Nutrients, in addition to nutrients, sensory or ballast, also contain anti-nutritive substances that may impede the use of nutrients from food. Naturally occurring anti-nutritional substances in food products include oxalic acid, which hinders the optimal use of minerals from food by forming sparingly soluble salts with them.
Oxalic acid is an organic, two-carbon carboxylic acid. Consumption of products rich in oxalic acid leads to the formation of water-insoluble salts – oxalates – with calcium and magnesium ions, iron or manganese, which reduces the bioavailability of these ingredients from food. Excess supply of oxalic acid along with food, in addition to nutrient absorption disorders, may lead to the formation of kidney stones, heart problems and arthritis.
Occurrence of oxalic acid
The highest amount of oxalic acid is found in plant products. The content of oxalic acid in plants depends on the stage, conditions, season of the plant’s growth, type of crop as well as the method and time of storage and food processing (an example can be cooking, which reduces the amount of oxalates in the product by up to 50%), coffee – smoking techniques and tea – brewing time. In addition to the supply of food, oxalic acid is also produced in the human body as a product of the metabolism of mainly vitamin C and amino acids such as serine, glycine and hydroxyproline.
Anti-nutritional action in relation to calcium
One of the main minerals, the deficiency of which can cause too much oxalic acid to be consumed is calcium. For anti-nutritive effects
Oxalic acid, in addition to its amount in the product, has a molar ratio of oxalic acid content to calcium. The higher the ratio of oxalic acid to calcium, the lower the bioavailability of this element from the product. The highest molar ratio of oxalic acid to calcium is shown
Rhubarb 8.5 mEq (COOH) 2 / mEq Ca
Sorrel 5.6 mEq (COOH) 2 / mEq Ca
Spinach 4.3 mEq (COOH) 2 / mEq Ca
Botwina 2.45 mEq (COOH) 2 / mEq Ca
3.9 mEq (COOH) 2 / mEq Ca coffee
Cocoa 2.6 mEq (COOH) 2 / mEq Ca
Black tea 1.13 mEq (COOH) 2 / mEq Ca
The products rich in oxalic acid also include nuts, mushrooms and soy. The molar ratio of oxalic acid to calcium above 1.0 indicates that calcium in such products occurs in the form of insoluble oxalates and is not absorbed by the body. It also reduces the absorption of calcium from other food products, due to the higher content of oxalic acid than calcium in the product.
Sources of oxalic acid in food
Due to the wide distribution and high intake, oxalic acid is a source of coffee and tea (80-90% of oxalate supply with diet). This is evidenced by the fact that products rich in oxalic acid such as rhubarb, sorrel, spinach are not consumed in as often and as much as coffee and tea. Among teas, the highest concentration of oxalic acid is represented by black teas (about 110 mg / 100 ml), followed by red teas (about 104 mg / 100 ml). Least oxalic acid contains green tea (about 48.5 mg / 100 ml).
The daily allowable food intake (ADI) for oxalic acid is about 250 mg / day.
Effect of consuming oxalic acid on health
With rare and low food intake, oxalic acid does not affect your health. However, a high, continuous supply of this ingredient along with a diet with low supply of calcium and vitamin D may cause calcium absorption disorders in the body and, as a consequence, disturb its balance, which may lead to diseases such as osteomalacia, osteoporosis, and tetany.
Excess oxalates in the diet may also be a factor in developing kidney stones by forming insoluble calcium oxalate deposits in the kidneys. According to the American Dietetic Association, patients diagnosed with kidney stones and those at risk of developing it should limit the supply of oxalates with food to 40-50 mg per day. Calcium oxalate crystals may also appear in other organs, including in the joints causing their inflammation.
Prevention of anti-nutritive effects of oxalic acid
– limiting the consumption of products rich in oxalic acid or completely eliminating them from the diet in people suffering from kidney stones
– adding calcium-rich products to dishes containing a large amount of oxalic acid – (milk for coffee, black and red tea, natural yoghurt for sorrel soup)
– cooking products and casting used water
Consuming a large amount of oxalic acid with food is not recommended, but controlling the diet and the appropriate selection of food products can prevent its harmful effects. The anti-nutritive effects of oxalic acid should be especially taken care of by children, people with identified abnormalities in the work of the kidneys, liver and pancreas, as well as vegetarians and pregnant women. It is recommended to supplement the diet with calcium-rich products, as well as to eat products whose molar ratio of oxalic acid to calcium is below 1.0 which means that in addition to the high content of oxalic acid, the product contains a large amount of calcium.
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