Tuesday, September 2, 2014

Sodium, in the spotlight for next week's #NephJC

In August, the NEJM pushed out three articles examining the role of sodium in human disease. These are the subject of September 9's #NephJC.

The first article is the Association of Urinary Sodium and Potassium Excretion with Blood Pressure. This question used the large epidemiologic study, Prospective Urban Rural Epidemiology (PURE) to answer the question.

PURE enrolled 157,543 adults age 35 to 70 from 18 low-, middle-, and high-income countries on 5 continents.

The study collected 102,216 fasting first morning urines. The authors used the Kawasaki formula to extrapolate 24 hour urine sodium and potassium from the samples. They collected 24-hour samples on 1,000 patients and found that they over estimated sodium intake by about 7%:

The mean sodium excretion was 4.9g and the mean potassium excretion was 2.1 grams.
It was difficult for me to understand the difference between the Observed excretion and Usual excretion but the authors seemed to reference the Usual excretion as the definitive curve.

Sodium excretion was higher in rural areas and in lower income countries. The reverse was true for potassium, higher in cities and higher in higher income countries.

The meat of the paper was the positive association between sodium intake and blood pressure. For every additional gram of sodium excretion the systolic blood pressure went up 1.46 mm Hg and the diastolic rose 0.54 mm Hg (P less than 0.001). Statistical mumbo jumbo increased those numbers to 2.11 systolic and 0.78 mm Hg diastolic. This relationship was non-linear with increased blood pressure effect as the sodium excretion rose over 5 grams.

Potassium had the opposite effect with systolic blood pressure falling 0.75 systolic (1.08 after statistical adjustment) and diastolic dropping 0.06 (0.09 adjusted) mm Hg for every gram increase in potassium excretion. 

Older people showed larger changes in blood pressure with increased sodium excretion.

The sodium effect on blood pressure was a lot larger that the 0.94 mmHg systolic and 0.03 mmHg diastolic found in the landmark INTERSALT study but still seems like a pretty small effect given the difficulty in getting to a low a salt diet. Look at the bell curve showing only 0.2% of samples hitting the WHO goal of less than 2.3 g a day.
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