Calcium & phosphorus
Calcium is a mineral that is essential for a strong skeleton. Because the metabolism of calcium is very closely related to that of phosphorus, these two minerals are often discussed together.
Calcium absorption in the intestines can occur in three different ways:
- The first way is an active form, a transcellular – transport through the cell – process, which mainly takes place in the duodenum and jejunum – the middle part 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 a passive form of calcium absorption, which are independent of the amount of vitamin D present in the body. These absorption methods 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. Of all the calcium present in the body, about 99% is found in the skeleton (bones and teeth). The remaining 1% is found in the 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%, the majority is found in soft body tissues.
Calcium is necessary for the structural integrity and strength of the skeleton. Calcium also contributes to maintaining proper blood calcium levels through resorption and release. Although a large amount of calcium is present in the bones and teeth, calcium is not constant/static; when bone growth and/or repair occurs, calcium is mobilized or deposited. Additionally, an animal’s need for plasma calcium is not constant, so mobilization or release of calcium is also necessary for this.
Calcium in the body has multiple functions beyond providing skeletal strength and contributing to proper blood calcium levels:
- It is necessary for muscle contraction
- It plays a role in nerve impulse transmission
- It is needed for the activation of certain enzymes
- It is required for blood clotting
- It plays a role in the normal functioning of the heart
- It is necessary to maintain normal cell membrane permeability
The calcium requirement differs for puppies and adult dogs. Research suggests that growing puppies, especially those of large and giant breeds, have a higher calcium requirement than adult dogs. Adult dogs have a minimum calcium requirement of 0.40% on a dry matter basis, while weaned puppies require 0.59% on a dry matter basis (based on a diet with 4,000 kcal/kg). The calcium requirement of large and giant breed puppies is the highest, ranging from 0.7 to 1.2% on a dry matter basis (based on a diet with 3,800 kcal/kg).
The calcium requirement for 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.
You have already read that phosphorus in the body is present in combination with calcium. This compound is known as hydroxyapatite. Like calcium, phosphorus provides:
- Structural support for the skeleton
- Phosphorus is released into the blood in response to homeostatic mechanisms
Phosphorus is not only present in the bones; the mineral is also found in the soft tissues of the body where it has various functions. Phosphorus is involved in almost all metabolic processes in the body; it is a component of DNA, RNA, various B-vitamin coenzymes, and phospholipids of the cell membrane, which are important for regulating solutes in and out of cells.
Phosphorus is also essential for cell growth and differentiation, energy use, transport of fatty acids, and amino acid and protein formation.
Generally, animals can absorb phosphorus from animal sources better than phosphorus from plant sources because phosphorus in animal material is mostly in organic form. Furthermore, animals are not able to absorb all the phosphorus from their diet; they are usually able to absorb about 60 to 70% of the phosphorus content in the diet.
The kidneys and intestines are necessary for phosphorus absorption. These organs regulate phosphorus levels and ensure intestinal absorption of phosphorus. When phosphorus absorption from the diet is not optimal or low, intestinal absorption is optimized; the kidneys minimize phosphorus loss through urine and increase renal phosphorus transport. When too much phosphorus is absorbed from the diet, the intestines reduce phosphorus absorption, and the kidneys increase phosphorus excretion via urine.
As with calcium, phosphorus requirements depend on the life stage of an animal. Adult dogs have a phosphorus requirement of 0.4 grams on a dry matter basis, while puppies require at least 0.7 grams on a dry matter basis.
Not only the amount of calcium and phosphorus is essential for dogs, but also the ratio between these two nutrients is very important. The minimum calcium to phosphorus ratio is 1/1. For adult dogs, the maximum calcium/phosphorus ratio is 2/1, while for puppies the maximum ratio is 1.6/1 or 1.8/1.
The phosphorus requirement for adult cats is at least 0.26 grams on a dry matter basis. Kittens have a higher phosphorus requirement; their diet must contain at least 0.84 grams on a dry matter basis. For cats, the minimum calcium/phosphorus ratio is also 1/1, with a maximum calcium/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 too much calcium forms an insoluble complex with phosphorus.
Excess phosphorus is also undesirable because it negatively affects/reduces calcium absorption. When the ratio between calcium and phosphorus is not correct, it results in an imbalanced calcium or phosphorus level, leading to skeletal problems.
