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The calcium:phosphorous connection


Calcium and phosphorus are essential for sound and normal bone development – bone structure is 35% calcium and 17% phosphorus.

Together they help compose bone, and they play major roles in metabolic activity. Hormones tightly regulate their levels in the body to maintain a balance between the two, as both calcium and phosphorus dance between different parts of the body as needed via bone, blood and tissue and back again.Calcium ranks as the most common of the body’s minerals, while phosphorus is the next most abundant.

In addition, calcium is essential for normal muscle contraction where it is needed to activate potassium ion channels. It is a cell membrane transmitter and regulates enzymes and their actions. Nearly all the body’s calcium is in the bones (99%) and the remaining 1% circulates in the blood stream.

Phosphorus is essential for energy production in cells. Approximately 80% of phosphorus is in the bones.

The calcium: phosphorus ratio and its interventions with other minerals and compounds

The calcium:phosphorus ratio is critically important for our horse’s nutritional wellbeing, as well as the absolute amount of each mineral fed. All diets should have more calcium than phosphorus – most requirements yield ratios of 1.2:1 to 1.4:1. Every gram of phosphorus eaten by the horse must be matched by calcium before the calcium can be absorbed across the intestinal wall.

Diets high in grain, bran and alfalfa will affect the ratio - if the dietary level of calcium is low the efficiency of absorption increases. If there is excess calcium in the diet, less will absorbed by the gut and more is excreted in the urine. If this isn’t confusing enough then consider the role other minerals play:

  • high magnesium levels increase calcium absorption, but excess phosphorus will decrease calcium absorption.
  • High zinc levels will decrease calcium and copper absorption, while high calcium levels will interfere with copper, manganese, zinc and iron absorption.
  • High calcium levels limit phosphorus absorption, with high sodium and chloride levels increase phosphorus absorption by 30-60%.Regulation of calcium levels in the blood and bone

Providing our horses have a healthy, adequate amount of forage to browse on, the system seems to regulate mineral levels fairly well. Calcium levels in the body are regulated by Vitamin D and by the hormones calcitonin and parathyroid. Blood levels of calcium are maintained in a very narrow range to ensure normal neuromuscular activity. The body will sacrifice optimal bone strength to maintain normal blood calcium levels.

If daily calcium requirements aren’t sufficient from the diet, then parathyroid hormone is released to take calcium from the bones to circulate in the bloodstream. If the blood calcium levels get too high, calcitonin and parathyroid hormone act to decrease gut absorption and increase urinary excretion.

Vitamin D is needed to help with absorption of calcium and to a lesser extent phosphorus from the gut. Signs of calcium deficiency and excess signs include shifting lamenesses, weak bones, ostepaenia (crooked bones and enlarged joints), spontaneous fractures of bones, tying up, and poor performance. These are non specific signs that can mimic other conditions so a thorough veterinary check is essential.

A horse with calcium depletion can take up to 12-months to correct the problem. Low blood calcium levels can present as staggers and thumps - both medical emergencies and needing veterinary intervention. Staggers is a repeated prolonged contraction of muscles, especially of the face and limbs, caused by low blood calcium.

Staggers can occur in horses with marginal blood calcium levels where they start twitching, spasming and have rigid, stiff legs. Thumps are a synchronous diaphragmatic flutter where there is very low blood calcium, potassium and chloride from excessive sweating. The phrenic nerve becomes hyperactive and irritable and the diaphragm thumps in synchrony with the heartbeat.

Consequences of phosphate deficiency and excess

High phosphorus levels will lead to chronic calcium deficiency which leads to nutritional hyper-parathyroidism (big head). Deficiency of phosphorus will lead to soft bones.

Dietary sources of calcium and phosphorus

Most grassy hays contain low to moderate levels of these minerals whilst cereal grains are low in calcium and high in phosphorus. Bran is also a rich source of phosphorus but very low in calcium. Alfalfa has a high level of calcium and very low phosphorus – if alfalfa represents more than 50% of the feed ration there will be excessive calcium and protein in the diet. How much calcium and phosphorus a horse requires depends upon the horse’s age, stage of growth, workload, and pregnancy/lactation status in a mare.

It is generally accepted that an average 500kg adult horse on a maintenance workload requires approximately 20g calcium and 14g phosphorus per day for maintenance needs. This equates to calcium being 0.21% and phosphorus being 0.15% of the daily diet.

However, a horse in very hard work requires 0.31% calcium and 0.23% phosphorus in the diet (a 48% increase in calcium needs and 53% increase in phosphorus needs over maintenance levels).

Senior horses over 20 years may require more phosphorus than adult maintenance. Excess calcium intake should be avoided in older horses.

Foals and youngstock needs are again different, i.e. foals (4mo’s) require 0.62% calcium and 0.34% phosphorus in the diet; 2yr-olds need 0.28% calcium and 0.15% phosphorus.

A pregnant mare in the last 90 days of gestation requires a diet with 0.4% calcium and 0.3% phosphorus. A lactating mare with a foal up to three months of age requires 0.47% calcium and 0.30% phosphorus in the diet.