There’s a lot of confusion over whether kale is high or low oxalate, whether it’s OK to have it raw in a smoothie, whether it’s best to steam or boil, or even, as Bulletproof suggests, whether adding a calcium and/or magnesium supplement to your blender can reduce the oxalate content!
Time to hunt down some facts. Kale is part of the Brassica oleracea species of vegetables, of which cauliflower, cabbage, broccoli are also members – all low oxalate. So far so promising. According to nutrient data provided by the US Department of Agriculture (USDA), a 100 gram serving of chopped kale contains only 20 milligrams of oxalates, putting it in the low category. Note however that this data is from 1984, and much has changed in the testing methods since. A more recent study (2008) by the Department of Nutrition at the Harvard School of Public Health showed kale to have only 2mg oxalate per cup, or 3mg per 100g. Apparently still fine.
Until you look at the Yahoo low oxalate groups data table (you’ll have to register to become a member). This is by far the best data source, led by researcher Susan Owens, with an up-to-date, incredibly comprehensive food list that has been tested with the newest, most accurate techniques. You’ll immediately see that several factors affect the classification, as kale appears in the low, medium and high sections!
First of all, you’ve got to be clear about which variety of kale you’re cooking, as the different kinds have differing contents – and unhelpfully, the US and UK names seem completely different too. Russian/red kale (baby kale in the UK), purple and lacinato/dino (known as cavolo nero in the UK) have lower scores, whereas curly kale (raw) moves into the high list (27.8mg per 100g).
The cooking method will also affect the content, with general rule of thumb being that raw is highest, followed by steaming (it condenses the leaves), whereas boiling leaches the oxalate away, so ½ cup of boiled kale is classed as low, but the same ½ cup, steamed, moves up into the medium list.
So what’s the take-out on all this? As there are so few leafy green vegetables available to us low-oxalaters, and kale is the veg du jour (!) it would be a shame to miss out on all those beautiful Insta salads. Clearly, raw kale smoothies are out, as you don’t know the serving size or the variety (probably curly), and the raw form is higher in oxalates. Secondly, try and choose an open-leaf variety (Russian/baby or purple), pull out all the tough stalks, shred the leaves and pour 2 full kettles of boiling water over them. This will break down the cellulose and leach away some of the oxalate content. Then give the leaves a quick whizz in a salad spinner to get rid of the excess moisture, cool down, and pop into a salad served with a coconut-oil based dressing for maximum vitamin absorption. This way you can have 2-3 cups a week. If using the tightly knit curly kale, boil for 4-5 mins (any longer and it will turn to mush), and limit your serving size to 1/2 cup. And finally do try dino kale (cavolo nero) – even though the taste is quite different to the curly variety, it’s great as a substitute for spinach in stir fries, soups and stews.
Hope that helps!