constituent of proteins. Since sulphur is essential to the constitution of the body proteins, it is obviously important that sufficient of this element shall be supplied by the food; but all food proteins contain sulphur, and though the percentages of sulphur in individual proteins show considerable differences, the different proteins of the same food material usually tend to balance each other in this respect so that the sulphur content of the total protein (or the ratio of sulphur to nitrogen) is about the same for most staple foods as for the body.

Hence it is believed that under ordinary conditions food which supplies adequate protein will thereby supply adequate sulphur, so that usually sulphur need not be considered as a separate factor in determining food values, but may be regarded as sufficiently provided for when the protein requirement is covered.

When the digestion products of the food proteins are burned in the body, the greater part of the sulphur is oxidized to sulphuric acid and excreted as sulphates.

Phosphorus compounds. Phosphorus compounds are essential to all the tissues of the body, and it is important that they be adequately supplied by the food.

The various articles of food differ greatly in phosphorus content, nor does the amount of phosphorus run even approximately parallel with the protein content (as does the sulphur). Hence the phosphorus compounds of food materials should be carefully considered in forming judgments of nutritive values.

The phosphorus compounds of foods may be grouped into four general classes, one inorganic and three organic: (1) inorganic phosphates; (2) phosphorized proteins, including the phosphoproteins such as casein and the nucleoproteins characteristic of cell nuclei; (3) phosphorized fats or phospholipins, such as egg lecithin; (4) combinations of phosphoric acid with carbohydrates or with closely related substances such as inosite. This last group includes phytin and the related compounds.

All three groups of organic phosphorus compounds are more or less completely oxidized in the body, the phosphorus being finally excreted almost entirely in the form of phosphate. The phosphates of the food while entering and leaving the body in essentially the same form are nevertheless utilized in some very important nutritive functions such as furnishing material for bone structure and facilitating the maintenance of the normal neutrality or slight alkalescence of the blood and the body tissues. (The normal condition of the blood and tissues is described either as neutral or as slightly alkaline, according to the definition of neutrality used.)

Chlorides. Sodium chloride (common salt) is an essential and a prominent constituent of the blood and other body fluids. Carnivorous animals, eating the blood as well as the flesh of their prey, obtain in this way sufficient salt for their needs along with their organic foodstuffs; man and the herbivora take salt in addition to that naturally contained in their food. Salt is now such a cheap and popular condiment that it is commonly added to the food in quantities which make the natural chloride content of the food a matter of no practical consequence.

While sodium chloride enters and leaves the body in the same form, it performs important functions. From it the hydrochloric acid of the gastric juice is made and chiefly to it the normal solvent power and osmotic pressure of the blood and other body fluids are due. The nature of these functions makes plain the imperative need for salt, but also suggests that too much may

be objectionable. The quantities of salt now commonly eaten seem larger than necessary; whether larger than desirable is still an open question.

Sodium, potassium, calcium, and magnesium. Sodium occurs in the food chiefly in the form of sodium chloride; potassium chiefly as phosphate, as salts of organic acids, and perhaps in other organic combinations. The quantity of sodium present naturally in foods is usually not of great significance because of the large amounts added in the form of common salt used as a condiment. Potassium is particularly abundant in many of the vegetables. To a certain extent sodium and potassium appear to act antagonistically in the body, so that the large quantity of potassium taken in when such vegetables are eaten freely must be balanced by the taking of common salt.

There must also be maintained in the body a proper balance between sodium and calcium. For example, the rhythmical contraction and relaxation of heart muscle which constitutes the normal beating of the heart is dependent upon this muscle being bathed by a fluid containing the proper concentration and quantitative proportions of sodium and calcium.

In other directions there appear to be somewhat analogous balancings of calcium and magnesium, and of calcium and phosphorus.

Since these elements are not only not interchangeable but are in some respects mutually antagonistic, it is evident that each must be supplied in sufficient quantity to permit the proper performance of its specific functions. In the case of sodium, the liberal use of salt as a condiment insures a more than ample supply. Potassium and magnesium appear to be sufficiently abundant in most staple foods so that it is not usually necessary to specifically consider these elements in estimates of food values. Calcium is not always sufficiently abundant even when the food is freely chosen; hence the richness of a food in calcium is a factor affecting its nutritive value. American dietaries are

probably more often deficient in calcium than in any other chemical element. Milk and many vegetables are relatively rich in calcium, and for this reason as well as others they are deserving of a more prominent place in our dietaries.

Iron. Iron occurs in the food almost entirely in organic form as a constituent of certain proteins. The simpler forms, chiefly inorganic, in which iron is given medicinally may or may not have the same nutritive effect as the food iron.

The greater part of the iron in the body exists as an essential constituent of the hemoglobin of the blood, the remainder being chiefly in the chromatin substances of the cells. There is no considerable reserve store of inactive iron in the body corresponding to the stores of phosphorus and calcium in the bones. Hence if the food fails to furnish as much iron as is expended in the nutritive processes and excreted by the body, there must soon result a diminution of hemoglobin, which if allowed to continue is marked by a greater or less degree of anæmia. Thus although only small amounts of iron are contained in the food or involved in the nutritive processes, its function as a building material for the red blood cells is conspicuously important. Iron salts and other simple compounds of iron have long been used medicinally in the treatment of anæmia and are undoubtedly often helpful, but very extensive investigation leaves it still an open question whether hemoglobin is made from these forms as advantageously as from the iron-protein compounds of the food. Perhaps there are organic groups in these latter compounds which play a part in hemoglobin formation as well as the iron itself.

For the present at least it seems much better to look to the food and not to medicines or mineral waters for the supply of iron needed in normal nutrition and since freely chosen food does not always furnish enough iron to meet satisfactorily the requirements of the body, it follows that the iron content is a factor of some importance in the consideration of food values.