Calcium - Nutrition

What is Calcium?

Calcium is the mineral that is found in the greatest abundance in mammals, including dogs and cats, and also one of greatest importance because of its vital intracellular and extracellular functions as well as providing structural rigidity of the body. Most of the calcium in the body [99%] is found as calcium phosphate and stored in bones. A small, but vital portion is found in blood. Calcium in the blood [1% of total body calcium] exists in its free ionised form (iCa) [approximately 50%], protein bound [40-45%] and in a complexed or chelated state [5-10%].

Why is it Important?

The vital role of calcium relates to its key roles in numerous enzymatic reactions such as in coagulation, as a cell-signalling molecule, ligand for cell membrane transport, nerve conduction, neuromuscular transmission, muscle contraction, vascular muscle tone, hormone secretion, cell growth and division and for bone formation.

Roles in the Body

In terms of its structural role, the primary role of calcium is, when combined with phosphorus, in the formation of hard structures such as bones and teeth. Low dietary levels are therefore associated with severe skeletal problems. Calcium (especially in its ionised form) is also involved in numerous key intracellular and extracellular processes such as blood clotting, the transmission of nerve impulses, muscle contraction, maintenance of vasomotor tone, and cellular signalling. The level of calcium in the blood plasma is crucial to these functions and is under tight homeostatic control. Dietary calcium absorption is controlled by parathyroid hormone (PTH), which regulates the renal production of 1,25-dihydroxyvitamin D. This vitamin stimulates the absorption of calcium from the intestine, so by carefully controlling its production, calcium absorption can be regulated up at low intake and down at high intake. Nevertheless, Dobenecker reported that beagle puppies younger than 4 to 5 months were unable to adjust the digestibility of calcium in relation to either excess or insufficient intakes. This makes it particularly important to adjust the supply of calcium in line with the dog’s requirements.

Consequences of Calcium Deficiency

A 2 month old Great Dane puppy with a history of eating an unbalanced diet. The bone density is overall decreased with a poor corticomedullary distinction.

Dog:

The classic syndrome of calcium deficiency in dogs is known as nutritional secondary hyperparathyroidism (NSHP), which is caused by a persistent decrease in circulating calcium in the blood (hypocalcaemia). This, in turn, stimulates the release of parathyroid hormone (PTH), which increases the reabsorption of bone to provide the calcium required to maintain a normal ionised calcium (iCa) blood concentration. The inevitable consequence is a major decrease in bone mineral content, which can lead to skeletal problems such as pathologic fractures (Dan to provide a radiograph). This is particularly serious for young puppies, where active growth of the skeleton is occurring. The condition typically results from feeding diets deficient in calcium such as those composed mainly of meats, since all meats (including offal) have a very low calcium content. The ratio of calcium to phosphorus is also important and it is the low ratio in these foods (i.e. Ca:P much less than 1:1) that contributes to the adverse effects because the high phosphorus level inhibits calcium absorption.

Recognised Syndromes related to Calcium Deficiency

1. Skeletal malformation: Hazewinkel et al. described severe skeletal problems in Great Dane puppies fed diets with 0.55% calcium on a dry matter basis (DM) (hotline to definitions/glossary of terms). However Nap et al. found no abnormalities in miniature poodle puppies (13-25 weeks) fed 0.33% calcium on DM. Nevertheless, in the same experiment, NSHP was observed when a very low level of 0.05% calcium was fed. It is clear that giant, rapidly growing breeds are more sensitive to restrictions in dietary calcium.
2. Eclampsia: Also called lactation tetany or milk fever, eclampsia is a disease occurring during the later stages of pregnancy or soon after parturition (1-3 weeks postpartum). It is characterised by loss of consciousness, convulsions or both and is associated with severe hypocalcaemia, although its aetiology is not completely understood. Proposed mechanisms include poor dietary intake, excessive calcium losses during lactation and abnormal parathyroid gland function. In addition, there may be a predisposition of this condition in toy breeds. Clinical signs in dogs are typically not seen until total calcium concentration drops below 1.6 mmol/L [6.5 mg/dL] or iCa drops below 0.7 mmol/L [2.8 mg/dL].

Cat:

The effects of calcium deficiency in cats are broadly similar to those in dogs in terms of effects on bone formation, although there is little if any breed variation. This is to be expected given the much narrower bodyweight range in the cat. As with dogs, the calcium requirement of kittens decreases with increasing age. Cats can also suffer from eclampsia but this is not as common as in dogs.

Toxicity

Dog:

Excess dietary calcium has caused bone abnormalities in growing dogs but this seems to be confined to puppies of large breeds. Hazewinkel et al. found that Great Dane puppies fed a diet containing 3.3% calcium DM developed osteochondrosis and stunted growth. In contrast, when Nap et al. fed a similar diet to miniature poodle puppies no significant skeletal problems were observed. These effects of excess calcium intake appear therefore to be confined mainly to puppies of large breeds. Nevertheless, Dobenecker et al. investigated the effects of calcium excess on beagles and foxhound crossbred puppies between 6 and 27 weeks of age. X-ray measurements of bone lengths and widths showed a growth-reducing influence only in beagles, i.e. the smaller breed, although there were no effects on general health or clinical parameters of skeletal health in either group. There are apparently no reports of adverse effects of calcium excess in adult dogs. A more general effect of very high calcium intakes is interference with the absorption of other minerals and trace elements, such that deficiencies can occur at dietary levels that would otherwise be regarded as adequate. Such an effect has been observed in cats (see below).

Cat:

There is some evidence of adverse effects of high dietary calcium in cats. Howard et al. found that kittens fed a dietary calcium level of 2.3% DM had depressed food intake, decreased growth and a higher requirement for magnesium. Nevertheless, these effects are not linked to severe skeletal abnormalities as seen in the giant dog breeds.

Dietary Sources

The raw materials commonly used as sources of calcium in dog and cat foods are bone meal or meat and bone meal – for example meat and bone meal contains around 10% calcium. Supplements of calcium include inorganic salts of calcium carbonate (limestone) and calcium phosphate. Others forms such as calcium hydroxide, calcium chloride and calcium sulphate (gypsum) can also be used.

Diagnosing Calcium Deficiency

In animals with skeletal abnormalities, radiography can be used to assess bone density (see image). Assessment of total blood calcium and iCa is also required. Normal total blood calcium in dogs ranges from 2.2-3.8 mmol/L [9.0-11.5 mg/dL] and 2.0-2.6 mmol/L [8.0-10.5 mg/dL] in cats, respectively. In dogs, the iCa range is 1.2-1.5 mmol/L and for cats it ranges from 1.1-1.4 mmol/L.