Oxalic Acid and Calcium

Oxalic acid blocks calcium absorption. That is a big concern to hermit crab keepers who realize the importance of calcium in their pet’s diet. Calcium is necessary for formation and maintenance of exoskeleton. Therefore the question arises: What are safe levels of oxalic acid and how often should foods high in oxalic acid be offered?

Extensive research has been done on oxalic acid’s affects on humans since kidney stones are thought to be formed by crystals formed when oxalic acid combines w/ calcium, iron, and other minerals. These crystals (oxalates) form in the digestive system and once they enter the bloodstream the calcium particles are too large to be absorbed and are eliminated as waste through the kidney and bladder. How this process translates into hermit crab physiology is uncertain, but calcium that is rendered unusable is reason for concern.

Some cautiously avoid all high oxalic foods altogether, but this may not be the best course of action because many of the foods in this class are otherwise highly nutritious. Many of the ingredients recommended for beginning crab keepers are somewhat high in oxalic acid. So, what is one to do? Some sources say that the oxalic acid affect can be reduced or eliminated by heating, but that could harm many of the other valuable nutrients in the plant. Success rests on understanding the complete nutrient content of the high oxalic acid foods you are considering and weighing the risk vs. benefits. Spinach, for instance, is very high in both oxalic acid and calcium. Mathematically, the oxalic acid content probably renders nearly all of the calcium unavailable. However, if you figure in the fact that B6 and magnesium also present in spinach retards the chemical binding of the calcium*, the typical result is that hermit crabs will still get some calcium from spinach as well as a whole host of nutrients like beta carotene, zeaxanthin, A, and potassium. So, from that point of view, your crab should eat spinach--unless he happens to be prone to kidney stones!

It is important to determine how much oxalic acid is in the foods that you feed so that you can offset it with other sources of calcium, iron, and other minerals. If you know you are feeding a high oxalic food, consider dusting it with powdered cuttlebone and offering a low oxalic food the following day. High oxalic foods are best avoided during pre-molt and post-molt. Unripe fruits and vegetables tend to have higher oxalic content, so a ripe (red) bell pepper is better than a green bell pepper. Also, young shoots tend to have more oxalic acid than mature leaves as in the case of spinach.

High Oxalic Acid Content:
Buckwheat, starfruit, parsley, poppy seed, rhubarb, amaranth, beet leaves, spinach, Swiss chard, beets, banana, plantain, ginger, cassava, nuts, berries, beans, bell peppers, sweet potatoes, soybeans, tomatillos, oats, pumpkin, cabbage mango, eggplant, lentils, parsnips.

Low Oxalic Acid Content:
Dandelion greens, most ripe fruits, kale, watercress, escarole, mustard greens, turnip greens, broccoli, tomatoes, asparagus, cabbage.

*Here is the formula I’ve developed:
oxalic acid (mg) X .097 = x, x - total calcium (mg) = actual absorbable calcium