Green, leafy vegetables are one of the most nutrient-dense foods you can put on the table. In fact, when the Centers for Disease Control and Prevention stratified foods based on their nutrient density per calorie, watercress, Chinese cabbage, and chard topped the list. All are green, leafy vegetables that have a health halo. Leafy greens are versatile too. You can enhance a salad with a generous serving of greens or saute a plate of them and enjoy a delicious side dish. Plus, you can add greens to sandwiches, wraps, smoothies, soups, and stews.
The Oxalate Conundrum
Despite the nutrient density of greens, some leafy greens are high in oxalates. What are oxalates? They’re organic compounds naturally found in certain foods. Science shows that oxalates bind to minerals. When you eat a plate of greens, the oxalates attach to the minerals in foods and block their absorption. The bound minerals are then eliminated from your body and you don’t get the benefits since they pass out of your body without the body processing them. However, the oxalates in greens don’t interfere with the absorption of calcium from dairy foods, just plant-based foods.
Should you be concerned about the impact oxalate has on mineral absorption? After all, it’s important to get enough minerals in your diet, especially calcium, magnesium, iron, and zinc. As it turns out, not all leafy greens are an abundant source of oxalates. Some, like kale, have very little of these natural, mineral-binding compounds. Leafy greens that are highest in oxalates include beet greens, purslane, rhubarb, spinach, chard, endive, and dandelion greens. In contrast, kale, turnip greens, and collard greens have relatively low levels of oxalates. For example, beet greens have 916 milligrams of oxalate per serving, whereas collard greens only have 74 milligrams/serving. Therefore, greens vary widely in their oxalate content.
Leafy greens aren’t the only foods that contain oxalates. Rhubarb, almonds, cashews, peanuts, and dry cocoa powder are high in oxalates too. You can find charts online that list the oxalate concentration of common foods.
Should You Be Concerned about the Oxalates in Leafy Greens?
Despite the ability of oxalates to bind to minerals, it’s unlikely that you’ll develop a mineral deficiency by eating oxalate-rich greens, especially if you consume a nutrient-dense diet. However, it’s best to eat a diversity of greens rather than eating only greens that are high in oxalates. In other words, don’t eat platefuls of spinach, Swiss chard, and beet greens every day since they have the highest levels of oxalates. Enjoy the nutritional abundance that lower oxalate greens offer, like kale, turnip, and collard greens.
What about cooking green, leafy vegetables? Do certain cooking methods reduce oxalate content? Most cooking techniques, like sautéing, baking, steaming, or roasting, have only a minor impact on the oxalate content of greens. At most, these cooking methods reduce oxalates by 10 to 15%. However, the cooking method matters. A study published in the Journal of Agricultural Food Chemistry found that boiling vegetables high in oxalates can lower their oxalate content by 30 to 90% a bit more by boiling leafy greens for a long time, but you’ll also lose a significant amount of the nutrients you’re eating them for, particularly vitamin C and some B-vitamins.
Who Should Avoid Oxalate-Rich Foods?
If you’re healthy, the minerals you don’t absorb when you eat greens high in oxalates are unlikely to have an impact on your health, especially if you eat a nutrient-dense diet that contains sufficient minerals. However, there are people who should avoid eating oxalate-rich foods.
If you have a history of kidney stones, stick to leafy greens that are lower in oxalates, like kale and collard greens. Most kidney stones (around 80%) are made of calcium oxalate, and they form when oxalate binds to calcium in the kidneys. Stones form when so much calcium oxalate builds up that the urine becomes supersaturated with it and it clumps together. Urologists who treat kidney stones typically tell their patients to consume less than 50 milligrams daily of oxalates.
The take-home message? Reduce the amount of oxalate in your diet if you have formed calcium oxalate stones in the past. Also, drink lots of water to reduce the formation of kidney stones. It may also be prudent to adopt a low-oxalate diet if you have a strong family history of kidney stones.
It’s controversial, but some health experts also believe people with inflammatory conditions or autoimmune diseases may be sensitive to oxalates and should limit them. The theory is that men and women with inflammatory or autoimmune conditions have gut inflammation. When the lining of the gut is inflamed, more oxalates can be absorbed. But this is more a theory than something that is proven by science.
If you have an inflammatory condition, particularly if you have joint pain, try a low oxalate diet for a few weeks and see if your symptoms improve. If you’ve had calcium oxalate kidney stones in the past, it’s also wise to limit oxalates in your diet. Fortunately, that doesn’t mean you can’t eat leafy greens. It means you’ll need to be more selective and choose ones low in oxalates.
The Bottom Line
Unless you have a history of calcium oxalate kidney stones, consuming oxalate-rich leafy greens in moderation is unlikely to be a problem. There are too many good reasons to eat your greens! Still, make sure you’re eating a diversity of fruits and vegetables and not eating multiple servings of high oxalate greens, like beet greens, each day. A more diverse whole food diet is healthier, anyway. Unless you have a history of kidney stones, there’s no need to eliminate greens entirely from your diet. They’re too nutrient dense not to enjoy! And, even if you have a history of calcium oxalate stones, you can still munch on greens lower in oxalates.
· Centers for Disease Control and Prevention. “Defining Powerhouse Fruits and Vegetables: A Nutrient Density Approach”
· Korean J Urol. 2014 Dec; 55(12): 775–779.
· J Agric Food Chem. 2005 Apr 20;53(8):3027-30.
· National Kidney Foundation. “What are Oxalates and Why are They a Concern for Kidney Disease Patients?”