VN August 2023

Vetnuus | August 2023 40 Calcium and phosphorous are usually discussed together because their metabolism and the homeostatic mechanisms that control their levels within the body are closely related. Calcium is the principle inorganic component of bone. As much as 99% of the body’s calcium is found in the skeleton; the remaining 1% is distributed throughout the extracellular and intracellular fluids. Phosphorus is also an important component of bone, with around 85% of the body’s phosphorus found in inorganic combinations with calcium as hydroxyapatite in bones and teeth. Most of the remaining porting of this is found in soft tissues. Calcium in bones provides structural integrity to the skeleton and contributes to the maintenance of proper blood calcium levels through ongoing resorption and deposition – it is constantly being mobilized and is not in a static state, as the body’s need for plasma calcium fluctuates.The level of circulating plasma calcium is strictly controlled through homeostaticmechanisms and is independent of the animals dietary intake of calcium. Circulating calcium has essential roles in nerve impulse transmission, muscle contractions, blood coagulation, activation of certain enzyme systems, maintenance of normal cell-membrane permeability and transport, and cardiac function. Like calcium, phosphorous in bones lends structural support to the skeleton and is also released into the bloodstream in response to homeostatic mechanisms. The phosphorus found in soft tissue has a wide range of functions and is involved in almost all of the body’s metabolic processes. In addition to having common homeostatic mechanisms in the body, calcium and phosphorus also have an important relationship with each other within the diet. Once adequate levels of calcium and phosphorus have been included in the diet, it is important to consider the ratio of the amount of calcium and phosphorus. Excess dietary calcium forms an insoluble complex with phosphorus, resulting in decreased phosphorus absorption. Similarly, high levels of phosphorus in the diet can inhibit calcium absorption. The recommended ratio of Ca:P in pet foods is between 1:1 and 2:1(1). Feeding animals diets that have an improper Ca:P ratio or supplementing balanced foods with high amounts of either of these minerals can lead to calcium or phosphorus imbalance. Such problems are usually manifested as skeletal disease in growing and adult animals. For calcium (Ca), both FEDIAF (European Pet Food Industry Federation) and AAFCO (Association of American Feed Control Officials) give a recommended MIN-RL of 0.5 and MAX-RL of 2.5 g/100 g of dry matter (DM) of dog food In terms of phosphorus (P) content, both FEDIAF and AAFCO give the same MIN-RL and MAX-RL (0.4 and 1.6 g/100 g DM, respectively) Both the FEDIAF and AAFCO nutritional guidelines recommend that the ratio should be no less than 1:1 and no more than 2:1 (2) It appears that the mechanisms of calcium homeostasis that prevent puppies from absorbing excess amounts of calcium are not fully functional prior to 6 months of age. Therefore, the safety margins for optimal calcium and phosphorous intake early in life are relatively narrow, especially for large- and giant-breed dogs. (3) Growth diets formulated for large breeds should contain lower percentages of calcium and phosphorus because of the lower energy densities of these diets and the need to carefully control calcium intake to support proper skeletal development. In summary, it is of utmost importance to ensure diets have the correct amounts and ratio of Calcium and Phosphorus, as both deficiencies and excesses can cause problems in dogs and cats. Calcium, Phosphorous and the Ratio between them (Dr Debby Bain) Slight ↓ plasma Calcium Release of PTH Synthesis of active Vitamin D ( kidney) ↑ Ca Resorption ↓ P Resorption (Kidney tubules ) ↑ Resorption of Ca and P ( bone) ↑ Mobilization of Ca (bone) ↑ Ca and P absorption (intestine) Normal plasma Ca levels Chronically this will cause a decrease in growth rate and a decrease in mineral content and strength leading to brittle bones. Can cause rickets, OCD Slight ↑ plasma Calcium ↑ Osteoclastic activity Calcitonin secreted (thyroid) Inhibit PTH ↓ Calcium levels Chronic elevation in calcitonin supresses bone resorption for an abnormal period of time resulting in thickening of cortical bone. This interferes with normal bone remodelling and development in growing animals. It also affects cartilage maturation leading to OCD (flaps)