In brief:

Assists in bone and teeth formation, blood health, blood pressure, muscle contraction, nervous system, metabolism, hormone and enzyme function, utilisation of fat carbohydrates, proteins and nutrients. Construction of DNA, RNA & ATP.

Calcium and phosphorous are often considered together due to interactions. Calcium and phosphorous are a major part of mineral content in bone.

Chemical properties and distribution

• 99% of calcium found in bone and teeth, the rest in extracellular fluid and within cells
• Calcium often low in grains while abundant in forages. Soils in humid tropics are often acid soils high in iron and aluminium making phosphorous unavailable to plants

Metabolism

• A calcium:phosphorous ratio of 1:1 to 2:1 is required in the system, the precise ratio depends on age and workload etc, of the horse. In a horse, excessive calcium has little effect on phosphorous absorption as calcium and phosphorous are absorbed from different regions of the gastro-intestinal tract (Briggs, 1999)
• In the horse, the colon is the major site for phosphorous absorption
• Calcium and phosphorous absorption is both active and passive
• Major function for vitamin D to maintain blood calcium and phosphorous for normal mineralisation and physiological functions (Deluca and Zierold, 1998)
• Calcium absorption is related to demand and is higher in early lactation
• Calcium absorption is favoured by acid conditions (lactose promotes)
• Phosphorous absorption is influenced by dietary level of phosphorous, source of phosphorous, intestine (pH), age, intestinal parasitism and intakes of calcium, iron, aluminium, manganese, potassium and magnesium
• Phytates decrease absorption for both calcium and phosphorous
• Fatty acids may form insoluble calcium soaps

Control of calcium and phosphorous homeostasis

Blood calcium is maintained within very narrow limits by three hormones: parathyroid (PTH), 1,25 dihydroxycholecalciferol. Vitamin D and thyrocalcitonin (calcitonin)

Physiological functions

Structure of bone

• Calcium and phosphorous make up 70% of bone minerals
• Ca : P relationship nearly constant
• Mineral salts deposited in organic matrix, mixture of proteins (mostly ossein)
• Bone length takes place at the junction of the epiphysis and diaphysis
• Cartilage (protein) in between is a temporary formation that is replaced by calcified bone
• Cortical bone is composed of densely packed layers of mineralised collagen which provides rigidity and is a major component of tubular bones
• Trabecular (cancellous) bone is spongy, provides strength and elasticity (major part of axial skeleton)
• Bone undergoes a continuous process of resorption and formation
• Goal should be made to build up bone mass with nutrition and activity as aging decreases bone mass

Calcium and phosphorous in soft tissue

• Blood clotting
• Enzymatic reactions, activities ATPase
• Secretion of a number of hormones and hormone-releasing factors
• Muscle contraction
• Maintenance for normal blood pressure
• Synaptic nerve transmission
• Low calcium levels can affect hypomagnesia situations due to the interaction with magnesium
• Needed for weight gains, feed utilisation, milk production

Phosphorous is involved in almost all, if not all metabolic reactions, therefore the most versatile of them all. Involved with every aspect of feed metabolism and utilisation of fat, carbohydrates, protein and nutrients. Phosphorylation and dephosphorylation regulate cell activities, including functions of enzymes, hormones and transcription of genetic information.

High energy phosphate bonds (ATP) and nucleic acids (RNA, DNA). Essential for buffer systems in blood and other fluids. Phosphorous declines dramatically as forage matures.

Deficiency

Deficiency of calcium and phosphorous or Vitamin D can result in rickets, reduced bone calcification, stiff and swollen joints, stiffness of gait and bone deformities. In mature horses, severe lameness. Low calcium : high phosphorous can result in a condition called nutritional secondary hyperparathyroidism, this causes calcium to remove from facial bones following fibrous connective tissue invading the area.

Phosphorous deficiency can result in – eating bones, wood, hair, rocks and clothing and health issues surrounding deficiency can be weakness, failure to exhibit estrus and low conception rate while affecting milk production.