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Blood pressure responses to changes in dietary salt and potassium intake remained consistent in two assessments separated by nearly 5 years, researchers found.

The original and follow-up protocols were identical, and included a 7-day low-sodium and a 7-day high-sodium feeding, as well as a 7-day high-sodium feeding with oral potassium supplementation, according to , chair of the department of epidemiology at Tulane University School of Public Health and Tropical Medicine in New Orleans, and colleagues.

The correlation coefficients, which are measures of how two linear entities move in lockstep (with 1.0 being perfect), were "moderate but highly significant" (P<0.0001 for all) for absolute systolic blood pressure levels at the two time points:

• 0.77 at baseline
• 0.79 low sodium
• 0.80 high sodium
• 0.82 high sodium and potassium

The correlation for absolute diastolic blood pressure and mean arterial pressure levels followed a similar upward slope from baseline to high salt and potassium intake, but were less moderate, ranging from 0.65 to 0.66 and 0.72 to 0.76, respectively (P<0.0001 for all), researchers wrote in the study published online in Hypertension: Journal of the American Heart Association.

While the correlation coefficients for the changes in blood pressure at both time points were low, they were still highly significant (P<0.0001). They ranged from 0.34 to 0.24 for systolic blood pressure responses to low, high, and high sodium with potassium intake, 0.22 to 0.14 for diastolic blood pressure responses, and 0.27 to 0.18 for mean arterial pressure responses.

The results "indicate that blood pressure responses to changes in dietary sodium and potassium are not random phenomena but stable and reproducible human characteristics during a relatively long time period," He and colleagues said.

Such a proclamation "is perhaps a little overstated," wrote Peter W. de Leeuw, MD, PhD, and Abraham A. Kroon, MD, PhD, of Maastricht University in Maastricht, The Netherlands, in an accompanying editorial.

They noted the low correlations of the blood pressure responses and said the results from the first assessment can explain only about 20% of the results from the follow-up assessment.

However, with so many "potential mechanisms involved, it would be surprising to find a much greater concordance," they pointed out.

Two potential mechanisms reside on chromosomes 1 and 18, according to a study by Toru Nabika, MD, PhD, of Shimane University in Izumo, Japan, and colleagues, that was published in May in Hypertension.

They found fragments on these chromosomes in a rat model that harbored major . "The salt-sensitive blood pressure increase was implied to play a key role in the stroke susceptibility," they wrote.

He and colleagues said the relatively weak correlation for blood pressure responses is "most likely because of their greater random error."

Leeuw and Kroon suggested blood pressure response to salt may have a setpoint but is not fixed in time. Good reproducibility, therefore, is a matter of the "relative constancy of the compensating mechanisms." Consequently, the focus of research should be on those mechanisms that determine the salt-pressure setpoint.

Prior studies of the reproducibility of salt sensitivity have produced inconsistent results potentially due to differences in participant characteristics, salt-sensitivity cutoffs, and blood- pressure-reading methods.

Therefore, He and colleagues enrolled 487 Chinese adults from rural areas in northern China in the , initially from October 2003 to July 2005 and then from August 2008 to November 2009. The median age was 37 and participants were equally divided between men and women.

Trained and certified observers measured blood pressure on each of the 3 days of baseline observation and on days five, six, and seven of the initial and follow-up study. The average time between each assessment was 4.4 years.

The same observer obtained readings using the same sphygmomanometer at about the same time each day for each study participant.

One technician at the calibrated the random-zero sphygmomanometers. Other protocols were followed to minimize error.

Leeuw and Kroon praised the study authors for avoiding the dichotomous "salt-sensitive or resistant" approach and instead considering the "degree of salt sensitivity as a continuous variable." This helps in learning whether being salt-sensitive is a "fortuitous phenomenon" or not, they said.

From the American Heart Association:

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