You have probably heard that foods containing oxalates (or oxalic acid) will block calcium absorption and that most vegetables have this naturally occurring compound. Well, you are right on both accounts, BUT there is hope. A study (1) showed that kale, bok choy, and broccoli have a greater calcium bio-availability (your body’s ability to absorb a nutrient) than the calcium in milk.
What about spinach? If you boil spinach, it will reduce the soluble oxalate content, the form that is easily absorbed by your body, up to 30% to 80%. Soluble oxalate is the major concern to those with kidney stones as 75% of stones are composed of calcium oxalate. Steaming does not reduce the oxalate as much as boiling.(2) Other dark green leafy vegetables with a high oxalate content such as Swiss chard can be boiled as well.
1. Heaney, et al., “Absorbability of Calcium from Brassica Vegetables: Broccoli, Bok Choy, and Kale”, Food Science, vol 58, issue 6, pp 1378-80, 1993.
2. Chai and Leibman, “Effect of different cooking methods on vegetable oxalate content”, J Agric Food Chem, 2005 Apr 20;53(8):3027-30.