Sodium

Sodium is an essential macronutrient. It is one of the components of table salt (sodium chloride) and the those two ions are paired in many of their functions. Sodium is the princiapl cation of the extracellular fluid and thus plays a major role in osmotic regulation and thus extracellular fluid and plasma volume.

Absorption
Pure metallic sodium is not found naturally and reacts exothermically with water. Sodium is absorbed as an ion and found in ionic compounds such as sodium chloride (primary source), sodium bicarbonate, and additives including sodium phosphate and sodium carbonate. Absorption occurs in the small intestine and it is approximately 98% across a wide intake range. Almost all absorbed sodium is later excreted via sweat and urine.

Deficiency
The effects of deficiency of sodium are not well known, though there are a number of potential concerns including adverse affects to blood lipids, insulin resistance, and cardiovascular disease risk. The daily recommendation takes a normal amount of sweating into account, so only those who work in hot and physically demanding environments may be in danger from sweating enough to be deficient in sodium. Without substantial sweating, one only loses 0.18g sodium per day, so they can survive on extremely low sodium intakes.

Overdose
It is difficult to overdose on sodium, as it is a water-soluble ion eliminated through urine. However, high daily intakes of sodium increase the risk for many conditions. For adults, the upper limit is 2.3 g/day. High sodium intake is one of several dietary factors that contribute to increased blood pressure, along with excess weight, potassium deficiency, high alcohol consumption, and suboptimal dietary pattern. Increased sodium intake increases its excretion, and along with it the excretion of calcium which may lead to calcium deficiency, though this is unlikely because the two ions are usually consumed together.