A total of 12,126 subjects with normal kidney function were involved in the study. The subjects visited the hospital for their annual physical check-up between July 2008 and December 2015.
Their salt intake was assessed by estimating 24-hour urinary sodium excretion.
As researchers studied the health records of these subjects throughout the years, they found that those with higher urinary sodium excretion were more likely to develop kidney impairment.
For instance, the risk of developing kidney impairment is 29% higher in individuals with a higher salt intake (mean intake of 11.5g) that is approximately two times higher than those with lower salt intake (mean intake of 6.2g).
In addition, as salt intake increases with each gram, the risk of developing kidney impairment increases by approximately 4.5%.
At present, a salt intake level of lower than 8g/day for men and 7g/day for women is recommended by Japan’s Ministry of Health, Labour, and Welfare.
Researchers acknowledged that the observational study is unable to explain the underlying mechanisms between salt intake and kidney function.
However, since the kidneys is responsible for excreting sodium, researchers believe that excess salt intake could result in a pressure and volume overload of the kidney.
This in turn causes glomerular hypertension – a condition where chronic high blood pressure causes damages to kidney tissue. Proteinuria, a condition where abnormal amount of proteins is found in the urine, may also occur.
“The findings of the present study propose an important concept, namely that dietary salt intake is closely associated with future kidney function in the general population,” the researchers said.
“The findings of the present study indicate that the lower the dietary salt intake at baseline, the better the kidney function going into the future,” the researchers concluded.
The researchers noted a few limitations of this observational study.
First, as participants were undergoing an annual physical check-up, their urine sample was only obtained once a year.
Second, a 24-hour urinary measurement is the gold standard for estimating salt intake.
However, in this study, only spot urine test was conducted, since the 24-hour approach is not practical for large scale studies.
As such, the present study might underestimate sodium excretion in participants with relatively high sodium excretion.
Source: Kidney and Blood Pressure Research
Dietary Salt Intake is a Significant Determinant of Impaired Kidney Function in the General Population
Authors: Sugiura et al