Doesn’t it seem like when it comes to nutrition there are more opinions than facts to go around? Every day we hear new theories about diets, and supplements, and the best foods to eat. My role is to take the mystery out of good nutrition, and look at the science. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger. And I’m here to bring you an evidence-based approach to the best way to live a healthier longer life.
Dark green leafy vegetables are the healthiest foods, with more nutrients per calorie than any other food on the planet. There are a few, though, you can overdo, for example spinach.
Even though dietary oxalates like those in spinach may have a limited effect on kidney stone risk in most people, there are some predisposing factors that can put anyone at risk.
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.” “Even 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 instead 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 may 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 may 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 is too much? I’ll answer all those questions, next.
Given their oxalate content, how much is too much spinach, chard, beet greens, chaga mushroom powder, almonds, cashews, star fruit, and instant tea?
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. So can taking megadoses of vitamin C. This guy went into kidney failure juicing spinach and beet greens, but he was also taking about 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 tea. 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.
To be clear, I encourage everyone to eat huge amounts of dark green leafy vegetables every day, the healthiest food on the planet. But if you follow this advice (and you should!), then just choose any of the other wonderful greens. If you eat regular boring amounts of greens (like a serving a day), then it doesn’t matter which you choose. I continue to eat spinach, beet greens, and chard all the time. It’s just that you can overdo those three, so when I’m trying to hit my pound-a-day green leafy quota, I personally do mostly kale, collards, and arugula, which also happen to have the added benefit of being cruciferocious!
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