Sodium
is the primary positive ion found in the blood and body fluids; it is also found in every cell although it is mainly extracellular, working closely with potassium, the primary intracellular mineral. About 60 percent of body sodium is in the fluids around cells (extracellular), 10 percent is inside the cells, and around 30 percent is found in the bones. Sodium is one of the electrolytes, along with potassium and chloride, and is closely tied in with the movement of water; "where sodium goes, water goes." Sodium represents about 0.15 percent of the body weight. Approximately 90-100 grams are present in the body, most of which occurs in combination with chloride as salt, or sodium chloride.
Sodium chloride is present in solution on a large part of the earth's surface in ocean water. In common usage, the word "salt" refers mainly to sodium chloride, but in chemistry, a salt is any combination of a positive and a negative ion in crystalline form or in solution. Sodium chloride is only 75 percent of the salt in seawater, which also contains potassium chloride (KCl), calcium chloride (CaCl2), and calcium phosphate (Ca(CPO4)2), as well as other mineral salts.
Sodium, or salt, has been valued throughout history. The word "salt" is the source of the English word "salary," which originally referred to money paid to soldiers to buy salt. Yet this value placed on salt has possibly led to its overuse in industrial society. For millions of years, the human species lived on a natural diet containing less than 1 gram per day of sodium, and elevated blood pressure was very rare. Nowadays, 6-12 grams and even higher amounts of salt per day are consumed by people eating processed and snack foods or as salt added in cooking and preparing foods. Salt itself is 40 percent sodium and 60 percent chloride. Therefore, 5 grams of salt (about one teaspoon) contain approximately 2 grams of sodium.
High blood pressure is now epidemic in our society as well as in all other cultures eating high-salt diets. Where natural foods are the only source of sodium, there is almost no hypertension. These foods contain more potassium, which is found in high amounts in plant cells as well as in human cells. There is still some controversy about the relationship between salt and high blood pressure; the sodium-potassium ratio may be even more important in controlling blood pressure than the actual amount of sodium. Certain people seem to be more sensitive to sodium and its effects on blood pressure, although it is not clear whether this is due to genetic or other physiologic factors. Restricting sodium may significantly help the estimated 20-30 percent of the population that is salt sensitive. Reducing salt intake may have less effect on the blood pressure of other people. In any event, eating a low-sodium diet on a long-term basis may be one of the best ways to prevent hypertension; this will likely be even more effective if the diet is low in fat as well. Research indicates that other minerals, including calcium, magnesium, and chloride, may also be implicated in high blood pressure.
Sodium, like potassium, is very soluble and, therefore, is easily absorbed from the stomach and small intestine-nearly 100 percent of the sodium consumed gets into the body. It goes into the blood and is circulated through the kidneys, which can reabsorb or eliminate it in order to maintain stable blood sodium levels. About 90 percent of the sodium consumed in the average diet is in excess of body needs and must be eliminated in the urine. Therefore, urine levels reflect dietary intake. Aldosterone, a hormone made and secreted by the adrenal cortex, acts on the kidneys to regulate sodium metabolism.