Human Nutrition 3(3+0)
Lesson 44 : Bioavailability and Ion Absorption Electrolyte Balance


Sodium is the major positive ion of the extracellular fluid. It is important for maintaining

    • Fluid balance
    • Muscle irritability
    • Acid-base balance
    • Conducting nerve impulses and osmotic pressure.

When combined with chloride, the resulting substance is sodium chloride or table salt. Excess sodium (such as that obtained from dietary sources) is excreted in the urine. Fluid balance throughout the body depends partly on varied sodium concentrations among the water-containing compartments in the body. In a normal man the extracellular fluids contain 2175 mEq. of Na and intracellular fluids about 375 mEq. In addition about 900 mEq of Na is present in the bones, making a total of about 3450 mEq. (200g. of Nacl) in the whole body.

Intake of sodium:
Dietary intake of sodium is very variable. Most normal people ingest 5 to 20g. NaCl/day, corresponding to 70 to 350 mEq. of Na. The food as served contains about 200 mEq. Na/day. In general, foods as such, contain relatively little sodium, but large amounts may be added in cooking. In most forms of processing and preservation, salt is added. Butter and many other foods are commonly salted in preparation.

Output of sodium:
Sodium is lost through sweat, faeces and urine. In normal conditions, the losses through the first two routes are minimal, and a bulk of it is lost in urine. The sweat loss is variable and can be decreased by adaptation to heat. Most of the daily output of sodium is in the urine which usually contains 100-150mEq./24 hrs., and reflects the dietary intake.

Table : Sodium balance in man (mmoles/day)


Sodium input


Sodium output

Food and water









a---- Variable with environmental temperature and humidity and physical activity
b---- Negligible in basal state but can be high in diarrhoeal diseases.

Source: West J.B. (1990).

Men doing hard work in hot humid climates, as in mines suffer from heat cramps, i.e., intense and painful contractions of skeletal muscle. This is due to NaCl deficiency caused by loss of NaCl from the body by excessive sweating.
  • Increased sodium level in the blood (hypernatremia) occurs whenever there is excess sodium in relation to water. There are numerous causes of hypernatremia; which include kidney disease, too little water intake, and loss of water due to diarrhea and/or vomiting.

  • A decreased concentration of sodium (hyponatremia) occurs whenever there is a relative increase in the amount of body water relative to sodium. This occurs in some diseases of the liver and kidney, in patients with congestive heart failure, in burn victims etc.

A normal blood sodium level is 135 - 145 milliEquivalents/liter (mEq/L), or in international units, 135 - 145 millimoles/liter (mmol/L).
Last modified: Wednesday, 15 February 2012, 6:16 AM