Tannins iron absorption

Tannins decrease iron absorption if tea is consumed with meals. 

Some authors caution that tannins in St. John s wort (as well as those in fever few, borage, saw palmetto, and chamomile) might inhibit iron absorption. Study is needed to determine if this is a concern. 

Black tea has been shown to reduce iron absorption, especially in individuals who have an increased risk of anemia (Disler et al. 1975 Merhav et al. 1985). The postulated mechanism of action is the formation of iron complexes with black tea tannins in the gut (South et al. 1997). However, in a review of clinical trials and epidemiological studies of black... 

Silicate is determined spectrophotometrically with ammonium molybdate and ammonium vanadate. The pH of the sample must be adjusted to 7-8. Potassium cyanide is added to prevent interference of heavy metals. Oxalic acid is added to destroy mo-lybdophosphate and vanadophosphate and to bind aluminum in a complex. As in all spectrophotometric determinations, high and variable optical absorption of the sample (due to color or turbidity) at the wavelengths of investigation causes errors tannin, iron, and sulfide also interfere. To avoid contamination, all contact surfaces should be of polyethene. It is also possible to determine silicate using FAAS, in which case nitrous acid and ethyne must be used as flame gases. As silicates are present in colloidal form, the sample must be introduced into the AAS equipment using an ultrasonic nebulizer. Such a nebulizer is also used when silicate is measured by ICP-AES. 

Rapeseed meals frequently contain tannins. These are polyphenolic compounds that complex with proteins and carbohydrates to form enzyme-resistant substrates with a consequent lowering of digestibility. This may also result from the combination of the tannins with digestive enzymes, with a consequent loss of activity. Tannins may cause damage to the intestinal mucosa and are known to interfere with iron absorption. 

Non-heme iron exists in plant products and its bioavailability is compromised by the concurrent ingestion of tannins, phytates, soy, and other plant constituents, that decrease its solubility in the intestinal lumen. Bioavailability of non-heme iron is increased by concurrent ingestion of ascorbic acid and meat products. Nonheme iron is reduced from the ferric to the ferrous form in the intestinal lumen and transported into enterocytes via the divalent metal transporter (DMT-1). Once inside the enterocyte, iron from heme and nonheme sources is similarly transported through the cell and across the basolateral membrane by the ferroportin transporter in conjunction with the ferroxidase hephaestin after which it can be taken up by transferrin into the circulation. The regulation of iron across the basolateral membrane of the enterocyte is considered the most important aspect of iron absorption. 

In the commoner cases the base is a hydrated metallic oxide (of aluminium, tin, lead, zinc or, less often, chromium, iron, copper, antimony) to which the colouring matter (if acid) is united by true chemical combination tannin lakes are also made (with basic colouring matters). In other lakes the base is an inert substance (barium sulphate, precipitated alumina and silica, chalk, gypsum, kaolin, etc.), on which the colouring matter is fixed by simple mechanical absorption. Lakes of the former kind may be mixed, either fraudulently or for the purpose of attenuating the colour, with inert materials. 

Ascorbic acid increases absorption (see above) but its use (200 mg/day) is not clinically important in routine therapy desferrioxamine binds iron and reduces absorption (see Poisoning, below) tea (tannins) and bran reduce absorption. 

The interactions between different foods have sparked some interest. For exam-pie, if rice is consumed with orange juice, the orange juice can enhance the absorption of the iron in the rice. This effect results from the chelation of the iron by the ascorbate in the juice and the increased absorbability of the iron from the complex, On the other hand, if rice is consumed with tea, the tannins in the tea can reduce the absorption of the iron in the rice because the iron in the iron-tannin complex is not readily available. In general, including meat in the diet can increase the availability of iron (iom other foods. The mechanism of this effect is not dear. The availability of heme iron is not much influenced by other components of the food known to influence availability of nonheme iron. These components include ascorbate, tannins, phytate, phosphates, and fibers. 

The iron in meats is in the form of heme, which is readily absorbed. The non-heme iron in plants is not as readily absorbed, in part because plants often contain oxalates, phytates, tannins, and other phenolic compounds that chelate or form insoluble precipitates with iron, preventing its absorption. Conversely, vitamin C (ascorbic acid) increases the uptake of non-heme iron from the digestive tract. The uptake of iron is also increased in times of need by mechanisms that are not yet understood. Iron is absorbed in the ferrous (Fe ) state (Fig. 44.6), but is oxidized to the ferric state by a ferroxidase known as ceruloplasmin (a copper-containing enzyme) for transport through the body. 

Tannins are thought to form insoluble complexes with iron and thus re-duee its absorption. " ... 

Fortifying foods with minerals and vitamins is becoming more and more common. Mineral deficiency is one of the most important nutritional problems in the world. The best method to overcome this problem is to make use of an external supply, which may be nutritional or supplementary, like the fortification of foods with highly bioavailable mineral sources. Major interests of mineral encapsulation are linked to the fact that this technique enables to reduce mineral reactions with other ingredients, when they are added to dry mixes to fortify a variety of foods, and it can also incorporate time-release mechanisms of the minerals into the formulations. For example, iron is the most difficult mineral to add to foods and ensure adequate absorption, and iron bioavailability is severely affected by interactions with food ingredients (e.g., tannins, phytates, and polyphenols). Additionally, iron catalyses the oxidative degradation of fatty acids and vitamins (Schrooyen et al., 2001). 

The composition of these materials has been investigated further by IR and NMR spectroscopy, and the absorption of a tannin as a representative polyphenolic compound to iron-containing and iron-free clays investigated. A sample of low grade oil sand, containing a considerable percentage of clay, was also examined for the presence of clay-bound organic materials. 

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