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A new study, for the first time, compared the relationship between sodium intake (a component of table salt) and mortality, based on various methods for assessing usual sodium intake; mean measured sodium showed a direct linear relationship with mortality.
he et al suggest that an incorrect way of estimating sodium intake may help explain the paradoxical results reported in some cohort studies. Image credit: Philipp Kleindienst.
There is a strong linear relationship between sodium intake and blood pressure, but does this relationship extend to an increased risk of mortality? Recent cohort studies have challenged this relationship.
"Sodium is hidden – you often do not know how much you eat, which makes it difficult to estimate how much a person has eaten from a food questionnaire," said Professor Nancy Cook, a researcher at the Division of Prevention Medicine, Department of Medicine, Brigham and Women's Hospital.
"Sodium excretion is the best measure, but there are many ways to collect it, and in our work we used several measurements to get a more accurate picture."
Sodium intake can be measured using a localized test to determine the amount of salt excreted in a person's urine sample.
However, sodium levels in the urine can fluctuate throughout the day, so an accurate measurement of a person's sodium intake on a given day requires a full 24-hour sample.
In addition, sodium consumption may vary from day to day, which means that the best way to get a complete picture of sodium intake is to take samples over several days.
While previous studies have used spot samples and Kawasaki's formula, Professor Cook and his co-authors have evaluated sodium intake in several ways, including estimates based on this formula and those based on the standard method. non-consecutive urine samples.
"Sodium consumption has been evaluated in four ways," they explained.
"(I) measured mean (gold standard): average of three to seven 24-hour urine measurements during the test periods;
(ii) mean estimate: mean of three to seven estimated urinary excretion of 24 h of sodium from sodium concentration in 24 h urine using the Kawasaki formula;
(iii) first measured: 24 hour urine sodium measured at the beginning of each test;
(iv) first estimate: 24-hour urine sodium estimated from the sodium concentration of the first 24-hour urine using the Kawasaki formula. "
The authors of the study evaluated the results for participants in the Trials of Hypertension Prevention, which included 2,974 individuals aged 30-54 years with pre-hypertension.
The reference method showed a direct linear relationship between the increase in sodium intake and the increase in mortality risk.
Scientists found that Kawasaki's formula suggested a J-shaped curve, which would imply that low levels and high levels of sodium consumption were associated with increased mortality.
"Our results indicate that inaccurate measurement of sodium intake could be a major contributor to the J-shaped paradoxical results reported in some cohort studies," they said.
"Epidemiological studies should not associate health outcomes with reliable estimates of sodium intake."
The results were published in the International Journal of Epidemiology.
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Feng J. He et al. The errors in the estimation of usual sodium intake by the Kawasaki formula alter its relationship with mortality: implications for public health. International Journal of Epidemiology, published online June 22, 2018; doi: 10.1093 / ije / dyy114
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