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Sodium and chloride

Sodium and chloride are secreted intensively into the digestive tract with saliva, pancreatic juice, and intestinal secretions. Up to 99% of the content of these minerals is absorbed in the hind-gut. The gut is not an excretory organ. Any dietary surplus of sodium and chloride is instead excreted in the urine. Dietary potassium passes through the gut wall of the small intestine (65-70%). In the large intestine, potassium may both be absorbed from the gut contents and secreted from the blood into the gut contents.

Calcium is absorbed in the small intestine. The efficiency of the absorptive process is regulated according to the bodily needs for calcium. Absorption is high when calcium losses are increased, such as during lactation, or when bone mineralization requires large amounts of calcium in young growing horses. Calcium absorption is stimulated by vitamin D and impaired in vitamin D deficiency. 

Magnesium metabolism is much less tightly controlled than calcium metabolism. Magnesium is absorbed more or less proportionately to the magnesium content in the diet. Magnesium surplus is excreted with urine. 

Phosphorus is secreted into the digestive tract with saliva and other digestive juices. In roughage-rich rations the secretion of phosphorus with the digestive juices may exceed the dietary intake of phosphorus. In such cases it is first in the hind-gut that there is net absorption of phosphorus. In low-fibre rations, the secretory loss of phosphorus to the intestine is smaller, and net absorption takes place in the small intestine. Calcium is absorbed only in the small intestine, while phosphorus can be  absorbed both in the small intestine and the hind-gut. High phosphorus contents relative to calcium may reduce calcium absorption in the small intestine, and thus cause secondary calcium deficiency. Phosphorus is less influenced by the calcium content of the diet.