There has long been much speculation concerning the optimum content of protein in the dietary. The vegetarian generally eats a relatively low protein diet; the meat eater a high protein diet. Experiments have shown that even a single large dose of histidine, tyrosine, or cystine - three digestion products common to most proteins - when injected into the blood of a dog will seriously injure the kidneys. Experiments on rats have shown that high protein dietaries, continued over a considerable period, tend to produce nephritis. Hinhede states that the occurrence of nephritis parallels the protein consumption in Greenland, Iceland, Faroe Islands, and Denmark. These areas are named in the order of decreasing protein intake.

A standard argument against a luxus consumption of protein is based on the excessive putrefaction of undigested protein residues in the colon with the consequent absorption of a series of products of bacterial origin which have no place in physiological processes and which are regarded by some as physiological abominations. The prevailing view among experimenters appears to be that a relatively low protein dietary furnishing proteins of high biological value is more likely to promote well-being than is the dietary containing an excess of protein.

The Calcium Content of Foods. All seed products, such as cereal grains, peas, beans, all tubers and fleshy roots, all meats and eggs, are too poor in calcium to supply the physiological needs of growing young. The only foods rich in calcium are the leaves of plants and the milk of animals. All seed products, meats, and eggs contain an excess of phosphorus in proportion to calcium. This ratio, as already stated, is unfavorable to the calcification of the bones. In order to provide a diet which is appropriately constituted in respect to calcium and phosphorus, therefore, the diet should contain an appropriate amount of either a leaf vegetable or of milk.

Meats are deficient in all vitamins except G, the anti-pellagra principle. Milk contains all the vitamins, but is somewhat deficient in B. Egg yolks contain the full quota of vitamins. All foods are deficient in vitamin D; but many of them contain traces of the mother substance - ergosterol - which is converted into the vitamin, provided one is exposed sufficiently to sunlight of good quality, that is, sunlight containing some ultra-violet rays.

During the last 100 years there has been a steady increase in the consumption of sugar. This is solely an energy food, since it contains no protein, no mineral elements, and no vitamins. The consumption of sugar is now approximately 115 pounds per person per year, not including molasses, corn syrup, and honey. This is about ten times the amount consumed in the United States one hundred years ago. While sugar is not inimical to health when eaten in such large amounts, a food so deficient in necessary principles tends to crowd out of the diet significant amounts of other foods which would supply the indispensable nutrients.

Since about 1880 the practice of using a highly refined white flour has been general. This is deficient in all the vitamins, is almost lacking in calcium, contains proteins of relatively poor quality, an acid ash, and is especially rich in potassium and phosphorus. Because of improved keeping qualities, degerminated corn meal is now extensively used, and wherever rice is eaten, polished rice, which is deprived of its vitamins and much of its mineral content, is the rule. The diet of many people in America and Europe may be accurately described as one consisting largely of products made from white flour and other cereal products having similar dietary qualities, meat, potatoes, and sugar. No combination of these foods makes an adequate diet. This list and any modification of it containing roots, tubers, and fruits is supplemented in an effective way by the inclusion of suitable amounts of milk or of leafy vegetables. It is for this reason that milk and the leafy vegetables have been distinguished from other foods as the protective foods.

The sensitiveness of certain tissues to deficiency of antiscorbutic vitamin makes it a matter of great importance that the diet should contain regularly suitable amounts of uncooked vegetable food, or of food canned by a process which preserves this vitamin.

It is beyond the scope of this chapter to discuss in detail the planning of menus which will be complete so as to promote optimum well-being. The objective was to set forth the principal scientific discoveries concerning foods and the nutritive needs of the body. The application of this knowledge to the planning of the diet will be found discussed in full elsewhere.1

1 McCollum, E. V., and Nina Simmonds,"Food, Nutrition, and Health," 2d ed., The Macmillan Company, New York, 1922.