Q: I know that calcium is a necessary mineral and one that, if properly assimilated, is useful in many pain situations and is good for bone health. As such, I have been adding kale, spinach, beet greens, and chard to protein shakes.
But I have read that kale and spinach, among other foods, contain oxalic acid and as such interfere with calcium absorption in the intestines. I have also read that casual consumption is okay. Can you talk about this in terms of whether adding kale to my morning shake inhibits all calcium absorption?
A: Calcium, which is an essential nutrient for strong bones, can be found in many green leafy vegetables, such as collard greens, spinach, mustard greens, kale, and Swiss chard. However, oxalic acid, a type of antinutrient found in spinach, chard, beet greens, and moderately in kale, among other plants, binds with the calcium they supply and reduces its absorption, so in their raw form, these should not be considered a good source of calcium. Oxalic acid are also found in some legumes and grains.
Does this mean you should not add kale or other greens that contain oxalic acid to your smoothies? Not necessarily. Cooking or steaming these vegetables can significantly reduce the amount of oxalic acid present, which will help with calcium absorption (make sure to drain your greens thoroughly as the oxalates go into the water).
One suggestion would be to precook your leafy greens and store them in the freezer (in individual portions) for quick access. When you’re ready to make your smoothie, just grab a portion and throw it in! Cooked and drained, kale is also a great addition to soups, stews, and even pizza! Other methods to reduce their antinutrient content include sprouting and fermenting.
Another suggestion would be to eat foods known to contain oxalic acid 2 hours apart from dietary calcium sources. Doing this will allow enough time for the body to absorb it.
Other foods that will boost the calcium content in your smoothie, are unsweetened Greek yogurt (which also contains a good amount of protein) and flax seeds.
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WEAVER, C.M., HEANEY, R.P., NICKEL, K.P. and PACKARD, P.I. (1997), Calcium Bioavailability from High Oxalate Vegetables: Chinese Vegetables, Sweet Potatoes and Rhubarb. Journal of Food Science, 62: 524-525. https://doi.org/10.1111/j.1365-2621.1997.tb04421.x