One of the two “Macro Minerals” the other being Phosphorous, calcium is the most abundant element in the body and associated with bones and teeth for good reason as both require it for their physical structure and development. It is a critical component of Calcium Hydroxyapatite the mineral compound from which bone is formed around its collagenous fibrous tissue and cartilage matrices. About 99% of total body calcium is found in bone.
Calcium is also an essential mineral for proper function of nerve and muscle cells and for blood clotting. Without it, nerves cannot transmit messages and muscles cannot contract and relax normally. Skeletal calcium acts as a reservoir to maintain serum calcium at normal levels and the circulating calcium level is tightly controlled within narrow margins. The hormones Calcitonin and Parathyroid hormone control calcium flows into and out of bone respectively. If serum calcium falls below normal, disturbances such as muscle weakness or tremors soon develop such as may be seen in the hypocalcaemia associated with “Milk Fever”, more common in cattle and dogs than horses.
Calcium is absorbed mainly (60% to 67%) in the small intestine. It needs to be in ionic form as a divalent cation Ca2+ to be absorbed, and as such other divalent cations like copper and zinc may compete for transport into the body. Acid suppressor drugs like Omeprazole may reduce absorbtion. Oxalic acid found in tropical grasses forms an insoluble salt, calcium oxalate, which reduces digestibility and absorbtion, potentially leading to a functional calcium deficiency and ‘’Big Head’’ (see Bran Disease). Phytic Acid found in cereal grains like oats and barley may also form insoluble complexes with calcium and reduce digestibility. The amount of dietary Phosphorous present does not affect calcium absorbtion and any dietary calcium surplus to immediate requirements is only poorly absorbed. Accretions of calcium in the gut can form stones called enteroliths, fortunately rare in Australia. Excess calcium in the body is excreted through the kidney and under certain conditions may also be associated with kidney and bladder stones (urinary calculi).