Network diagram ■ Oxalic acid in spinach will be greatly reduced during cooking and heating, so there is no need to worry about stones
■ Pregnant women should not avoid eating spinach, because spinach contains not only oxalic acid, but also folic acid
■ The effect of eating spinach to supplement iron is not good, because the iron contained in spinach is non heme iron, and the absorption and utilization rate of this iron by the human body is very low
Some people say that spinach cannot be eaten together with tofu. Eating "spinach tofu soup" will lead to stones. Because spinach is rich in oxalic acid and tofu is rich in calcium, oxalic acid and calcium combine to produce calcium oxalate precipitation, so eating spinach tofu is easy to form stones in the body.
Oxalic acid in spinach is indeed related to the formation of stones, but the formation of stones is not simple addition. The key to stone formation also depends on whether the "raw materials" are sufficient and the proportion is appropriate. Oxalic acid in spinach will be greatly reduced during cooking and heating, so there is no need to worry about stones. Scientific experiments show that spinach blanched in boiling water for 60 seconds can remove 80% oxalic acid.
Soybean is also a food with high oxalic acid content, but when making bean products such as tofu, there is little oxalic acid left. When making tofu, magnesium chloride (brine) or calcium sulfate (gypsum) should be added. This process not only reduces oxalic acid, but also significantly increases the content of calcium and magnesium. Investigation shows that magnesium is very beneficial to prevent kidney stones. Calcium is another important substance for stone formation, so some people think that "tofu cannot be matched with spinach". In fact, calcium in tofu is also beneficial to prevent stones. Research has found that calcium in food can combine with oxalic acid, thus reducing the absorption of oxalic acid by the human body and preventing kidney stones.
Others say that spinach can supplement iron, and people with iron deficiency should eat more spinach. Is there any scientific basis for this statement?
The effect of eating spinach to supplement iron is not good, because the iron contained in spinach is non heme iron, and the human body's absorption and utilization rate of this iron is very low. The iron in the diet is divided into heme iron and non heme iron. The heme iron is easy to be absorbed by the human body and mainly exists in red meat and liver blood of animals; In vegetable food, beans, cereals and eggs, non heme iron is generally not easy to be absorbed by the human body. In contrast, if the absorption rate of total heme iron in animal food is 10 points, the absorption rate of iron in plant food is less than 1 point, only a fraction of zero, a difference of more than ten times.
In theory, the absorption rate of iron in spinach is lower than that of non heme iron in other plant foods, because oxalic acid and other substances will affect the absorption of iron. Moreover, the iron content of spinach is not high in vegetables. Therefore, the effect of spinach iron supplementation is not good, and it does not have much significance.
Therefore, there is no need to avoid spinach, no matter with tofu or other food, it is OK. Although the iron supplement effect of spinach is not ideal, it is rich in vitamin C, which can promote the absorption of iron, and the spinach itself tastes good. When cooking spinach, you should blanch it in water first, so you don't have to worry about oxalic acid. Pregnant women should not avoid eating spinach, because spinach contains not only oxalic acid, but also folic acid.
(The author Yu Kang is the chief physician of the Nutrition Department of Peking Union Medical College Hospital)
