Abstract
Oxalic acid and its salts occur as end products of metabolism in a number of plant tissues. When these plants are eaten they may have an adverse effect because oxalates bind calcium and other minerals. While oxalic acid is a normal end product of mammalian metabolism, the consumption of additional oxalic acid may cause stone formation in the urinary tract when the acid is excreted in the urine. Soaking and cooking of foodstuffs high in oxalate will reduce the oxalate content by leaching. The mean daily intake of oxalate in English diets has been calculated to be 70-150 mg, with tea appearing to contribute the greatest proportion of oxalate in these diets; rhubarb, spinach and beet are other common high oxalate-content foods. Vegetarians who consume greater amounts of vegetables will have a higher intake of oxalates, which may reduce calcium availability. This may be an increased risk factor for women, who require greater amounts of calcium in the diet. In humans, diets low in calcium and high in oxalates are not recommended but the occasional consumption of high oxalate foods as part of a nuritious diet does not pose any particular problem.
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The oxalic acid content is variable within some species;some cultivars of spinach (Universal, Winter Giant) contain 400 to 600 mg/100 g, while others range from 700 to 900mg/100 g. Oxalic acid accumulates in plants especially during dry conditions. A study comparing two cultivars of spinach, Magic (summer) and Lead (autumn), revealed that the summer cultivar contained greater amounts of oxalate(740 mg/100 g fresh weight, FW) than the autumn cultivar(560 mg/100 g FW). Reports of some tropical leafy vege-tables revealed that dry vegetables had higher oxalate con-centrations than did fresh vegetables.
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Toxic effects of oxalates
The ingestion of 4–5 g of oxalate is the minimum dose capable of causing death in an adult, but reports have shown that 10–15 g is the usual amount required to cause fatalities. Oxalic acid ingestion results in corrosion of the mouth and gastrointestinal tract, gastric haemorrhage, renal failure and haematuria. Other associated problems include low plasma calcium, which may cause convulsions, and high plasma oxalates. Most fatalities from oxalate poisoning are apparently due to the removal of calcium ions from the serum by precipitation. High levels of oxalate may interfere with carbohydrate metabolism, particularly by succinic dehydrogenase inhibition; this may be a significant factor in death from oxalate toxicity caused by animals grazing in pastures which contain high levels of H. glomeratus.
Although sorrel is a herb and not normally consumed in high quantities, there has been one report of fatal oxalate poisoning after a man consumed an estimated 6–8 g of oxal-ate in vegetable soup containing 500 g of sorrel. Both fatal and non-fatal poisoning by rhubarb leaves is thought to becaused by toxic anthraquinone glycosides rather than byoxalates as corrosive gastroenteritis was not observed. These anthraquinone derivatives may also occur in the roots and stems of rhubarb or sorrel grass.
Experiments involving the consumption by eight women of more than 30–35 g/day of cocoa, a high oxalate foodstuff, provoked symptoms of intoxication including loss of appetite, nausea and headaches. However, cocoa contains theobromine(1500–2500 mg/100 g) and tannic acid (4000–6000 mg/100g), both of which are more toxic than the oxalic acid present(500–700 mg/100 g).3There appears to be a great deal of confusion as to what was responsible for these poisonings and it would be unwise to assume only one factor was the cause.
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Normal urine is usually supersaturated with calcium oxalate. The normal urinary Oxalate content of foods and its effect on humans. excretion of oxalate is less than 40–50 mg/day with less than 10% coming from the diet. Intakes of oxalate exceeding 180 mg/day lead to a marked increase in the amount excreted. Small increases in oxalate excretion have pronounced effects on the production of calcium oxalate in the urine, implying that foods high in oxalate can promote hyperoxaluria (high oxalate excretion) and increase the risk of stone formation. As well as causing significant increases in urinary oxalate excretion in healthy individuals, rhubarb, spinach, beet, nuts, chocolate, tea, coffee, parsley, celery and wheat bran have been identified as the main dietary sources in the risk of kidney stone formation. However, it has been reported that black tea increased oxalate excretion by only 7.9%, compared with increases of 300 and 400% for spinach and rhubarb, respectively.
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Conclusions
Foods high in oxalates should be consumed in moderation to ensure optimum intake of minerals from the diet. Although some foods are reported to be high in calcium and other essential minerals, the amount available may be limited due to the presence of oxalates. For instance, spinach is a high calcium food (93–111 mg/100 g FW), yet due to its high oxalate content (779 mg/100 g FW) the calcium availability is almost negligible. The availability of magnesium, iron, sodium, potassium and phosphorus may also be restricted. It is recommended that high oxalate foods be accompanied by calcium-rich foods such as dairy products and shellfish.
High oxalate foods should be cooked to reduce the oxalate content. Soaking raw foods will also reduce the oxalate content but other useful nutrients such as vitamin C may also be lost at the same time. Oxalates tend to occur in higher concentrations in the leafy parts of vegetables rather than in the roots or stalks.
Vegans, vegetarians, women or inhabitants of tropical countries should be aware that some foods contain high levels of oxalates. The diets of vegans and those with lactose intolerance may be low in calcium due to the exclusion of dairy products, unless supplemented by some other high cal-cium food products. If high oxalate foods were to be con-sumed in conjunction with a low calcium diet, then the consumer may be at risk of hyperoxaluria and stone forma-tion. It appears that leafy tropical plants tend to contain higher levels of oxalates than plants from temperate climates.People living in these areas are at possible risk of stone formation due to hyperoxaluria, and mineral deficiencies if sufficient minerals are not consumed.
Women tend to be more susceptible than men to calcium and iron deficiencies, while osteoporosis is of concern to females, especially after menopause. Therefore, women should eat red meats, which are low in oxalate, rather than vegetables, which can be high in oxalates, to satisfy their iron intake. The risk of stone formation is three times greater in males and thus, they should avoid eating excess amounts of high oxalate foods. Sufferers of hyperoxaluria and kidney stones are also advised to restrict their diet to low-oxalate foods because although urinary oxalate arises predominantlyf rom endogenous sources, it can be influenced by dietary intake. People suffering from fractures should also be aware of the potential effects of oxalates on mineral availability,given that high calcium is required for bone repair.
The occasional consumption of high oxalate foods as part of a mixed diet does not pose health problems. Problems are more likely to occur in people with diets of little variety
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