The standard nutrition labels you see on most packaged foods say your daily sodium intake should be less than 2,400 mg. A diet of average American food can easily contain much more sodium, mostly in the form of ordinary table salt. So how much is really too much?
One research group tested the effects of different salt concentrations in the body by measuring blood pressure after an intravenous infusion of 7,000 mg of sodium (in the form of NaCl, 18 g) for 378 healthy volunteers and 198 subjects with high blood pressure. This is 3 times the value recommended on our food labels. To get a baseline blood pressure, researchers then gave the test subjects a low sodium diet and a diuretic to flush out the salt.
For 26% of the healthy volunteers, the difference in blood pressure between high and low salt content was 10 mmHg or more. These were defined as salt sensitive. For the subjects with high blood pressure, 51% were found to be salt sensitive by this definition.
Salt sensitivity can also be defined in terms of the difference in blood pressure for the subject on a high salt diet (5,700 mg sodium per day) compared to low salt diet (200 mg sodium per day). For the overall population of adults who do not have high blood pressure, one third are reported to be salt sensitive. African Americans and elderly subjects are more likely to be salt sensitive than younger white adults.
Researchers in Japan identified a defect in kidney function as the cause of salt sensitivity. They suggest their findings may form the basis for development of drugs to treat this condition. However, for many people, an increase in dietary potassium is enough to suppress salt sensitivity.
Our kidneys work best with more potassium than sodium in our diets, according to researchers at the University of California. Potassium is abundant in fruits and vegetables, but not grains (bread, pasta, etc.) The average American diet may have more issues with potassium deficiency than with salt excess.
Dietary salt is a controversial subject. For a brief history of the debate, see http://www.empowher.com/media/reference/shaking-conventional-thinking-about-sodium-and-hypertension