Mineral Availability and Techniques for Its Measurement.

In the study of mineral metabolism it is generally recognized that the total content of an element in a particular dietary source has little significance unless it is qualified by a factor indicating the biological ‘utilization’ of the element. However, there appears to be less agreement as to what constitutes ‘utilization’ and how it should be expressed. This has resulted in a number of terms, e.g. percentage availability (true and apparent), percentage utilization, percentage digestibility (true and apparent), percentage absorption, percentage retention, and such like, many of which have been used as synonyms by some authors though being given precise definition by others. Apart from any confusion in terminology there appear to be two different concepts of availability, though these are by no means mutually exclusive. In animal nutrition, particularly that concerned with the ruminant, more emphasis appears to be placed upon the degree to which the element can be extracted from different dietary sources. This implies that ‘availability’ is a characteristic of a particular source. Thus, many comparisons of feeds and feed supplements have been made using animals of the same age, with identical body stores and total intakes of the element; the resulting differences in utilization, often very considerable, are valid in this context. In other work, particularly studies with man, it is clear that the term ‘availability’ is being used to characterize the metabolic state of the body rather than the dietary source. Studies in which comparisons are made between subjects of different ages or with varied body status of the element, all receiving identical amounts of the same dietary source, clearly reflect this view of ‘availability’. The difference in degree of emphasis upon these two concepts is understandable in view of the very considerable mineral demands of high-producing farm livestock. When an animal, such as a dairy cow, may secrete amounts of calcium or phosphorus of 20-30 g or more per day, it is probable that the degree of extraction of these elements from the dietary source is the limiting factor. In the subsequent discussion of ‘availability’ reference will be made to the simplified flow scheme which, along with the relevant definitions of nomenclature,