Sodium intake and public health -- methodological pitfalls in assessing the relationship between sodium intake and health outcomes

Background and aims The findings on the relationship between sodium intake and health outcomes such as cardiovascular disease and all-cause mortality have been controversial. Some studies found the relationship between sodium intake and all-cause mortality was linear while others found a U-shaped or J-shaped relationship. This study aimed to identify the methodological issues contributing to the conflicting findings. Methods and results The present study investigated methodological gaps in assessing the relationship between sodium intake and health outcomes (hypertension, cardiovascular disease, and all-cause mortality). The contradictory findings appear to stem from flawed methods used in the published studies: (1) Both spot and 24-h urinary sodium collection methods underestimate the adverse effects of low sodium intake and overestimate the harmful effects of high sodium intake, (2) the linear relationship between sodium intake and all-cause mortality appears to be a result of random chance due to small sample sizes, and (3) the divergent temporal trends of sodium consumption and hypertension prevalence indicate sodium intake was not the primary cause of the worldwide hypertension epidemic. Conclusion Considering, (1) sodium is an essential nutrient, (2) the adverse effects of low and high sodium intake appear to be under- and over-estimated, respectively, (3) large studies have found a U-shaped or J-shaped relationship between sodium intake and all-cause mortality, and (4) sodium consumption is unlikely to be the major driver behind the worldwide hypertension epidemic and has little effect on the blood pressure of most normotensive individuals, the recommendation for population-wide low sodium intake merits further evaluation.