Availability biological

Although modern analytical techniques have considerable precision and sensitivity, food composition tables carmot be considered to give more than an approximation to vitamin intake. Apart from the problems of biological availability (Section 1.1.2), there is considerable variation in the vitamin content of different samples of the same food, depending on differences between varieties, differences in growing conditions (even of the same variety), losses in storage, and losses in food preparation. 

When foods have been enriched with vitamins, because of the requirement for the food to contain the stated amount of vitamin after normal storage, manufacturers commonly add more than the stated amount - so-called overage. One of the problems in the debate concerning folate enrichment of flour (Section 10.12) is the relatively small difference between the amount that is considered desirable and the amount that may pose a hazard to vulnerable population groups, and the precision to which manufacturers can control the amount in the final products. In pharmaceutical preparations, considerable latitude is allowed the U.S. Pharmacopeia permits preparations to contain from 90% to 150% of the declared amount of water-soluble vitamins and from 90% to 165% of the fat-soluble vitamins. 

Many vitamins are absorbed by active transport tbis is a saturable process, and, therefore, the percentage that is absorbed will decrease as the intake increases. 

The fat-soluble vitamins (A, D, E, and K) are absorbed dissolved in lipid micelles, and, therefore, absorption will he impaired when the meal is low in fat. Gastrointestinal pathology that results in impaired fat absorption and steattorhea (e.g., untreated celiac disease) will also impair the absorption of fat-soluble vitamins, because they remain dissolved in the unabsorbed Upid in the intestinal lumen. Lipase inhibitors used for the treatment of obesity and fat replacers (e.g., sucrose polyesters such as Olestra ) will similarly impair the absorption of fat-soluble vitamins. 

Many of the water-soluble vitamins are present in foods bound to proteins, and their release may require either the action of gastric acid (as for vitamin B12, Section 10.7.1) or specific enzymic hydrolysis [e.g., the action of conjugase to hydrolyze folate conjugates (Section 10.2.1) and the hydrolysis of biocytinto release biotin (Section 11.2.3)]. 

The state of body reserves of the vitrimin may riffect the extent to which it is absorbed (by affecting the synthesis of binding and transport proteins) or the extent to which it is metabolized after uptake into the intestinal mucosa [e.g., the oxidative cleavage of carotene to retinaldehyde is regulated by vittunin A status (Section 2.2.1)]. 

Compounds naturally present in foods may have antivitamin activity. Many foods contain thiaminases and compounds that catalyze nonen-zymic cleavage of thiamin to biologically inactive products (Section 6.4.7). 

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Sulfur deficiency usually is not a problem for mminants because the mminal microflora can utilize sulfur-containing amino acids. A deficiency can occur, however, when an NPN source is fed. L-Methionine [63-68-3] is the most biologically available source of sulfur (21). Various sulfates are intermediate in sulfur avadabiHty, and elemental sulfur is the least available source of sulfur. 

Silver compounds having anions that are inherently toxic, eg, silver arsenate and silver cyanide, can cause adverse health effects. The reported rat oral LD values for silver nitrate, silver arsenate [13510-44-6] and silver cyanide are 500—800 (29), 200—400 (29), and 123 mg/kg (30), respectively. Silver compounds or complexes ia which the silver ion is not biologically available, eg, silver sulfide and silver thiosulfate complexes, are considered to be without adverse health effects and essentially nontoxic. 

Subgroup on Metals of the Tri-Academj Committee on Acid Deposition, Acid Deposition Effects on Geochemical Cjcling and Biological Availability of Trace Elements, National Academy Press, Washington, D.C., 1985. 

The destiny of most biological material produced in lakes is the permanent sediment. The question is how often its components can be re-used in new biomass formation before it becomes eventually buried in the deep sediments. Interestingly, much of the flux of phosphorus is held in iron(lll) hydroxide matrices and its re-use depends upon reduction of the metal to the iron(ll) form. The released phosphate is indeed biologically available to the organisms which make contact with it, so the significance attributed to solution events is understandable. It is not clear, however, just how well this phosphorus is used, for it generally remains isolated from the production sites in surface waters. Moreover, subsequent oxidation of the iron causes re-precipitation of the iron(lll) hydroxide floes, simultaneously scavenging much of the free phosphate. Curiously, deep lakes show almost no tendency to recycle phosphorus, whereas shallow... 

Other d-metals are also vital to health. For example, chromium(III) plays a role in the regulation of glucose metabolism. Copper(I) is an essential nutrient for healthy cells and is the only biologically available Lewis acid with a + 1 charge. 

Upon administration, release the active ingredient for full biological availability... 

W. H. Barr, Factors involved in the assessment of systemic or biologic availability of drug products, Drug Inf. Bull, 3, 27—45 (1969). 

Price, T. D., J. H. Burton, and R. A. Bentley (2002), The characterization of biologically available strontium isotopes ratios for the study of prehistoric migrations, Archaeometry 44(1), 117-135. 

Nishita, H. and Larsen, K. H. (1957). Summary of Certain Trends in Soil-Plant Relationship Studies of the Biological Availability of Fallout Debris, Report No. UCLA-401 (National Technical Information Service, Springfield, Virginia). 

Montgomery, J.R., M. Price, J. Thurston, G.L. de Castro, L.L. Cruz, and D.D. Zimmerman. 1978. Biological availability of pollutants to marine organisms. U.S. Environ. Protection Agen. Rep. 600/3-78-035. 134 pp. Mora, M.A. and D.W. Anderson. 1995. Selenium, boron, and heavy metals in birds from the Mexicali Valley, Baja California, Mexico. Bull. Environ. Contamin. Toxicol. 54 198-206. 

Neel, J.W. and K.H. Larson. 1963. Biological availability of strontium-90 to small native animals in fallout patterns from the Nevada test site. Pages 45-49 in V. Schultz and A.W. Klement, Jr. (eds.). Radioecology. Reinhold, New York. 

Most metals, for which compounds are carcinogenic, are from IV group of Periodic Table of Elements. In biological systems, carcinogenic metals can form stable complexes and biological availability of these complexes determines the carcinogenic potential of various metal compounds. 

Luoma, S. N. (1989). Can we determine the biological availability of sediment-bound trace elements Hydrobiologia, 176/177, 379-396. 

The species of components present will also be affected by oxidation-reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation-reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods. 

Water and cations trapped between layers are held so strongly that they do not exchange with water and cations in the surrounding environment and thus are not biologically available. For instance, ammonium trapped between the clay layers is not a source of nitrogen for plants. Also, it will not be oxidized by bacteria to nitrite and nitrate, and thus will not be a potential source of nitrate or nitrite pollution. 

Polycarbonyl and polyamino compounds include chelates, which are manmade compounds, and siderophores, which are biologically derived and more specialized. They can help dissolve inorganic ions, keep ions in one specific ionic state, and maintain them in biologically available forms. Because of their ability to bring ions into solution, they are often used to extract specific ions in specific forms from soil. Cationic micronutrients, particularly iron and zinc, are often applied in chelated form as a foliar spray to alleviate nutrient deficiencies. This is particularly true of plants growing in basic soil and is done even when plants do not show micronutrient deficiencies. Typically, both foliar and soil applications are effective in this regard. 

The structures show the lone pairs of electrons that form bonds with metal cations. Both chelates and siderophores hold micronutrient cations in solution and in a biologically available form until the cations can be used by the... 

Because of its common occurrence and biological importance, it is an essential micronutrient for most organisms. Thus, a number of analytical procedures for the analysis of iron species have been developed and typically concentrate on biologically available species [9],... 

However, many analyses will require extraction of the component of interest before analysis is carried out. It may be essential to remove it from the soil matrix and/or interfering components before reliable analyses can be made. Extraction may be designed to extract only a portion of the analyte of interest. For example, extraction of a particular oxidation state may be important because oxidation states can affect the mobility and biological availability of chemicals. It may also be necessary to concentrate the extract after isolation. 

One of the questions that arises in soil analysis is whether a determination of the total amount of a component in soil is desired or if just the biologically available amount is more relevant. Related to this is the question of which species of the component is present. In some cases, speciation is of utmost importance. For instance, chromium can be present as Cr(III) or Cr(VI). Cr(VI) is more toxic and thus of greater concern [15], This concern is also related to the biological availability of a specific species. In this case, while knowing the total chromium content (i.e., the sum of Cr(III) and Cr(IV)... 

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