Integrity Matters' vets continue their look at electrolytes, this time with a focus on calcium and phosphorus.

What is calcium?

Calcium is an important electrolyte that has a positive charge when dissolved in the fluid inside the horse's body.

Calcium plays a vital role in most of the body's functions including energy production, muscle contraction, fluid balance (along with sodium and other electrolytes), blood clotting, nerve impulse transmission and regulation of the heartbeat.

Calcium is also the main substance that makes bones and teeth strong. Calcium is only found in certain feedstuffs. 

What is phosphorus?

Phosphorus is essential for life. phosphorus is highly reactive so it is converted to the more stable phosphate in the body.

Phosphates are found in the nucleus of all cells as DNA and RNA and they form an integral part of every cell membrane. The body uses a special phosphate containing molecule. Phosphate is very common in feedstuffs.

Both calcium and phosphorus have roles in almost all the physiological processes in the horse’s body.

What do calcium and phosphorus do in the horse's body?

Most of us think of the bones when we think of calcium in the body.

It is true that nearly 35 per cent of the total body calcium in stored in the skeleton at any one time. Phosphate combines with calcium to form crystals that make up the hard part of bone.  Approximately 15 per cent of the skeleton is made up of phosphate. 

Bone is constantly being broken down and rebuilt in response to both exercise and calcium demands. The levels of available calcium in the body is very precisely controlled by a complex interplay of hormones. These hormones regulate the release and absorption of calcium (and phosphates) from the skeleton continuously to ensure that there is sufficient free calcium available.

Calcium is under such strict control because it is so important to the body.  

Where do calcium and phosphorus come from?

Phosphorus is present in forage (grass hay, chaff, pasture) and grains. It is particularly high in bran, and soybean meal.

For many years it was believed that horses did not absorb this organic form of phosphorus very well but recent work by researchers have shown that the bacteria in the horse's hind gut are quite efficient at processing this phosphorus into a form that is available to the horse.

The common practice of adding additional inorganic phosphates into grain-based feed mixes is now being questioned.

Calcium is not as abundant in feedstuffs as phosphorus. Even when calcium is present in a foodstuff, it is not always available to the horse.

Calcium is often chemically bound to other molecules that make it impossible for the horse's gut to absorb and/or the ratio of calcium to phosphorus is too low.

Too much phosphorus in the diet actively inhibits the absorption of calcium by the horse's gut.  The ideal ratio of dietary calcium to phosphorus (Ca:P) for horses is between 1:1 and 2:1. This is especially important for young horses, mares that are pregnant or lactating and horses that are racing.

What happens if calcium and phosphorus are out of balance?

Too much or too little calcium and phosphorus in the growing animal causes limb abnormalities.

Long periods of low calcium availability or high phosphorus in the diet will result in weaker bones even in mature horses. Horses grazing on grasses containing oxalates can develop calcium deficiency as oxalates inhibit the absorption of calcium.

Acute low calcium levels in the blood can occur in stressed lactating mares, horses with severe infections, exhausted animals and horses that have been treated with alkalinising agents such as sodium bicarbonate.

Synchronous diaphragmatic flutter (SDF or "thumps") is a sign of low calcium levels. Another sign is a “lights are on but nobody's home” aspect to the horse's interaction with its environment, in that the horse will react to stimuli but simultaneously appearing unaware of its surroundings.

Signs of low blood calcium levels should be addressed as soon as possible as it can lead to muscle tremor, tetany, stilted gait, cardiac arrhythmias and ultimately seizure and death. 

It is relatively rare for horses to suffer high levels of calcium. It is most often associated with kidney disorders (so excretion of calcium is impaired) or specific cancers of the parathyroid gland, which makes the hormones that control calcium.

There are some plants that increase the quantity of Vitamin D in the blood stream and this will increase concentrations of both calcium and phosphorus in the blood. Overdosing with Vitamin D will have the same effect. Signs of high calcium blood levels include cardiac arrhythmias, depression, poor performance and weight loss.

Low levels of phosphorus are seen in the same diseases that cause high levels of calcium. Low level of blood phosphorus is also seen in animals that are starving or have disorders of the pancreas.

Animals given alkalinising agents may also exhibit low levels of phosphorus. High levels of blood phosphorus are found in horses with acute kidney failure, tying up, injury or disease that is causing cell breakdown, parathyroid disease and overdosing with Vitamin D.

How do you ensure your racehorse gets the right amount of calcium and phosphorus? 

It is not just the amount of calcium or phosphorus, but also the relative proportion of each of these important electrolytes that determines how much of these electrolytes your horse needs.

It is much easier to feed too much phosphorus than too much calcium, especially as the amounts of both elements vary so much in different feedstuffs.

Overfeeding other minerals can also interfere with the absorption of calcium and/or phosphorus.

It is important to review the ingredients on any commercial feed mixes and supplements (including those administered via injection) to ensure that both the availability and the ratios of calcium and phosphorus is correct for your horse.

Special care must be taken to consider the overall balance of these electrolytes if injectable or paste forms of concentrated electrolytes are used as well as dietary sources.

Current recommendations for calcium intake (based on a 500kg horse in work) is 45 grams + 0.2g/kg to replace calcium lost in sweat, while phosphorus intake is between 25-30 grams in proportion with calcium intake.

Consideration of calcium and phosphorus intake is an important part of ration formation. Consultation with a trained equine nutritionist can assist in this process.