Magnesium - Nutrition

What is Magnesium?

Magnesium is a mineral that is the second most common cation (positive ion) in intracellular fluid, after potassium. Its other main site of storage is in bone, where about 50% of the bodys' magnesium is found. Like other minerals, magnesium is a reactive metal that is not found in its free state in nature but combined with other elements to form salts.

Why is it Important?

Magnesium is an important cofactor in enzyme reactions involved in major metabolic pathways. Examples include those associated with cellular respiration and the transfer of phosphate between adenosine triphosphate (ATP), diphosphate (ADP) and monophosphate (AMP). It is also vital for normal nerve, muscle and heart function and bone mineral formation.

Roles in the Body

Although large amounts of magnesium are stored in bone, it is not readily mobilised as a reserve for the animal when the supply of dietary magnesium is insufficient. The absorption of magnesium is via an active transport process but passive diffusion from the digestive tract can occur if there is a concentration gradient from the gastrointestinal lumen to the extracellular space. Extracellular magnesium is vital for normal nerve function. A common sign of magnesium deficiency is hyperexcitability caused by a reduction in the resting potential of the nerve membrane, which allows the action potential to be triggered prematurely.

Consequences of Magnesium Deficiency

Dog:

Studies evaluating magnesium deficiency in dogs have had to rely on using purified diets containing artificially low magnesium content, as a naturally-occurring deficiency in magnesium is not achievable.

Recognised Syndromes Related to Magnesium Deficiency

1. Joint problems and paralysis: Weaned mongrel puppies fed a purified diet with no added magnesium developed hyperextension of the carpal joints and hind-leg paralysis. The animals recovered on a diet containing 0.024% magnesium in dry matter (DM). Similar signs were seen in puppies fed dietary magnesium at less than 0.014% DM, in adult beagles fed 0.017% DM magnesium and in Beagle puppies fed a dietary magnesium content of 0.014% DM.
2. Effect of phosphorus: Bunce et al. showed that the adverse effects of magnesium deficiency were exacerbated by increasing dietary phosphorus from 0.4% to 0.9% DM. As the nutritional recommendation for phosphorus in puppies is around the higher of these two values, it is particularly important to ensure that the dietary content of phosphorus and magnesium are correctly balanced.

Cat:

As with dogs, studies of magnesium requirements in cats have used semi-purified diets in which the concentrations of magnesium and other minerals have been controlled.

Recognised Syndromes Related to Magnesium Deficiency

1. Muscle weakness: Clinical signs of magnesium deficiency in kittens include muscle tremors, lack of muscle tone and muscle weakness, initially in the hind limbs. There was also depressed food intake.
2. Effect of calcium: In the same study, the effect of increased dietary calcium on magnesium deficiency was also investigated. The deficiency signs described above in section (1) occurred only when kittens were fed 0.01% magnesium and a high calcium level of 2.3% calcium (both on a dry matter (DM) basis). However, kittens receiving 0.01% magnesium and a normal dietary calcium of 0.6% DM showed a lower serum magnesium than those on higher magnesium levels. Kittens receiving magnesium at 0.02% DM or higher and a normal dietary calcium showed no adverse effects. As observed in the dog studies, it is important that the balance of magnesium and other minerals is appropriate. This effect on magnesium requirement may be due to the formation of insoluble calcium-magnesium-phosphorus complexes that lower the bioavailability of magnesium.

Toxicity/Excess

Dog:

There is no direct information on dietary toxicity of magnesium in dogs. It has been reported that diets containing up to 0.2% DM magnesium (providing 3 to 5 times their requirement) have no adverse effects in adult dogs.

Cat:

There is ample evidence that increasing dietary magnesium is associated with urinary tract disease in cats, notably the formation of uroliths composed of struvite (i.e. magnesium ammonium phosphate) [DC to provide image of uroliths]. Dietary magnesium above 0.1% DM had previously been implicated in urolithiasis, however it is thought that this is an indirect effect of magnesium salts, increasing urinary pH which is permissive for struvite stones, rather than a direct effect of magnesium on struvite formation. An increase in urine volume can have some ameliorating effect but acidification of the urine (pH around 6.5) substantially increases the solubility of struvite and is the most effective way of dealing with struvite uroliths.

Dietary Sources

Many ingredients used in dog and cat foods contain reasonable amounts of magnesium. The best sources include meat meals, meat and bone meals, cereal products and soybean meal. There are also a number of magnesium salts used as supplemental sources, including oxide, sulphate, carbonate and chloride. Ground limestone also contains magnesium and if used as a source of calcium will normally also supply sufficient magnesium for nutritional requirements.