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Kidney stones affect as many as 1 in 10 people in their lifetime and can cause excruciating pain. Makes me cross my legs just thinking about them! Oxalate stones are the most common type, forming when the oxalate concentration in your urine gets so high it basically crystallizes out of solution, like rock candy. Some foods, like spinach, have lots of oxalates in them. Should we try to reduce our intake of oxalates to lower our risk? It turns out that people who do get stones don’t seem to eat any more oxalates on average than people who don’t get stones. It may be less what you eat, and more what you absorb. People who are predisposed to kidney stones just appear to be born with a higher intestinal oxalate absorption. Their guts just really suck it up: “so-called ‘super absorbers'” assimilate up to “50% more oxalate than non–stone formers.”

Overall, “the impact of [typical] dietary oxalate” on the amount of oxalates that end up in the urine “appears to be small.” “[E]ven a massive dose” of dietary oxalates typically only “results in a relatively mild increase” in the amount that makes it into your urine. A 25-fold increase in oxalate consumption doesn’t even double the concentration of oxalates flowing through your kidneys, so it’s really more determined by genetics than diet. But still, until you get your first stone, how do you know if you’re a super absorber or not? Is it safer to just generally avoid higher-oxalate fruits and veggies? People who eat more fruits and vegetables may actually tend to get fewer kidney stones. When researchers put it to the test and removed produce from people’s diets, their kidney stone risk indeed went up.

Removing fruits and veggies can make your dietary oxalate intake go down, but your body produces its own oxalate internally as a waste product, that you have a more difficult time getting rid of without the alkalizing effects of fruits and vegetables on our urine pH. This may help explain why those eating plant-based get fewer kidney stones (but it also may be due to their cutting animal protein intake, which can have an acid-forming effect in the kidneys). We’ve known this for 40 years. Just a single can of daily tuna fish can increase your risk of forming stones 250%. And even just cutting back on animal protein may help cut kidney stone risk in half.

Surely there’s some level of oxalate intake that could put people at risk regardless. There have been a few rare cases reported of people drinking green juices and smoothies getting oxalate kidney stones, though most had extenuating circumstances. This case describes a woman whose kidneys shut down after a 10-day juice cleanse, which included two cups of spinach a day.

Normally we might not expect a cup or two of spinach to cause such a violent reaction, but she had two aggravating factors—she had gastric bypass surgery (which can increase oxalate absorption) and a history of “prolonged” antibiotic use. There’s actually a friendly bacteria you want in your colon, called oxalobacter, that eats oxalate for breakfast, leaving even less for us to absorb, but it can get wiped out by long-term broad spectrum antibiotic use.

She still probably wouldn’t have run into a problem, though, if she would have used something other than spinach or beet greens or Swiss chard, the trifecta of high-oxalate greens. Kale has hundreds of times less oxalates than spinach. She would have had to have juiced in excess of 650 cups of kale a day to get a comparable dose, so over those ten days more than 6,000 cups of kale. But are the three high-oxalate greens only a problem for people with extenuating circumstances or who are otherwise at high risk? And what if you cook the greens? How much would be too much? I’ll answer all those questions, next.

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