Sodium

Sodium (Na) is the predominant cation in extra cellular fluid and its concentration is under tight homeostatic control. Excess dietary sodium is excreted in the urine. The mineral is very efficiently reabsorbed by the kidney when intakes are low or losses are excessive. Sodium acts in consort with potassium, to maintain proper body water distribution and blood pressure. Sodium also is important in maintaining the proper acid-base balance and in the transmission of nerve impulses.

Deficiencies: Persons who experience pronounced losses of sodium through diarrhea, heavy perspiration or inability of the kidney to reabsorb it may experience decreased blood volume and a fall in blood pressure that could result in shock.

Diet recommendations: The Estimated Minimum Requirement of Healthy Persons for sodium from the National Academy of Sciences ranges from 120 mg/day for infants to 500 mg/day for adults and children >10 years. Recommendations for the maximum amount of sodium that can be incorporated into a healthy diet range from 2,400 to 3,000 mg/day or 6 to 7.5 grams of table salt/day. Individuals with hypertension should see their physician to determine if a sodium-restricted diet is appropriate for them.

The usual dietary intake of sodium in the U.S. and other populations where salt is readily available ranges from about 2,300 to 4,500 mg/day. Mean sodium intake for the entire U.S. population is 3,280 mg Na or 8.2 grams salt /day; however, discretionary salt intake is not included. Intake of discretionary salt, that added during cooking and at the table, averages 2.7 g/day according to the National Health and Nutrition Examination Survey. A diet based on Asian foods, such as soy sauce and monosodium glutamate, may contain the equivalent of 30 to 40 grams salt/day.

Food sources: Sodium added to processed foods account for the majority of sodium (75 %) in the U.S. diet. The remainder comes from discretionary salt (15 %) and the sodium that occurs naturally in foods (10%). High amounts of sodium are found in table salt and soy sauce, followed by foods in brine such as pickles, olives and sauerkraut. Salty or smoked meats and fish, salted snack foods, bouillon cubes, bottled sauces, processed cheeses, and canned and instant soups also contain significant levels of sodium.

Toxicity: Acute toxicity results in edema and hypertension and can cause death in an infant because of limited excretory ability of the immature kidney. However, sodium is generally nontoxic for healthy adult individuals because it is excreted readily in the urine. High salt intakes have been correlated with hypertension. Meta analysis suggest that a reduction in sodium intake of 2,300/day would lower systolic blood pressure by about 5-6 mm Hg and diastolic pressure by 1-2 mm Hg among hypertensives. However, salt-responsive individuals are estimated to be only 20% of the population. The expected impact of a similar restriction among the normotensive population is considerably smaller (1-2 mm Hg systolic and 0-1 mm Hg diastolic). The appropriateness of current recommendations for the general healthy population to restrict sodium intake has been a matter of debate in the public health community.

Recent research: A study of nearly 2,600 males with mild to moderate hypertension showed that those in the lowest quintile for sodium excretion had significantly greater incidence of coronary heart disease than subjects in the upper quintile. Low sodium diets have also been associated with elevated serum lipids.

For further information:

Muntzel, M. & Tilman, D. (1992) A comprehensive review of the salt and blood pressure relationship. Am J. Hypertension 5: 1S-42S

National High Blood Pressure Education Program (1993) Working Group Report on Primary Prevention of Hypertension. NIH Publication No. 93-2669. National Institutes of Health, Washington, DC.

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