Calcium & phosphorus
Calcium is a mineral that is essential for a strong skeleton. Because calcium metabolism is closely linked to that of phosphorus, these two minerals are often discussed together.
Calcium can be absorbed in the intestines in three different ways:
- The first is an active form. It is a transcellular process – transport through the cell – that mainly takes place in the duodenum and jejunum – the middle section of the small intestine. This active form of calcium absorption is influenced by age, pregnancy, lactation, and the amount of calcium and vitamin D present in the body.
- The other two ways are facilitated absorption and passive calcium absorption, both of which are independent of the amount of vitamin D present in the body. These methods of absorption depend on the amount of calcium present in the diet and its availability.
Calcium is an important component of bones; it is even essential for a strong skeleton. Approximately 99% of all the calcium present in the body is found in the skeleton (bones and teeth). The remaining 1% is found in extracellular and intracellular fluids.
Like calcium, phosphorus is essential for a strong skeleton. As much as 85% of the phosphorus present in the body is found combined with calcium in the bones and teeth. Of the remaining 15%, most is found in soft body tissues.
Calcium is needed for the structural integrity and strength of the skeleton. It also contributes to maintaining the correct blood calcium level through resorption and release. Although a large amount of calcium is present in the bones and teeth, it is not constant or static; when bone growth and/or repair takes place, calcium is mobilized or deposited. In addition, an animal’s need for plasma calcium is not constant, which also requires the mobilization or release of calcium.
Calcium in the body has several functions beyond providing skeletal strength and contributing to a healthy blood calcium level:
- It is needed for muscle contraction
- It plays a role in the transmission of nerve impulses
- It is needed for the activation of certain enzymes
- It is needed for blood clotting
- It plays a role in the normal functioning of the heart
- It is needed to maintain normal cell membrane permeability
The calcium requirement differs between puppies and adult dogs. Research suggests that growing puppies, particularly puppies of large and giant breeds, have a higher calcium requirement than adult dogs. Adult dogs are considered to have a calcium requirement of at least 0.40% on a dry-matter basis, while weaned puppies have a calcium requirement of 0.59% on a dry-matter basis (based on a diet containing 4,000 kcal/kg). Puppies of large and giant breeds have the highest calcium requirement: 0.7 to 1.2% on a dry-matter basis (based on a diet containing 3,800 kcal/kg).
The calcium requirement of adult cats is 0.4 grams on a dry-matter basis. Here too, kittens have a higher calcium requirement than adult cats. Kittens have a minimum calcium requirement of 1.0 gram on a dry-matter basis.
As you read earlier, phosphorus is present in the body combined with calcium. This compound is known as hydroxyapatite. Like calcium, phosphorus then contributes to:
- Structural support of the skeleton
- Phosphorus being released into the blood in response to homeostatic mechanisms
Phosphorus is not found only in the bones; the mineral is also present in the body’s soft tissues, where it has various functions. Phosphorus is involved in almost all metabolic processes in the body, is a component of DNA, RNA, various B-vitamin coenzymes, and the phospholipids of cell membranes; these are important for regulating dissolved substances entering and leaving cells.
Phosphorus is also essential for cell growth and differentiation, energy utilization, fatty acid transport, and amino acid and protein formation.
In general, animals can absorb phosphorus from animal-based material better than phosphorus from plant-based material; this is because phosphorus in animal-based material is generally present in organic form. In addition, animals are unable to absorb all the phosphorus in their diet. They are generally able to absorb approximately 60 to 70% of the phosphorus in their food.
The kidneys and intestines are needed for phosphorus absorption. These organs regulate phosphorus levels and ensure the intestinal absorption of phosphorus. When dietary phosphorus intake is not optimal or is low, phosphorus absorption in the intestines is optimized; the kidneys ensure that phosphorus loss through the urine is kept as low as possible and also increase renal phosphorus transport. When too much phosphorus is absorbed from the diet, the intestines naturally reduce phosphorus absorption; the kidneys increase the excretion of phosphorus through the urine.
As with calcium, the phosphorus requirement depends on an animal’s life stage. Adult dogs have a phosphorus requirement of 0.4 grams on a dry-matter basis, while puppies have a phosphorus requirement of at least 0.7 grams on a dry-matter basis.
Not only the amounts of calcium and phosphorus are essential for dogs; the ratio between these two nutrients is also very important. The minimum calcium-to-phosphorus ratio is 1/1. For adult dogs, the maximum calcium-to-phosphorus ratio is 2/1; for puppies, the maximum ratio is 1.6/1 or 1.8/1.
The phosphorus requirement of adult cats is at least 0.26 grams on a dry-matter basis. Kittens have a higher phosphorus requirement; their food must contain at least 0.84 grams on a dry-matter basis. Cats also have a minimum calcium-to-phosphorus ratio of 1/1, with a maximum calcium-to-phosphorus ratio of 1.5/1 for kittens and a maximum ratio of 2/1 for adult cats.
When there is an excess of calcium in the diet, phosphorus absorption is negatively affected. This is because excess calcium forms an insoluble complex with phosphorus.
Excess phosphorus is also undesirable. This is because too much phosphorus negatively affects and reduces calcium absorption. When the calcium-to-phosphorus ratio is incorrect, this results in an imbalance of calcium or phosphorus, leading to skeletal problems.
