http://www.nhlbi.nih.gov/health/prof/heart/hbp/salt_sum.htm
Here's the relevant quote from the above link - looks like about 50% of people will have their BP affected by sodium, with the percentage increasing as they age.
Sodium
Typically, studies to determine individual differences in blood pressure response to sodium intake have used a very low level of sodium chloride (10 to 20 meq/d) for several days followed by a very high sodium intake, provided either as a saline intravenous infusion or a high sodium chloride dietary intake over several days. As reviewed by Weinberger9 in one study of 19 hypertensive individuals, 9 were categorized as salt sensitive (SS), that is, a decrease then an increase of >=10 mm Hg when a very low sodium diet was followed by a saline infusion. In a study of 82 normotensive individuals following a diet moderately reduced in sodium, 42 percent were considered to be SS, that is, a blood pressure change >=3 mm Hg; 18 percent were salt-resistant (SR); and the remaining 40 percent were considered to be indeterminate. Investigators have also found that some individuals appear to change classification from SS to SR or vice-versa.10 In a study of 28 individuals, blood pressure response to change from a 10 meq sodium chloride intake compared with a high sodium chloride infusion was observed twice within a 12-month period. Reproducibility was reflected by a moderate correlation (R=0.56). On restudy, 18 of 28 were consistent in their responses, 4 changed salt responsivity classification, and 6 were classified as indeterminate (69 mm Hg); 3 were initially classified as resistant (<=5 mm Hg).11
Also, age appears to influence SS. In a study of 660 adults, a progressive increase in SS was seen in hypertensive persons with increasing age. In normotensive individuals, increased SS was seen among those 60 and older. After age 60, there was no significant difference between the SS responses of normal and hypertensive persons.11
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