In metabolism and nutrition, calcium and phosphorus are such closely related substances that they are best discussed together. They constitute much the greater part of the mineral matter of the body and are found in greatest amounts in the bones.

For proper nutrition, these elements must be present in the food in adequate amounts; equally important, it is necessary that they be present in proper ratio. The ratio of 1.2 parts of calcium to 1 part phosphorus is considered optimum for maximum utilization of these minerals. When the intake of either is too high or too low, the absorption of the other from the intestine is interfered with.

The metabolism of calcium and phosphorus and that of vitamin D are closely interrelated. When these interrelations are abnormal, rickets or pseudorickets tend to develop owing to failure to deposit proper amounts of calcium and phosphorus in the bones. When the supply of vitamin D is adequate, less favorable calcium-phosphorus ratios do not cause trouble. When the ratio is unfavorable and vitamin D is present in inadequate amounts, deficient bone calcification and rickets will result. Vitamin D has the power to take calcium from the bones as well as to deposit it in them.

Therefore it is essential to clinically differentiate between a vitamin D and a calcium deficiency.  Dietary supplementation of vitamin D3 or the systemic injection of vitamin D2, to a calcium deficient animal without simultaneously providing more dietary calcium would promote a rapid demineralization and  could precipitate hypocalcemic seizures.  (the same seizures from the same cause as in canine eclampsia)

It is generally accepted that an all meat diet does not supply the proper Ca:P ratio and requires supplementation. Growing dogs, pregnant or lactating bitches require liberal amounts of calcium and phosphorus in their diets. The amount retained varies with age; rapidly growing puppies need the most generous supply.

Investigators report that the amount absorbed and retained is fairly constant during the first 200 days of life despite an approximate six-fold increase, body weight. Retention appeared slightly higher during the third and fourth months of the growth cycle, reaching a peak calcium retention of 73 mg. /lb. of body weight  Retention of 35 mg./lb.  per day was more common, however.

Growing Puppies apparently require at least 0.37% of available calcium (0.60% of total calcium in the ration) for satisfactory growth , development and skeletal structure.  Various studies indicate that approximately 0.75% of total calcium in the diet would suffice if one assumed a 50% availability.  A 50% may not cover all situations since up to 80% utilization  has been reported.

There is no evidence that a calcium level of 1.0%, or even higher, in the ration produces any harmful effects in dogs if the proper calcium-to–phosphorus ratio is maintained. Calcium, not utilized, is excreted mainly in the feces.

Although it has been demonstrated repeatedly that rations low in phosphorus lead to poor calcification of bones, rickets, and other problems, requirements for phosphorus are not well defined. The requirement for  phosphorus was established by Jenkins and Phillips at 0.33% in practical type ration.. Under conditions of these studies, a 76% retention was observed indicating a minimum available need of 0.25% of available phosphorus in the ration. High levels of dietary fat increased the phosphorus requirement about 20%.

Under normal conditions and assum-ing a 50% biological availability of cal-cium and phosphorus, the requirements suggested in Table I for a ration (on a dry basis) are 1.1% calcium and 0.9% phosphorus, These amounts would fur-nish 253 mg. of calcium per kg. of body weight and 207 mg. of phosphorus per kg. of body weight. These amounts are recommended for normal adult dogs and about twice those amounts would be required by the growing puppy. The levels provide a wide margin of safety for estimating requirements for various types and breeds of dogs.

Calcium has several well known functions within the body. It is essen-tial for ossification of bone and teeth and is an important ingredient of inter-cellular cement. Calcium maintains the contractility, rhythm and tonicity of the myocardium and antagonizes the action of the sodium and potassium ions upon the heart. The calcium ion is required for normal coagulation of blood. Calcium is necessary for proper nerve irritability and appears to be essential for selective cellular perme-ability.

The bones of the body use calcium and phosphorus as structural elements. In addition, the bones serve as a storage place for the deposition of calcium. Large amounts of these minerals are utilized, in the process of ossification in which cartilage is converted to cal-cified bone in the growth process. This bone growth occurs at the epiphyseal junction of the long bones. In addition to being required for production of compact bone, calcium and phosphorus are normally deposited in the spongy bone formations on either side of the epiphyseal junction. These calcium re-serves are deposited for mobilization against some future need such as preg-nancy, lactation or periods of deficient intake.

Like calcium, the absorption of phosphorus is favored by factors which tend to hold it in solution. An acid pH favors the solution of phosphorus as well as calcium and tends to prevent formation of insoluble and unabsorbable tricalcium phosphate.

The excessive ingestion of magnesi-um, iron, aluminum and beryllium in-terferes with phosphorus absorption because these metals combine with it to form insoluble phosphates. An excess of calcium interferes with the ab-sorption of phosphorus and vice versa. A large excess of either substance pro-motes the conversion of the other into the insoluble tricalcium phosphate. Hence the ratio of 1.2:1 between cal-cium and phosphorus is necessary to promote absorption.

A disturbance of normal calcium and phosphorus metabolism may occur at any time of life when the supply of these minerals and vitamins A and D is inadequate. In the adult dog this is characterized by a negative mineral balance. In the growing animal a dis-turbance is characterized by inadequate retention of minerals. Continued loss of calcium and/or phosphorus leads to rarefaction of bones with frequent lameness and fractures in the adult (osteomalacia) and to slowed growth and misshapen bones in the immature animal (rickets or pseudorickets).

The bran portions as well as whole cereal grains contain phytic acid which may form insoluble calcium salts in the intestine. They in turn make a portion of the calcium unavailable for absorption. The degree to which these insoluble calcium salts are formed depends on the quantity of phytate present. Usually liberal amounts of calcium in-take will compensate for the presence of phytic acid.

In addition, an excess of fat in the intestine may retard calcium absorption through the formation of insoluble cal-cium compounds. It should be pointed out, however, that a small amount of fat in the intestine appears to improve calcium absorption.

Probably vitamin D, in addition to vitamin A, deficiency and an imbal-ance or deficiency of calcium and phosphorus are involved in most cases of pseudorickets. (Vitamin A deficien-cy has not been considered part of the rickets complex in man, hence use of the term pseudorickets has been sugges-ted. In dogs, however, both terms appear to be used interchangeably).

Rapid growth in large breeds of dogs predisposes to rickets. The soft tissues require and retain phosphorus so that in case of deficiency the bone may be depleted in order to supply the proper amount of phosphorus for the soft tissues. Rickets is a condition observed much more commonly in the heavy breeds of dogs (Great Dane, St. Ber-nard) than in small breeds. The ration of these heavier breeds must contain proportionately larger amounts of min-erals and vitamin D on a weight basis than are provided for small breeds. The proportionate rate of growth is also a factor and the formula of most com-plete and balanced dog foods has been adjusted to compensate for breed and growth differences. In large breeds the 1.2:1 ratio of calcium to phosphorus is especially important.

In treating rickets, attention should be given to the diet and to the admin-istration of the deficient nutrient in therapeutic amounts. When the dietary mineral content is faulty, Mayer suggests suitable supplementary sources of calcium and phosphorus-milk, steamed bone meal, dicalciurn phos-phate or prepared mineral mixtures. Exposure to sunlight is an aid in the treatment and prevention of rickets, particularly in short-haired dogs.

Excessive amounts of phosphorus in-terfere with absorption of calcium. Thus the development of rickets on a diet composed largely of meat has been ascribed to an excessive intake of phos-phorus. In such cases calcium gluconate or calcium lactate is preferable to di-calcium phosphate. Similarly, certain cereals have a high phosphorus content and a low calcium content and are rachitogenic.

In studies with cats it was found that the level of serum calcium controls the function of the parathyroid glands. Hypocalcemia may result from dietary deficiency, but is more commonly the result of excessive phosphorus feeding with low, normal or even high calcium levels in the diet. The important thing is that the diet is imbalanced in favor of phosphorus. Beef heart, a popular cat food, contains more than 20 times as much phosphorus as calcium. Still worse is the feeding of an imbalance of beef liver with an imbalance of 1:50 in favor of phosphorus. Young kittens; placed on a minced beef heart diet develop x-ray signs of generalized osteitis fibrosa within three weeks and folding fractures of the long bones occur a few weeks later.  Even at the advanced stage  when folding fractures occur, the cats  respond to treatment

Unless a favorable calcium-phosphorus ratio in the diet is known or can be established, it may be necessary  to administer vitamin D in doses well above the minimal amounts considered effective. This is especially true in larger breeds and in the winter months. When high dosages  are administered for any length of time, toxic effects must be guarded against.  It also is advisable to use a combination of vitamins A and D, rather than D alone.