A variety of vegetables are now in season and available fresh. While it's widely known that eating more vegetables is beneficial for health, many people also have questions: Vegetables contain oxalic acid, which can form precipitates with calcium. Can they be eaten together? If vegetables are high in oxalic acid, will eating too much of them lead to kidney stones? I've heard that oxalic acid is water-soluble. Should vegetables be blanched first to be safe?
Food experts state that research shows 75% of kidney stones are caused by calcium oxalate, so this concern is not entirely unfounded.
Food experts point out that when oxalic acid and calcium meet, they form insoluble calcium oxalate. Normally, the amount of calcium oxalate formed in the human body is small, is filtered by the kidneys, and excreted in urine. However, if the urine is too scanty and concentrated, or if a large amount of oxalic acid and calcium meet, it can lead to the gradual accumulation of calcium oxalate, forming crystals in the urine storage area, and eventually developing into kidney stones.
For preventing kidney stones, it is indeed important to reduce the amount of oxalic acid and calcium in the urine. However, food experts remind that these two substances are not as simple as "the more you eat, the more likely you are to get kidney stones; the less you eat, the less likely you are to get them." Therefore, it is necessary to clarify the following facts.
First, although oxalic acid is widely found in food, the human body's normal metabolism also produces it.
Second, not all oxalic acid from food is absorbed. Cooking and processing can remove some of the oxalic acid (for example, when blanching, oxalic acid dissolves in water). In the gastrointestinal tract, after oxalic acid binds with other substances like calcium, magnesium, and protein, the amount that actually enters the blood is further reduced. In addition, everyone's ability to absorb oxalic acid is different. Therefore, even if you eat the same amount of vegetables with the same oxalic acid content, the final amount of oxalic acid entering the body can vary greatly due to differences in cooking methods, food combinations, and gastrointestinal function.
Furthermore, the dietary intake of oxalic acid is not the main risk factor for kidney stones. Research has found that compared to the diets of healthy people without stones, the diets of kidney stone patients do not contain a higher intake of oxalic acid.
In addition, a large amount of oxalic acid in the kidneys must encounter a large amount of calcium to possibly form calcium oxalate stones. The less calcium excreted in the urine, the lower the risk of stones. Eating more green leafy vegetables to intake rich potassium and magnesium can improve calcium utilization and reduce the excretion of urinary calcium. Eating too much salt, meat, sweet drinks, etc., increases the amount of calcium excreted in the urine, which is not conducive to preventing kidney stones.
Finally, studies have shown that eating more calcium-rich foods is actually beneficial for preventing kidney stones. This may be because the calcium in food can bind with oxalic acid in the gastrointestinal tract to form insoluble calcium oxalate. Calcium oxalate is difficult to penetrate intestinal cells to enter the body and is instead excreted along with food residue.
These vegetables contain more oxalic acid
According to the U.S. Department of Agriculture database, green leafy vegetables rich in oxalic acid mainly include a few types: spinach has an oxalic acid content of 0.97%, amaranth 1.09%, chives 1.48 purslane 1.31%, and the highest is parsley (often used by restaurants to garnish plates), reaching 1.70%. To avoid excessive intake of oxalic acid, it is better to blanch these vegetables in boiling water to remove the astringent taste before eating.
Most vegetables, including common green leafy vegetables such as cabbage, rapeseed, kale, celery, Chinese cabbage, green lettuce, and broccoli, have low oxalic acid content, so there is no need to blanch them. Bamboo shoots, bitter melon, and water bamboo have moderate to high oxalic acid content. In addition, soybeans, all kinds of seeds, and nuts, which are not vegetables, also generally contain a considerable amount of oxalic acid.