Sodium is the major cation of extracellular fluid [ECF (1 mmol, or molar equivalent, corresponding to 23 mg of sodium)].
The mean body content of sodium in the adult male is 92 g,
half of which (46 g) is located in the ECF at a concentration of 135–145 mmol/L,
∼11 g is found in the intracellular fluid at the concentration of ∼10 mmol/L, and ∼35 g is found in the skeleton.
The concentration gradient between the ECF and ICF is maintained by the sodium–potassium pump activity, which transfers sodium and potassium, respectively, from inside to outside the cell and vice versa against the concentration gradient, using the energy supplied by ATP.
In these cells, sodium transport is associated with that of other substrates, e.g., phosphates, amino acids, glucose, and galactose.
Sodium balance in the body is closely linked to that of water and is finely maintained by the kidneys.
Sodium is an essential nutrient involved in the maintenance of normal cellular homeostasis and in the regulation of fluid and electrolyte balance and blood pressure (BP). Its role is crucial for maintaining ECF volume because of its important osmotic action and is equally important for the excitability of muscle and nerve cells and for the transport of nutrients and substrates through plasma membranes (1).
Deficiencies
Given the presence of added salt in a wide range of commonly used food products, a clinically relevant food deficit of sodium is extremely unlikely in healthy individuals. Indeed, a deficiency of sodium does not occur under normal conditions even with diets very low in sodium. In contrast, an excess of sodium in food is common to most populations worldwide, because of both the salt added to products during food processing and the widespread habit of adding additional amounts of salt in food preparation in the kitchen and at the table. This excess is a recognized causative factor of hypertension and cardiovascular diseases (CVDs) and also contributes to the development of chronic kidney disease, gastric cancer, calcium nephrolithiasis, and osteoporosis.
Toxicity
An acute toxicity from excess sodium intake with the possibility of fatal outcome has been reported in relation to the ingestion of huge amounts of sodium, such as 0.5–1 g of salt/kg body weight. In certain pathologic conditions (e.g., heart failure, decompensated liver cirrhosis, and renal failure), sodium intake to levels routinely present in our diet (≥10 g/d) may lead to a dangerous increase in ECF volume.
However, even under normal conditions, the intake of high amounts of sodium tends to favor, especially in predisposed individuals, an increase of ECF volume and BP. The Intersalt study (2) showed that the higher the habitual consumption of sodium in a given population, the stronger the average BP increase with age and the prevalence of hypertension.
Food Sources
(1 g of sodium corresponding to ∼2.5 g of salt).
The recent WHO guideline has set ≤2000 mg of sodium (5 g of salt) per day
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