Bioavailability oxalic acid

Hot water-extractable B and ammonium oxalate/oxalic acid-extractable Mo in 200 Egyptian arid soils ranged from 0.39-3.42 mg/kg and 0.030-0.395 mg/kg, respectively. DTPA-extractable Zn and Mn were from 0.377-11.93 and 7.28-51.87 mg/kg, respectively. EDTA-extractable Cu varied from 3.25-34.32 mg/kg. The average bioavailable B, Mo, Zn, Mn and Cu was 1.216, 15.02, 15.6, 0.185 and 1.53 mg/kg, respectively. These soils were sampled from both wheat and maize fields. The soils had 1-24.1% CaC03 and 7.57-8.95 pH. Bioavailable Zn varied considerably among the arid soils studied. 

In 16 soils sampled from maize fields, hot water-extractable B and ammonium oxalate/oxalic acid-extractable Mo were 0.702 0.208 and 0.100 0.088 mg/kg, respectively. Bioavailable B and Mn varied from... 

Review of studies on the effect of oxalic acid on calcium bioavailability in rats and in humans indicates that most of the research was done between 1930 and 1950. Decreased availability of calcium in young rats was reported when spinach containing oxalic acid was fed with low calcium diets. The extent of the effect of oxalic acid on calcium availability was shown to be related to levels of calcium and oxalic acid, as well as the presence of vitamin D in the diet. In human studies there was generally no effect of oxalic acid on calcium balance however, in a few studies decreased calcium balances were reported. There is recent evidence that oxalic acid consumed along with a moderately high level of fiber intake may have adverse effects on calcium balance of human subjects. 

Several comprehensive reviews on oxalic acid have been published in which effects on calcium metabolism were discussed (7-11). A review of studies on the effect of oxalic acid on calcium bioavailability in rats and humans is presented in this paper. 

In the studies on humans there appeared to be decreased calcium balances when 200 g or more of spinach per day was included in the diet. In two of the studies in which women were fed spinach, calcium intakes were below the Recommended Dietary Allowance of 800 mg/day (37). Some studies were conducted for short period of a week or less, which may not be sufficient time to adjust to a change in diet. From measurement of calcium excretion in urine after a test meal, it was shown that the calcium in oxalate-containing vegetables was less well-absorbed than that of milk or of vegetables not containing oxalic acid. However, this would not necessarily affect calcium balance, since the total amount of calcium in the diet would have to be considered. The effect of a combination of oxalic acid and fiber on calcium bioavailability should be further investigated. 

Little agreement has been reached as to which dietary components or which food processes physiologially affect mineral availability. Many plant foods contain phytic acid, oxalic acid or other dietary fiber components that can be shown to chelate minerals. The effect of these dietary substances upon the final bioavailability of the mineral in question will depend upon the digestibility of the chelate (106). 

Little information is available on the effect of oxalic acid on zinc bioavailability. In one study, Welch et al. ( fed weanling rats zinc-deficient diets with and without 0.75% sodium oxalate. The rats were dosed orally with zinc-labeled spinach leaves or zinc-labeled zinc sulfate. Dietary oxalate enhanced the availability of radioactive zinc from zinc sulfate, but had no effect on zinc from spinach leaves. Absorption and retention of zinc was greater from spinach leaves than from zinc sulfate. 

The reason for the differences in zinc balances between these two studies was not clear. In an effort to determine if the oxalic acid in spinach in the first study could be responsible for at least part of the decrease in bioavailability of zinc in the first study, we determined oxalic acid content of feces. Mean oxalic acid excretion in feces when the subjects were on the highest fiber diet in study 2 was about 1/2 that on the higher fiber diet in study 1 (210 and 423 mg/day, respectively). Another factor which might help explain the results was the length of the dietary periods the dietary periods in study 1 were 5 days longer than in study 2. 

Oxalic acid is poorly absorbed with a bioavailability of 2-5%. It is excreted unchanged in the urine. Normal urinary oxalic acid excretion ranges from 8 to 40 mg day... 

In the effort to expand the potential chemotherapeutic drug candidates, quinoxalines and related heterocycles were examined. Condensation of the 1,2-diaminobenzene with oxalic acid provides the quinoxaline tautomer in high yields. Treatment of this mixture of tautomers with phosphoryl trichloride provides the multifunctional ring. Displacing a chloride with sodium azide provides the tetrazole, which is considered as a planar acidic analogue of the carboxylic acid functionality. It can increase binding affinity,potency, and bioavailability.Condensation of the hydrazine with benzaldehyde completes these drug candidates. 

In the fields of environmental and exploration geochemistry, considerable use has been made of sequential extraction procedures in order to determine the bioavailability and geomobUity of trace metals (including V, Cr, As, Se, Cd and Pb). More recently, sequential extractions have been used to elucidate information relating to the speciation of metals in soils and sediments. The extraction protocol developed by Tessier " has traditionally been used (with modifications) and involves the sequential use of a variable cocktail of extractants including acetic acid, sodium acetate, magnesiumchloride, EDTA, acidified hydroxylamine hydrochloride, oxalic acid, sodium... 

Many other dietary factors have been reported to affect calcium bioavailability. Phytate, fiber, cellulose, uronic acids, sodium alginate, oxalate, fat (only in the presence of steatorrhea), and alcohol have been reported to decrease calcium bioavailability (15). Lactose and medium chain triglyceride increase it (15). FTuoride also affects calcium retention primarily by stimulating bone formation thereby decreasing calcium excretion (33-38). The effects of fluoride on calcium utilization have been variable (34,38,39). 

Bioavailability of Ca2+ is affected by numerous physiological conditions, including age, sex, genetic make-up, stress, hormonal status, health status, and nutritional habits. In addition to these intrinsic factors, certain dietary components, such as fiber and oxalate, form insoluble complexes with Ca2+ and interfere with its absorption. Another putative culprit in this category is phytic acid. 

Dietary Reference Intake (DRI) of Cu, 17-18% of the DRI of K, P, and Fe, and between 5 and 13% of the DRI of Zn, Mg, and Mn (Table 5.1). Potatoes are generally not rich in Ca, but can be a valuable source of trace elements, such as Se and I, if fertilized appropriately (Eurola et al., 1989 Poggi et al., 2000 Turakainen et al., 2004 Broadley et al., 2006). Moreover, since potato tubers have relatively high concentrations of organic compounds that stimulate the absorption of mineral micronutrients by humans, such as ascorbate (vitamin C), protein cysteine and various organic and amino acids (USDA, 2006), and low concentrations of compounds that limit their absorption, such as phytate (0.11-0.27% dry matter Frossard et al., 2000 Phillippy et al., 2004) and oxalate (0.03% dry matter Bushway et al., 1984), the bioavailability of mineral elements in potatoes is potentially high. 

Phosphorus extracted from sediment by NaOH has been related to non-occluded, surface-exchangeable, bioavailable forms (22). Hydrochloric acid extraction yields occluded phosphorus incorporated in hydrous metal oxides, carbonate and phosphate minerals of sediment. Hydroxylamine reagent specifically removes hydrous manganese oxides, while amorphous hydrous oxides of iron and aluminijm are removed by the oxalate reagent. Total available sediment phosphorus analyses includes sediment organic phosphorus components in addition to the inorganic portion determined by the selective extraction procedures. 

Other organic acids which have been tested for their impact on iron absorption include oxalic, succinic, fumaric and lactic acid (Table V). The iron from iron phytates was bioavailable only in the soluble monoferric form, whereas the less soluble diand tetra-ferric phytates were very poor iron sources (37,... 

Iron bioavailability is affected by valence state, form, solubility, particle size, and com-plexation which in turn may be affected by the food matrix. Complexation of iron has been found to have either a positive or negative effect on availability, with such compounds as ascorbic acid and fructose increasing availability and oxalates, phytates, phosphates and food fibers perhaps decreasing availability. Availability has also been shown to be directly correlated to acid solubility. We have found that acidity tends to increase ionization as well as favoring the ferrous state which has greater solubility at... 

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