Many people who need extra calcium in their diet may not be getting
amount they think they are getting. There is actually a few reasons for this.First thing is an acid called Oxalic acid. This is a naturally-occurring substance found in some foods that binds with important nutrients, making them inaccessible to body. Oxalic acid may combine with calcium, iron, sodium, magnesium, or potassium to form less soluble salts known as oxalates. Oxalates also occur naturally in plants.
Foods generally found include: apples asparagus, chocolate, cocoa, coffee, most berries (especially strawberries and cranberries), most nuts (especially peanuts), beans, beets, beet greens, bell peppers, black pepper, parsley, rhubarb, spinach, swiss chard, summer squash, sweet potatoes, and tea.
Plant foods with high concentrations of oxalic acid (over 200 ppm) include (but are not limited to): lamb's-quarter, buckwheat, star fruit, black pepper, purslane, poppy seeds, rhubarb, tea, spinach, plantains, cocoa and chocolate, ginger, almonds, cashews, garden sorrel, mustard greens, bell peppers, sweet potatoes, soybeans, tomatillos, beets and beet greens, oats, pumpkin, cabbage, green beans, mango, eggplant, tomatoes, lentils, and parsnips.
The good news is that cooking destroys oxalic acid therefore asparagus, beets, beet greens, chard, cranberries, green peppers, rhubarb, and spinach are all best eaten cooked.
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, oxalic acid in spinach and other foods does not interfere with absorption of calcium from other foods eaten at same time.
The second is a substance called Phytate. Phytic acid, found in bran of whole grains, nuts, and skins of legumes, can bind to calcium to form and insoluble complex, thereby decreasing absorption of calcium. Phytic Acid binds with calcium (and iron, magnesium, phosphorus, and zinc) in intestinal tract preventing absorbtion. Phytates/phytic acid are storage form of phosphorus bound to inositol in fiber of raw whole grains, legumes, seeds, and nuts. Although these foods have a high phosphorus content, phosphates in phytates are not released through digestive process. Phytates, particularly in such raw foods as bran, are a concern because they can bind a portion of iron, zinc, and calcium in foods, making minerals unavailable for absorption.
If one looks back at traditional cultures you will see that for thousands of years whole grains have been prepared by soaking or fermenting them prior to cooking. Phytic acid which occurs in unsprouted grains, seeds, and legumes, is particularly rich in bran. When bread is leavened by yeast, enzymes degrade phytic acid and phytates pose no problem. Phytic acid is also destroyed during baking and food processing.
Enzymes, called phytases, destroy phytates during certain food processes such as: yeast-raising of dough, sprouting of seeds, grains, legumes, roasting of nuts, presoaking beans, cooking, fermentation as in tempeh, miso, and natto, combining acidic foods with zinc-rich foods, etc.
Soybeans contain high levels of phytates; some researchers say more than other beans. Additionally, soy's phytates are so stable that many survive phytate-reducing techniques such as cooking. (The phytates in whole grains can be deactivated simply by soaking or fermenting. It is possible that only long periods of soaking and fermenting - as are used in making miso, natto, shoyu, tamari, and tempeh (but not tofu, soymilk, texturized soy protein, or soy protein isolate) - significantly reduce phytate content of soybeans.
The third reason is effects of a high protein diet. People are increasingly concerned about adopting healthier diets. However, many are prevented from necessary changes because of myths about certain nutrients. For example, people feel that they should eat increased amounts of meat in order to get adequate protein and large amounts of dairy products in order to get adequate calcium to avoid osteoporosis.
