Adding salt at the table: a marker of an unhealthy diet Free

This commentary refers to ‘Adding salt to foods and hazard of premature mortality’, by H. Ma et al., https://doi.org/10.1093/eurheartj/ehac208 and the discussion piece ‘Adding salt to foods as a new indicator for studying habitual sodium intake and health’, by L. Qi, https://doi.org/10.1093/eurheartj/ehac600.

Ma et al.¹ conclude that adding salt to food at the table is associated with a higher hazard of all-cause premature mortality and reduced life expectancy. However, several points suggest that these findings may be due to a Type 1 error, as the statistical significance probably occurred by chance.

First, three studies based on the same UK Biobank data showed either a trend towards an inverse relationship between sodium intake and mortality^2,3 or a U-shaped association between salt intake and mortality,⁴ or an inverse relationship between the Na/K ratio and mortality,⁵ thus being directly in conflict with the present interpretation. Second, a similar study based on the National Health and Nutrition Examination Survey (NHANES) showed that no table salt was associated with a higher risk of hypertension or hypertension and stroke.⁶ Third, in NHANES,⁷ there was no difference in salt intake based on 24 h urine samples (the gold standard of salt intake measurement) between those who reported to eat a reduced salt diet and those who did not. Fourth, although Figure 1 of Ma et al.¹ seductively suggests direct association between the salt adding groups and sodium excretion, it actually confirms NHANES findings because the difference between those who never and always add salt amounts to a minuscule 0.16 g (<4%).

In the PURE study, a difference in sodium intake of 1 g increased systolic blood pressure by 2.11 mmHg.⁸ Thus, an increase of 0.16 g sodium would elevate systolic blood pressure by merely 0.36 mmHg. Also, according to Lindley’s paradox,⁹ the nominal statistically significant risk effects of very small magnitude observed across different subgroups among those individuals adding salt to foods ‘sometimes’ or ‘usually’ may favour the null hypothesis (that no effect exists at all), rather than the alternative hypothesis (that some effect exists).¹⁰

We submit that people who routinely add salt to their food exhibit a widespread disregard for healthy nutrition. In the study of Ma et al.,¹ they consistently consumed more red meat, processed meat, less fish, less fruit, and vegetables than those who did not salt their food. The authors state ‘Compared with participants with lower frequency of adding salt to foods, participants with higher frequency were more likely to be male; non-white; and to have a higher BMI and Townsend deprivation index; they were less likely to have a healthy lifestyle (moderate drinking, non-current smoking, regular physical activity); and had higher prevalence of diabetes and cardiovascular diseases…’. Hence, the bad habit of thoughtlessly using the saltshaker at the table is simply a powerful marker for an absent-minded food intake and unhealthy lifestyle.

The study by Ma et al.¹ is a sophisticated and comprehensive exercise in attempting to adjust for multiple confounders but has little if anything to do with salt intake. There is no reason to believe that an increase in daily intake of a mere 0.16 g sodium could cause the powerful curtailment in life expectancy, which the authors observed.

Data availability

No new data were generated or analysed in support of this research.

References

Author notes