Raw spinach, some nuts like cashews, and other raw health foods can be high in oxalic acid. This is something of an anti-nutrient, which at best tends to reduce absorption of nutrients and at worst can build up in the kidneys and cause kidney stones.
As with all diet data though, YMMV. Some people eat loads of yaw spinach and never develop kidney stones.
Certainly if you have a familiar history of them, you should avoid raw spinach. Cooking it reduces the oxalates.
So... another case of "too much of a good thing?" I feel like the point comes up again and again... eat a variety of foods... everything in moderation? No single food is a cure-all or magically going to fix your problems. I'm sure if you eat too much of damn near anything, there will be some negative effects.
Yes. That's my take away after years of "paying attention" to diet and health. Countless articles read, videos watched, diets tried, trends analyzed.
Through it all, it seems a general rule of thumb is:
Eat whole foods. If it requires processing in a factory, avoid it. This includes many things that you might not intuitively think of. Vegetable oils, for example are a highly processed food.
Aside from avoiding processed foods, and eating whole ingredients... Yes, eat a variety of different foods. Eat vegetables, eat carbohydrates, eat meat. Just don't eat too much of any one thing, and if you can identify something that doesn't sit well with you, avoid it!
Spinach famously contains iron, but less famously also contains a substance that inhibits iron absorption. So IIRC, it actually lowers your iron, unless you blanch it and throw away the water.
Nothing indicates that the decimal point error ever was made, but the account about it will most likely live a long and colorful life, just like its parent myth
Spinach is a fictional plant, it was described by Linné as sort of a joke but then everyone just went with it.
But seriously, you described three layers of misconceptions in this comment thread, how is anyone supposed to know "the real truth" about anything food related, if spinach alone is such a hard subject?
Apparently there is no documentary evidence why an 1870 measure of spinach's iron content was exaggerated. The paper cited does not explore whether it ever was exaggerated, or what its actual iron content then or now might be.
I read various reports indicating that modern vegetables have much less of various nutrients than older, slower-growing or smaller-yielding varieties, and have no idea how I might evaluate such claims. Maybe spinach harvested in 1870 had more iron than highly-fertilized 1930 varieties, never mind 2021 varieties no one, to my knowledge, has bothered to measure. Or maybe not.
As with all diet data though, YMMV. Some people eat loads of yaw spinach and never develop kidney stones.
Certainly if you have a familiar history of them, you should avoid raw spinach. Cooking it reduces the oxalates.