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The tragic case in which a green smoothie cleanse shut down the kidneys of a woman who drank two cups of spinach a day for just 10 days is complicated by the fact that she had had a gastric bypass, and was on “prolonged” antibiotics, which can both increase absorption of the oxalates in spinach. So can taking megadoses of vitamin C. This guy went into kidney failure juicing spinach and beet greens, but he was also taking 2,000mg a day of vitamin C. “Vitamin C is metabolized to oxalate” inside the body, and likely played a role in his oxalate overload. In both cases, their juicing alone was giving them more than 1,200mg of oxalate a day, which is easy with spinach —just two cups a day — but practically impossible with most other greens, like kale, requiring more than six hundred cups a day.

There is one case of apparent dietary oxalate overload-induced kidney failure uncomplicated by surgery, antibiotics, or vitamin C. A man who had lost about 80 pounds eating a diet of greens, berries, and nuts, which evidently included spinach six times a day. Tragically, his kidney function never recovered.

Remember that study purporting to show a “massive” load of dietary oxalate didn’t have much of an effect on urine levels? That study went up to 250mg of oxalates a day. That is massive if you were talking about most greens. That would be 25 cups of collard greens, 60 cups of mustard greens, 125 cups of kale, or 250 cups of bok choy at a time. But, that’s less than one-half cup of spinach.

Spinach really is an outlier. Even though there’s small amounts of oxalates found throughout the food supply, spinach alone may account for 40% of oxalate intake in the United States. The Harvard cohorts found that men and older women who ate spinach eight or more times a month had about a 30% higher risk of developing kidney stones.

What if you cook it? Oxalates are water soluble, so, for example, blanching collard greens can reduce oxalate levels by up to a third; so those 25 cups at a time can then be 33! For low-oxalate greens, it doesn’t matter cooked or not, since they’re so low regardless.

Steaming spinach reduces oxalate levels 30%, and boiling cuts oxalate levels more than half. Boil the three high-oxalate greens — spinach, beet greens, and Swiss chard— and 60% of the oxalates are leached into the cooking water. They start out so high, though, even cooked would contain hundreds times more than low-oxalate greens like kale. For high-oxalate greens, it doesn’t matter cooked or not, since they’re so high regardless.

The bottom line is that anyone with a history of kidney stones, otherwise at high risk, or who eats cups a day should probably avoid the big three. This is especially important for those who juice or blend their greens, as oxalates appear to be absorbed more rapidly in liquid than solid form.

Another reason to give preference to low-oxalate greens is that they are less stingy with their calcium. While less than a third of the calcium in milks may be bioavailable (whether from a cow or a plant), most of the calcium in low-oxalate vegetables is absorbed. The calcium bioavailability in some greens is twice that of milk, but the oxalates in spinach, chard, and beet greens bind to the calcium, preventing the absorption.

Other high-oxalate foods that have been associated with kidney problems at high enough doses include chaga mushroom powder. Four to five teaspoons a day, and you can end up on dialysis. Four cups a day of rhubarb is also not a good idea. More than a cup a day of almonds, or cashews, and then star fruit, which I did a video on in the past. A single dose of about a cup and a quarter star fruit juice, or just 4-6 fruit…. Excessive tea consumption can be a problem, especially instant tea, which boosts urine oxalate nearly four times higher than brewed. Two cases of kidney damage have been reported, both of which were attributed to drinking a gallon of iced tea a day. Tea, like spinach, is super healthy; just don’t overdo it.

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