Biologically unavailable

Raw defatted cottonseed flours contain 1.2—2.0% gossypol [303-45-7] (7) (19). When cottonseed is treated with moist heat, the S-amino group of lysine and gossypol forms a derivative that is biologically unavailable thereby inactivating gossypol but further lowering the effective content of lysine. 

About 50% of copper in food is absorbed, usually under equitibrium conditions, and stored in the tiver and muscles. Excretion is mainly via the bile, and only a few percent of the absorbed amount is found in urine. The excretion of copper from the human body is influenced by molybdenum. A low molybdenum concentration in the diet causes a low excretion of copper, and a high intake results in a considerable increase in copper excretion (68). This copper—molybdenum relationship appears to correlate with copper deficiency symptoms in cattle. It has been suggested that, at the pH of the intestine, copper and molybdate ions react to form biologically unavailable copper molybdate (69). 

Alimentary biotin deficiency is rare. It may, however, occur in patients on long-term parenteral nutrition lacking biotin or in persons who frequently consume raw egg white. Raw egg white contains a biotin-binding glycoprotein, called avidin, which renders biotin biologically unavailable. Pharmacological doses of the vitamin (1-10 mg/d) are then used to treat deficiency symptoms. There are no reports of toxicity for daily oral doses up to 200 mg and daily intravenous doses of up to 20 mg [2]. 

Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

Selenate is the most mobile form of Se. Selenium becomes biologically unavailable by reduction to elemental Se or by formation of metal selenides or Se-sulfides. Inorganic Se compounds can be converted to volatile organic Se such as dimethyl selenide or dimethyl diselenide by... 

Sediment reduction t,/2(est.) = 1196 h, t,/2(exptl) = 825 h for chemical available phenanthrene and tA = 151 h for bioavailable phenanthrene for amphipod, P. hoyi in Lake Michigan sediments at 4°C. The average uptake clearance from sediment was (0.041 + 0.023)g of dry sediment-g-1 of organism-h-1, and the rate constants to become biologically unavailable was (0.0055 0.003) h-1 resulting a bioavailable t,/2 = 126 h (Landrum 1989) desorption t,/2 = 8.6 d from sediment under conditions mimicking marine disposal (Zhang et al. 2000). 

In terrestrial vegetation, molybdenum and sulfur interfere with copper-induced deficiencies (Gupta 1979). Copper poisoning in cattle and other ruminants is governed by dietary concentrations of molybdenum and sulfate (Lewis et al. 1967 Todd 1969 Buckley and Tait 1981 Eisler 1989). Molybdenum and sulfur in mammalian diets cause a decrease in the availability of copper because of the formation of the biologically unavailable copper-thiomolybdate complex (Aaseth and Norseth 1986). Cattle die when grazing for extended periods on pastures where the ratio of copper to molybdenum... 

Paraquat is strongly adsorbed to soils and sediments and is biologically unavailable in that form however, it is not degraded significantly for many years, except in surface soils. In surface soils, paraquat loss through photodecomposition approaches 50% in 3 weeks. In freshwater ecosystems, loss from water column is rapid about 50% in 36 h and 100% in 4 weeks. In marine ecosystems, 50 to 70% loss of paraquat from seawater was usually recorded within 24 h. 

The degradation rate of paraquat in certain soils can be slow, and the compound can persist for years — reportedly in a form that is biologically unavailable. But data are missing or incomplete on flux rates of paraquat from soil into food webs and on interaction dynamics of paraquat with other herbicides frequently applied at the same time. It seems prudent at this time to keep under close surveillance the residues of paraquat in soils in situations where repeated applications have been made over long periods of time (Summers 1980). 

Biologically available organic contaminants may find their way into the food chain and be toxic, but they are also more easily decomposed and are thus removed from the environment. Slowly decomposed biologically available organic compounds, such as pesticides, may have long lifetimes in soil and thus pose a hazard to animals and humans. Biologically unavailable compounds, such as tars, produce undesirable characteristics in soil, such as water... 

Encapsulated within the pores of soils are aqueous solutions, the so-called soil solutions. Radio-tracer experiments have shown that there is a rapid, dynamic equilibrium between metal ions in the soil phase and the soil solutions. Hence, the soil solution is regarded as the transport medium for moving the essential nutrients from soils to plants. The soil solutions are highly variable in composition and properties, e.g. the pH can vary over the range 2-11. Alkaline soils are notorious for causing metal ion deficiency in plants, principally because many of the micronutrients form insoluble hydroxides and so are biologically unavailable. 

The half-life of M-MT is dependent on the binding affinity of thionein for different metal ions. For instance, upon oxidation, Cu-MT forms insoluble polymers which are biologically unavailable and are eventually eliminated via biliary secretion. In contrast, thionein has lower affinity for Zn, making it more easily released from the protein and rendering the ion available for cellular processes. Furthermore, the rate of degradation may be influenced by differences in metal distribution between MT isoforms. It has been determined that MT degradation can occur in lysosomal and nonlysosomal (cytosolic) compartments. 

Unavailable Niacin in Cereals Chemical analysis reveals niacin in cereals (largely in the bran), but this is biologically unavailable, because it is bound as niacytin - nicotinoyl esters to a variety of macromolecules ranging between Mr 1,500 to 17,000. In wheat bran, 60% is esterified to polysaccharides, and the remainder to polypeptides and glycopeptides (Mason et al., 1973). In calculation of niacin intakes, it is conventional to ignore the niacin content of cereals completely. 

A proportion of the vitamin Be in foods may be biologically unavailable after heating, as a result of the formation of (phospho)pyridoxyllysine by reduction of the alditnine (Schiff base) by which pyridoxal and the phosphate are bound to the e-amino groups of lysine residues in proteins. A proportion of this pyridoxyUysine may be useable, because it is a substrate for pyridoxamine phosphate oxidase to form pyridoxal and pyridoxal phosphate. However, it is also a vitamin Be antimetaboUte, and even at relatively low concentrations can accelerate the development of deficiency in experimental animals maintained on vitamin Be-deficient diets (Gregory, 1980a, 1980b). 

Gross clinical deficiency of vitamin Be is extremely rare. The vitamin is widely distributed in foods (although a significant proportion in plant foods may be biologically unavailable Section 9.1), and intestinal flora synthesize relatively large amounts, at least some of which may be absorbed and hence available. 

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