Little weight can be attached to such statements regarding the iron content of foods as are based upon the data obtainable from the ordinary tables of ash analyses, since these have usually been obtained by methods which are likely to greatly overestimate the amount of iron. In the following table are shown the approximate amounts of iron now believed to be present in the average edible portion of typical food materials expressed (1) in milligrams per 100 grams of edible material, (2) in milligrams per 100 grams of protein, (3) in milligrams per 3000 Calories:

Iron In Typical Food Materials

Percentages of iron in some other foods will be found in the tables of ash constituents in the Appendix. Using these recent data for iron in food materials, approximate estimates of the amounts of iron contained in 150 American dietaries have been made. The majority of these were found to furnish 14 to 20 milligrams of iron per man per day. Apparently therefore the typical American dietary does not contain any such surplus of iron as would justify the practice of leaving the supply of this element entirely to chance. The available data rather indicate that foods should be selected with some reference to the kinds and amounts of iron compounds which they contain.

Meats

In meat as ordinarily eaten the iron exists largely as hemoglobin, due to the blood contained in the muscular tissue as usually sold and prepared for the table. Muscular tissue washed free from blood contains iron, but the amount is comparatively small. Since fatty tissue contains much less iron, the iron content of fat meat is much lower than that of lean, and in order to establish any useful estimate of the amount of iron in meat it is practically necessary to consider the lean tissue alone or to refer the iron to the protein content rather than to the gross weight of the meat. When expressed on the former basis, the results will still be influenced by the extent to which the blood has been either accidentally or intentionally removed from the muscle.

For fresh lean beef containing the full proportion of blood, the results obtained by most investigators are in satisfactory agreement, and the average figure, 0.00375 per cent iron in the fresh meat free from visible fat, can be accepted with little danger of serious error. This corresponds to about 15 to 16 milligrams of iron per 100 grams of protein in beef, and since no certain differences in iron content in the flesh of different species have been shown, it is assumed for the present that approximately the same ratio of iron to protein will hold for meats in general.

The iron of meat, as already mentioned, is largely due to the blood retained in the muscular tissue. The nutritive value of blood is often questioned. So far as the iron compounds of the blood are concerned, it seems to be established that hemoglobin and hematin may be absorbed and assimilated to some extent, but probably not to such good advantage as the iron compounds of eggs, milk, and vegetable foods.

Eggs

The edible portion of hens' eggs has shown as the average of several analyses 0.00303 per cent of iron. Whether the iron content of eggs can be increased by giving to poultry food rich in iron, is a disputed question.

There can be no doubt regarding the assimilation and utilization of the iron compounds of eggs, since they serve for the production of all the iron-holding substances of the blood and tissues of the chick, there being no possibility of the introduction of iron from without during incubation.

Milk

Analyses of samples of cow's milk of various origin have given results varying from 0.0002 to 0.0003 per cent, and averaging 0.00024 per cent of iron in the fresh substance.

It cannot be doubted that the iron of milk is readily absorbed and assimilated, since this constitutes the sole natural source of iron for all young mammals during a period of rapid growth. Moreover, metabolism experiments indicate that the iron of milk is likely to be utilized to especially good advantage, perhaps on account of its association with a high proportion of calcium.

The question of the iron supply of infants fed upon diluted or modified cow's milk may, however, be considered at this point. It is now generally recognized that the best substitute for mother's milk is obtained by diluting whole cow's milk or top milk with a solution of lactose or maltose. By varying the richness of the milk or top milk used and the amounts of water and sugar added, the composition of the modified milk can be controlled at will. In order to ascertain whether the iron compounds of milk tend to condense upon the fat globules or for any other reason are altered in their distribution by the rising of the cream, a sample of milk was allowed to stand, and after the cream had risen, the iron and nitrogen contents were determined separately in the upper half, containing all of the cream, and in the lower half, which consisted of skimmed milk. These analyses showed in the upper half 0.000277 per cent of iron and 0.54 per cent of nitrogen; in the lower half 0.000293 per cent of iron and 0.59 per cent of nitrogen. It is evident, therefore, that the ratio of iron to nitrogen was practically the same in the cream as in the milk. It is therefore important to recognize that the iron content of cow's milk is little if any higher than that of human milk, while the protein content is at least twice as high; that any modification of cow's milk which reduces its protein content will reduce the iron content in practically the same proportion, and that an infant fed upon cow's milk, modified or diluted to contain less than 3 per cent of protein, is probably receiving food poorer in iron than human milk. According to present estimates an infant fed on any modification of cow's milk must consume the equivalent of nearly a quart of undiluted milk or cream in order to obtain as much iron as is supplied daily in the milk of the average healthy nursing mother. Since no such quantity of cow's milk can safely be fed in early infancy, it is to be expected that during the first months of life the artificially fed infant will use up the surplus store of iron with which it was born more rapidly than will the child of the same age which receives the milk of a healthy mother.