- Calcium oxalate is an insoluble calcium salt of oxalic acid, with the chemical formula CaC₂O₄. It exists in several hydrated forms, most commonly as the monohydrate (CaC₂O₄·H₂O) and dihydrate (CaC₂O₄·2H₂O).
- This compound appears as a white, crystalline solid that is virtually insoluble in water, and it is known primarily for its biological and medical significance, especially in the formation of kidney stones in humans.
- Calcium oxalate is naturally found in many plants, including rhubarb, spinach, beets, sweet potatoes, and starfruit. In these plants, it forms needle-like or crystalline structures called raphides, which serve as a defense mechanism against herbivores. When consumed in excess or in raw form, plants high in oxalates can contribute to irritation of the mouth and digestive tract due to these sharp crystals. Some plants, such as taro and dieffenbachia, contain especially high levels of calcium oxalate and can be toxic if not properly prepared.
- In human physiology, calcium oxalate is most notably associated with the development of nephrolithiasis, or kidney stones. Approximately 70–80% of all kidney stones are composed primarily of calcium oxalate. These stones form when urinary oxalate and calcium levels become elevated, leading to precipitation of the poorly soluble salt in the kidneys or urinary tract. Risk factors for calcium oxalate stone formation include dehydration, high dietary oxalate intake, hypercalciuria (excess calcium in urine), and certain metabolic or genetic disorders. Management typically involves hydration, dietary adjustments, and sometimes medications to reduce calcium or oxalate levels in the urine.
- Although calcium oxalate is mostly recognized for its medical implications, it also has uses in laboratory and analytical chemistry. It can be produced through the reaction of calcium chloride or calcium nitrate with oxalic acid, and due to its low solubility, it is sometimes used to remove calcium ions from solutions or to demonstrate principles of precipitation reactions. Its crystalline properties also make it useful in microscopy studies and materials science.
- Calcium oxalate has no direct use as a nutritional supplement due to its low bioavailability and potential toxicity. In fact, high dietary oxalate can inhibit the absorption of calcium and other minerals by forming insoluble complexes in the gut. As such, individuals with a history of kidney stones or oxalate sensitivity are often advised to limit oxalate-rich foods and to increase calcium intake from dietary or supplemental sources, which helps bind oxalate in the gut and reduce its absorption.
