In experimental anemia in dogs due to blood loss the term "available iron" as determined by the dipyridyl test has no physiological significance. Iron salts (100 per cent available by dipyridyl) given in optimum dose (560 mg. per 2 weeks) will cause a net production of 50 to 55 gm. hemoglobin above the control base line in anemic dogs. This means that an iron salt which is rated as 100 per cent available by the dipyridyl test is only 35 per cent physiologically available.

The term "available iron (dipyridyl)" simmers down to iron not in the form of hematin compounds. The absorption of this "available iron" is conditioned by a great variety of factors, many unknown at this time.

Iron is indeed an elusive sprite whose "availability" or comings and goings cannot be determined in dogs by dipyridyl—perhaps only in part by studies of absorption and excretion.

Liver contains "available iron (dipyridyl)" but also organic factors influencing hemoglobin regeneration in anemia as liver ash contains only about 50 per cent the potency of the whole liver.

One can readily dissociate the iron from other potent factors in various tissues. Fractions of heart, liver, spleen, and kidney may contain very little iron yet cause much hemoglobin regeneration in anemic dogs.

No investigator has reported any condition of copper deficiency in man or dog. In fact, in anemias copper is usually above normal concentration in the liver. It is unlikely, therefore, that in experimental anemia in dogs and in the various anemias of man, any significance attaches to the intake of copper.

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