After the discovery of the vitamins it became obvious that a conventional chemical analysis of a food might give little indication of its real nutritive value, as the relative quantities of these accessory food factors usually are too small to be thus measured. As time passed it became increasingly plain that a similar philosophy applies to dietary constituents which can be measured; in addition to a chemical analysis, the utilization of these constituents can be examined by the methods of biologic assay. Through this device it has been shown that a wide variation exists in the availability to the organism of dietary essentials in various forms in our foods. A relatively simple method of determining the availability of iron, for instance, is to measure hemoglobin regeneration in an animal on an iron free diet to which the source of iron to be tested is added. Sherman, Elvehjem and Hart1
UNAVAILABLE NUTRIENTS. JAMA. 1945;128(16):1168. doi:10.1001/jama.1945.02860330036014
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