Reviewer: Liu Min (Central South University Third Xiangya Hospital)
Author: Peng Yonghan (Shanghai Changhai Hospital)
Many friends are very careful to prevent kidney stones through diet, and come to ask the doctor if they can eat tofu? When it comes to tofu, I can’t help but think of delicious pan-fried tofu, mapo tofu, stinky tofu… But some people say that tofu is untouchable, and think that “eat less tofu and less stones”. How did the soft tofu and the hard stone become a group?
Image source: Zhanku Hailuo
It’s a long story, everything in this world comes from various combinations of the periodic table. As a socially active molecule, the “calcium” element is quite high in soybeans, the raw material of tofu. Coincidentally, about 80% of kidney stones are composed of a component called “calcium oxalate”, and most people with this type of stone have an extremely high concentration of calcium ions in their urine.
Those who oppose tofu believe that eating tofu means taking calcium tablets. If there is more calcium in the body, more calcium will be excreted in the urine, and the risk of kidney stones will come. Therefore, “eat less tofu and produce less stones” sounds reasonable at first glance, so many people believe it to be true.
However, we don’t grow what we eat. The subtlety of the human body is that eating more does not mean absorbing more. When tofu enters the gastrointestinal tract, it is digested and decomposed into “calcium” ions, which are absorbed into the human body by the intestinal tract. In the intestine, a large amount of “calcium” ions are combined with “oxalic acid”, “citric acid”, “fatty acid”, etc., while only a very small amount of “calcium” ions are excreted from the urine under the regulation of hormones and kidneys in vitro.
It can be seen that under the arrangement of this mechanism, it is more difficult for free “calcium” ions in urine to form stones. Let’s look at the calcium content of tofu. Erliang tofu contains about 150 mg of calcium. Even eating tofu every day has little effect on urinary calcium concentration!
1. Also rich in calcium, milk even reduces the risk of kidney stones
Some foreign studies also support this view. However, Westerners are not studying tofu but milk. 100 ml of milk contains about 120 mg of calcium, which is similar to the calcium content of Erliang tofu. So, will drinking more milk increase the risk of stones?
Studies show that not only does it not increase, but drinking more milk reduces the risk of calcium-containing kidney stones, while deliberately low-calcium diets or extra calcium supplements can increase the risk of kidney stones disease risk [1].
Of course, milk is not the same as Chinese tofu, but in general, there is no reliable evidence that “eating less tofu will lead to fewer stones”, and eating in moderation may be beneficial.
Image source: Zhanku Hailuo
2. Patients with high urinary calcium may need to eat less tofu
However, pathological conditions can lead to abnormally high levels of calcium ions in the urine, increasing the risk of calcium supersaturation leading to the formation of kidney stones. The development of hypercalciuria is a disease, and the daily urinary calcium excretion is more than 200 mg, which is indeed the most common metabolic abnormality in patients with calcium-containing kidney stones. Therefore, people who are worried about tofu may wish to go to the hospital to check whether there is such hypercalciuria. If so, don’t really eat it.
All in all, tofu cannot be blamed for stones. Because eating tofu produces stones, it’s not just tofu.
Image source: Zhanku Hailuo
References
[1]. Taylor E N, Curhan G C. Dietary calcium from dairy and nondairy sources, and risk of symptomatic kidney stones[J]. The Journal of urology, 2013, 190(4) : 1255-1259.
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