June 25 (UPI) -- A more accurate test measurement showed a direct link between high salt intake and increased risk of death from cardiovascular disease, according to a new study.
Scientists at Brigham and Women's Hospital used multiple measurements to determine the effects of sodium. In their findings, published last week in the International Journal of Epidemiology, researchers said salt intake has been inaccurately determined in the past.
"Sodium is notoriously hard to measure," Dr. Nancy Cook, a biostatistician in the Department of Medicine at BWH, said in a press release. "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."
She noted there are numerous ways to study sodium excretions but they aren't always accurate.
Because sodium levels in urine can fluctuate throughout the day, an accurate measurement requires a full 24-hour sample. Samples should also be taken on multiple days, the researchers say, because sodium consumption may change from day to day.
Aside from using spot samples, which have been used to measure salt intake in other studies, the researchers used other methods that include the gold-standard of analyzing an average of multiple, non-consecutive urine samples.
The researchers analyzed records from participants in the Trials of Hypertension Prevention, which included 2,974 individuals with pre-hypertension aged 30-54 years. During a follow-up of 24 years, 272 people died.
The researchers found their method showed a direct linear relationship between increased sodium intake and increased risk of death. The average sodium intake measured using the gold-standard method was 3,769 mg/d, and the average overestimated sodium intake by 1297 mg/d.
Using the commonly available Kawasaki formula, they found low levels and high levels of sodium consumption both were associated with increased mortality in a J-shaped finding.
"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," the authors wrote. "Epidemiological studies should not associate health outcomes with unreliable estimates of sodium intake."