Oxalic acid, iron and calcium in foods

Calcium: It appears that oxalic acid only binds the calcium in foods in which is occurs.

• According to Finding Calcium Sources Outside the Dairy Case

  although spinach has a lot of calcium, it also contains a substance -- oxalic acid -- that binds up its calcium and prevents absorption of all but about 5 percent of it. However, the oxalic acid in spinach and foods like rhubarb does not interfere with absorption of calcium from other foods eaten at the same time.

• Calcium: It's everywhere - But is it enough?

  Oxalic acid found in spinach, chard and beet greens binds up the calcium present in these foods, making it unavailable for the body to use. It doesn't, however, affect the absorption of calcium in foods served at the same meal

Iron:

• Oxalic Acids and iron Heme iron is part of the hemoglobin and myoglobin molecules in animal tissues (blood and muscle). Nonheme iron comes from other sources. The body absorbs heme iron more easily. Oxalic Acids inhibits the absorption of nonheme iron. Consuming vitamin C with nonheme iron sources helps counter the nonheme iron binding effect of oxalic acid. Cooking breaks down oxalic Acids.

from Oxalic Acid Content of Selected Vegetables

Other links:

• Iron - Functions in the body, symptoms of defiency, sources.
• Calcium in fish diet.
• Oxalate-rich food items
• Oxalic Acids - Iguana diet article on oxylates.
• Rhubarb Poison Information - the leaves apparently contain a lot of oxalate which can be toxic if consumed in large amounts. The dose require to kill half of rats tested was 375 mg oxalate/kg body weight. Symptoms of poisoning.
• Vitamin C may help counter the nonheme iron binding effects of oxylates
• This page also says oxalic acid is broken down when food is cooked