History

Known since ancient times as essential for both animals and humans, animals have travelled long distances to salt deposits because of 'salt hunger'.

Chemical properties and distribution

• 0.2% sodium in the body, some skeleton (insoluble), but most in extra cellular fluids
• Chlorine in contrast, both within and outside cells, blood cells contain half as much as plasma

Absorption

• Sodium and chlorine is readily absorbed by the intestine
• Transport of sodium across intestinal epithelium involves a system of "pumps" and passive "leaks" in cell membranes
• 80% of sodium and chlorine entering the gastrointestinal tract arises from internal secretions (e.g., saliva, gastric fluids, bile and pancreatic juice)
• Loss via perspiration can be a major route at times of increased workload etc
• Losses also from vomiting and diarrhea

Regulation of body content

Any animal has a remarkable capacity to conserve sodium, amount of excretion can decline very rapidly to extremely low levels in an effort to conserve levels. Body sodium levels are controlled by hormones. Balance may be affected by saliva loss which is increased by temperature.

Physiological functions

• Sodium, chlorine and potassium all function in maintaining osmotic pressure and regulating acid-base equilibrium
• At the cellular level, involved in water metabolism, nutrient uptake and transmission of nerve impulses
• Sodium is over 90% of blood cations (positively charged elements)
• Sodium is a major component of salts in saliva to buffer acids
• Sodium has a major role in transmission of nerve impulses and proper heart and muscle contractions
• In respiration and regulation of blood pH, chlorine is transferred between plasma and erythrocytes through a process known as chloride shift
• Sodium and chlorine control passage of nutrients into cells and waste products out
• Sodium ions in intestine are needed for absorption of sugars and amino acids
• Absorption of riboflavin, thiamine, ascorbic acid nucleosides (from microbial breakdown of nucleic acid) is sodium-coupled
• Chlorine is a major nutrient of extra cellular fluid, also gastric secretions, part of HCL which is essential for protein digestion
• Chlorine is essential for activation of intestinal amylase

Requirements

Under tropical, hot and semiarid conditions horses can experience large losses of water and salt, requirements depend on sweating capabilities and their activity. Shade and air movement will lower salt requirements.

In addition to higher salt needs related to growth, lactation and warm climates, additional factors are:

• Level of potassium in diet
• Whether feed content is dry or green, horses fed silage typically consume more salt then those fed hay.
• Genetic differences – sodium in milk genetically controlled
• Body losses – depletion with diarrhea and vomiting, also kidney or adrenal or adrenal damage depletion of these elements

Deficiency

Sodium deficiency is most likely to occur:

• Rapidly growing young animals fed cereal-based or low sodium forages
• As a result of lactation
• Hot regions conducive to loss of sodium-chloride in sweat, particularly hard working horses

Initial signs of sodium and chloride deficiency is a craving for salt; avid licking of wood, soil, rocks, other objects, sweat from other animals and drinking water. With severe deficiency and considerable sweating, horses become fatigued and exhausted. When horses are deprived of salt, they tire easily, eventually stop sweating, decrease milk production and exhibit muscle spasms.

Supplementation

The most accurate indicator of sodium status in a horse is the level in urine and saliva.

Horses will not consume harmful amounts of salt if it is supplied if plenty of drinking water is available, obviously in this situation though high performance horses are monitored for intake. 'Salt poisoning' is largely a misnomer because the condition is in conjunction with water deprivation.