DEAR DR. BLONZ: I read an article saying that a large consumption of spinach can cause kidney stones. This is one of my favorite vegetables, so I want to know if this is something I need to be concerned about. -- S.T., Fremont, California
DEAR S.T.: Spinach is one of my favorite foods, as well. I am not sure what the article meant by a “large consumption” of this healthful food, but here is some information to put the issue in perspective.
Spinach, along with a number of other leafy greens and plant foods, contains varying levels of oxalic acid, a naturally occurring organic compound produced by these plants. There are also mineral elements in these plants, and when oxalic acid reacts with one of these, such as calcium, it becomes calcium oxalate -- an insoluble salt. There are different types of kidney stones (also called urinary calculi), but calcium oxalate is the predominant composition of these solid particles. The reason calcium oxalate stones are such a menace when they form in the kidney is that they block the ducts involved with the production and elimination of urine.
With spinach, we have a vegetable that does indeed contain oxalic acid, and it also contains calcium. One half-cup of spinach contains about 100 milligrams of calcium, but spinach is not really considered to be a good source of dietary calcium. This is because substances need to be in a soluble form to pass through the absorptive surface of the intestines. When eaten, the calcium in spinach is likely to get tied up as calcium oxalate, which means it is unlikely to get absorbed.
Compare this to kale, another leafy green. Kale contains calcium, but it only contains a fraction of the oxalic acid found in spinach. Thus, kale is considered a good source of calcium.
Spinach does have a bit more oxalic acid than calcium, so some of the oxalic acid excess can be absorbed. It is important to appreciate that oxalic acid is present in many foods, and it does not cause kidney stones to develop in an otherwise healthy individual. If, however, your body has displayed any tendency to form oxalate kidney stones, spinach would be on a “foods to avoid” list.
So, how to proceed? Assuming you are in good health, that there is no history of kidney stones in your family, and that your physician has expressed no concerns that you have a tendency to develop kidney stones, it seems dubious to avoid spinach on the mere chance that you “might” become a stone former.
For more information about kidney stones, see tinyurl.com/y7becp5d.
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