A new study, for the first time, has compared the relationship between sodium (a component of table salt) intake and mortality, based on various methods to assess usual sodium consumption; the average measured sodium showed a direct linear relationship with mortality.

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 contested this relationship.

“Sodium is hidden — you often don’t know how much of it you’re eating, which makes it hard to estimate how much a person has consumed from a dietary questionnaire,” said Professor Nancy Cook, a researcher in the Division of Preventive Medicine, the Department of Medicine, Brigham and Women’s Hospital.

“Sodium excretions are the best measure, but there are many ways of collecting those. In our work, we used multiple measures to get a more accurate picture.”

Sodium intake can be measured using a spot test to determine how much salt has been excreted in a person’s urine sample.

However, sodium levels in urine can fluctuate throughout the day so an accurate measure of a person’s sodium intake on a given day requires a full 24-h sample.

In addition, sodium consumption may change from day to day, meaning that the best way to get a full picture of sodium intake is to take samples on multiple days.

While previous studies have used spot samples and the Kawasaki formula, Professor Cook and co-authors assessed sodium intake in multiple ways, including estimates based on that formula as well as ones based on the gold-standard method, which uses the average of multiple, non-consecutive urine samples.

“Sodium intake was assessed in four ways,” they explained.

“(i) average measured (gold standard): mean of three to seven 24-h urinary sodium measurements during the trial periods;

(ii) average estimated: mean of three to seven estimated 24-h urinary sodium excretions from sodium concentration of 24-h urine using the Kawasaki formula;

(iii) first measured: 24-h urinary sodium measured at the beginning of each trial;

(iv) first estimated: 24-h urinary sodium estimated from sodium concentration of the first 24-h urine using the Kawasaki formula.”

The study authors assessed results for participants in the Trials of Hypertension Prevention, which included 2,974 individuals aged 30–54 years with pre-hypertension.

The gold-standard method showed a direct linear relationship between increased sodium intake and increased risk of mortality.

The scientists found that the Kawasaki formula suggested a J-shaped curve, which would imply that both low levels and high levels of sodium consumption were associated with increased mortality.

“Our findings indicate that inaccurate measurement of sodium intake could be an important contributor to the paradoxical J-shaped findings reported in some cohort studies,” they said.

“Epidemiological studies should not associate health outcomes with unreliable estimates of sodium intake.”

The results were published in the International Journal of Epidemiology.

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Feng J. He et al. Errors in estimating 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