Availability of minerals

Sources of metals or the mineral resources are distributed quite non-uniformly over the Earth s surface. No country can claim that it has domestic supplies of all the minerals it needs. The situation and outlook as to the availability of minerals thus varies from country to country and commodity to commodity. 

Although sesame seed is not known to contain any protease inhibitors or other ANFs, high levels of oxalic and phytic acids may have adverse effects on palatability (Ravindran and Blair, 1992) and on availability of minerals and protein (Aheme and Kennelly, 1985). Decortication of seeds almost completely removes oxalates, but it has little effect on phytate (Ravindran and Blair, 1992). Complete decortication is difficult because of the small size of the seeds. 

There is a lot of unanswered and important questions around bioavailability. In the literature there is a tendency to describe upper limits for availability of the minerals in the diet. Do the "bioavailabilities" represent reliable measurements of the amount of these elements available for absorption, or do they only give values for the average absorption for the narticular circumstances of the studies Is it possible to describe upper limits for availabilities of minerals Do we really increase the absorbed and utilazed amount of a mineral by fortification with the mineral in question Do we forget that there is a cooperation between all the elements present in the diet which determine the bioavailability ... 

Soil chemistry and structure determine the amount and availability of minerals and water to organisms. For instance, soil pH determines whether Ca2+, PO43-, and other essential ions are bound tightly to its particles or are free to be absorbed by organisms, or else leached away in water runoff. Likewise, plants tend to adapt to a specific soil pH. Soil is a restrictive environment for most animals, because relatively little food and oxygen are available, and movement through densely packed dirt, rocks, sand, and organic debris is difficult. On the... 

Determination of human requirements for dietary trace minerals necessarily includes knowledge of the factors which affect the availability of minerals for absorption Interactions of dietary minerals with organic constituents of the diet and with other minerals are complex Careful study of mineral absorption in human subjects is required to delineate dietary requirements and the factors which affect them ... 

In summary, the above observations and relationships indicate that the supply and availability of mineral surface area may be a primary control... 

SuGiuRA S, Dong F and Hardy R (1998) Effeas of dietary supplements on the availability of minerals in fish meal preliminary observations. Aquaculture 160 283-303. 

Bio-based nature and independence from fossil fuels. Because PHAs are produced from renewable resources, they are independent of the availability of mineral oils as feedstocks under the precondition that the generation of energy for the PHA production process itself is also based on renewable resources, the independence from fossil fuels is valid for the entire production process (see also Sect. 2.7). 

Methods of wet analysis were slow to develop as lack of acids made it difficult to bring many materials into solution. Mineral acids were not available until the Middle Ages, then first prepared by unknown alchemists. Nitric acid was first mentioned in the thirteenth century as aqua fortis (strong water) by which silver could be separated from gold. It was the availability of mineral acids that enabled wet methods of analysis to be developed for a wide range of materials. 

Plants need water and carbon dioxide along with sunlight for photosynthesis. Shortage of water in the soil and low insolation slow down photosynthesis. Crop yield depends also on the availability of minerals (fertilizer). The utiUzable water available to the plant is the difference between the retention capacity of the soil and the limit of extraction, and this capacity is dependent on the type of soil. In arid zones, the water which is available to the plant is only a fraction of the water received by the soil because the latter ends up in different places, for instance, as runoff water, seepage water lost or diverted, or water which is a constituent part of the soil and is not extractable by the roots. If the water extracted by the roots is insufficient, the plant will wilt and may eventually reach the permanent wilting point. Each plant requires a certain depth of soil for occupation by its roots and the water... 

Table 6.2 Examples of the relative availability (%) of mineral elements from mineral compounds...

The availability of mineral elements is commonly high in young animals fed on milk and milk products but declines as the diet changes to solid foods. An additional complication is that the absorption, and hence apparent availability, of some mineral elements is under homeostatic control (determined by the animal s need for them). Iron absorption, discussed in Chapter 8, is the clearest example of this effect, but in ruminants the efficiency of calcium absorption also appears to be dependent on the animal s requirements. 

Whilst no attempt is made here or in Chapter 8 to provide a complete list of factors affecting the availability of mineral elements, those mentioned serve to illustrate why availability coefficients are not included in tables of food composition. The availability of a mineral in a particular food depends so much on other constituents of the diet and the type of animal that average availability coefficients would be of little significance. 

Just as with other nutrients, a measure of the concentration of the element alone is not sufficient to describe its usefulness to the animal. Attempts have been made to assess the availability of minerals using chemical methods, such as solubility in water or dilute acids, but these have had little success. At present animal experiments are the only reliable way to measure mineral availability (see Chapter 10). 

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