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Everybody needs salt, but health experts generally agree that most Americans get too much of it. Excess salt can lead to excess fluid retention in the blood. This makes the circulatory system "fuller" and the pressure inside it greater. Salt can also make small blood vessels called arterioles contract, which effectively shrinks the circulatory system, again increasing pressure.
A Jan. 20 report in the New England Journal of Medicine concluded that if everyone in the United States consumed half a teaspoon less of salt every day, every year the number of new cases of coronary heart disease would drop 60,000; of stroke, 32,000; and of heart attack, 54,000 -- and the number of deaths from any cause each year would drop by 44,000.
But, in fact, the relationship between salt and high blood pressure is complicated. In some "salt-sensitive" people, blood pressure responds dramatically to changes in salt intake. In others, not so much.
Summarizing vast amounts of research in this area, authors of a 2006 article in the Journal of the American College of Nutrition (part of a special issue on the health effects of salt) wrote that in the general population, "careful observations using sophisticated techniques have revealed only a weak relationship between sodium intake/excretion and blood pressure."
It's hard to tell who's salt-sensitive. There's no simple test. But many factors seem to matter, including family history, age, weight, diet and race and ethnicity.(African Americans are more likely than others to be salt-sensitive.)
The reasons for salt sensitivity aren't well understood, but it's thought that if the body's own blood-pressure-regulating systems become compromised in some way, the effects of salt on fluid retention and blood vessel contraction are left to rule the day.
Given the ambiguities, most medical experts say that limiting salt consumption is a good general rule. Between 1,500 and 2,300 milligrams of sodium a day are considered plenty for healthy adults (that's a teaspoon of salt or less); if you're older than 50 or African American, or if you have other risk factors for salt sensitivity, you should stick to the low end of that range.
"That can be difficult, especially if you eat out often," says Dr. Mitra Nadim, director of the Hypertension Center at USC. More than 75% of the sodium in the average American diet comes from processed and prepared foods.