Sodium (Na) is the predominant cation in extracellular 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 consort with potassium, the chief cation of intracellular fluid, 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 US and other populations where salt is readily available ranges from about 2,300 to 4,500 mg/day. Mean sodium intake for the entire US 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 those seasoned with soy sauce and monosodium glutamate, may contain the equivalent of 30 to 40 grams salt/day.
The appropriateness of current recommendations for the general healthy population to reduce sodium intake has been a matter of debate in the scientific community. Public health scientists generally support the population-wide approach, while many clinically- or laboratory-oriented scientists are unconvinced that the population approach has sufficient benefit to offset the potential burden–to either consumers or industry– associated with sodium reduction. In addition, although sodium reduction to the levels recommended is presumed to be safe for healthy adults, the debate about this issue has been fueled by assertions that sodium reduction might have adverse effects on health. These assertions about adverse effects appear unwarranted.
Food sources
Sodium added to processed foods accounts for the majority of sodium (75 %) in the US diet. The remainder comes from discretionary salt (15 %) and the sodium that occurs naturally in foods (10%). A substantial portion of sodium in foods is hidden in the sense that it occurs in foods that are moderate in sodium content and that are not thought of as salty foods, e.g., processed grain and cereal products, but which are consumed regularly. Other contributors to high sodium intake are foods with high amounts of salt. High amounts of salt 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 analyses suggest that a reduction in sodium intake of 2,300mg/day would lower systolic blood pressure by about 5-6 mm Hg and diastolic pressure by 1-2 mm Hg among hypertensives, who comprise a sizeable proportion of the US adult population. In addition, many who are not yet hypertensive respond to sodium reduction, particularly those whose blood pressure is in the high normal range. The expected impact of a similar reduction among the normotensive segment of the population is considerably smaller (1-2 mm Hg systolic and 0-1 mm Hg diastolic on average). Sodium reduction among normotensives is primarily designed to effect a downward shift in the average blood pressure in the population, with consequent impact in preventing a large number of cardiovascular disease deaths. High sodium intake or intake of highly salted foods may also be related to asthma, to urinary calcium losses, and to gastric cancer.
Recent research
Phase I of the Trials of Hypertension Prevention (a multicenter, randomized controlled trial) demonstrated, over an 18 month period, the ability of moderate sodium reduction (from 155 to 100 mmol per day) to lower the mean systolic and diastolic blood pressure of 30-54 year old adults who had a high normal diastolic blood pressure at the time of enrollment. This group average blood pressure resulted in a 25% reduction in the number of individuals who developed high blood pressure during the study period. The Trials of Hypertension Prevention, Phase II, included a test of the ability of sodium reduction (without weight loss) to lower mean systolic and diastolic blood pressure in a group of adults who were overweight, over a longer follow up period (36-42 months). Compared to the usual care group (n=596), the chances of developing hypertension were lower in those assigned to sodium reduction (n=594) compared to those assigned to usual care (no dietary intervention) (relative risk or RR) for sodium versus usual care = 0.61, p=.04 at 6 months; RR=0.88; p=0.28 at 18 months; RR=0.88, p=0.09 at 36 months, and RR=0.86, p=.04 for those followed through 48 months. Urinary sodium excretion data collected at each time point suggest that the smaller effect on blood pressure from 18-months on reflects the difficulty in achieving sufficient long term adherence to sodium reduction.