Q: I have heard that there is a lot of sodium or salt in tap water. Is that true? How much sodium is in a glass of water? I would also like to know how potassium works in the body and what foods are high in potassium.
A: The sodium content of water depends somewhat on its source. For example, well water may contain more sodium than the surface water supplying Baltimore City and Baltimore County, where the amount of sodium in tap water is quite low -- approximately 2 milligrams per cup of water. It might be helpful to compare this with the sodium content of a few foods: 127 mg in a cup of skim milk, 142 mg in a slice of rye bread, and 46 mg in a teaspoon of margarine. Thus, water makes a minimal contribution to the sodium intake for people on unrestricted diets and even for those on diets severely restricted in sodium (500 mg per day). A reduced intake of sodium is recommended for individuals with fluid retention (edema) or high blood pressure.
Potassium is the major mineral in all body cells. Potassium is essential for normal growth, for the function of certain enzymes, and for maintaining the normal acid-base balance of body fluids. The relative amounts of potassium within and outside of cells regulates the excitability of nerves and muscles. The effects of low body potassium include weakness, temporary paralysis, a fall in blood pressure on standing up (postural hypotension), and excessive urination. High blood levels of potassium, most often due to kidney failure, can cause disturbances of the heart rhythm and sudden cardiac arrest.
Some of the foods rich in potassium are fruits (especially bananas, oranges, apples, pears, prunes, peaches, grapefruit; vegetables (especially potatoes, tomatoes, broccoli, carrots); and milk, turkey, chicken, fish and shellfish.
Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine and associate dean for faculty affairs at the school.