UK Biobank study
For the current study Dr. Qi and colleagues analyzed data from 501,379 people taking part in the UK Biobank study. When joining the study between 2006 and 2010, the participants were asked whether they added salt to their foods never/rarely, sometimes, usually or always. Participants were then followed for a median of 9 years.
After adjustment for sex, age, race, smoking, moderate drinking, body mass index, physical activity, Townsend deprivation index, high cholesterol, chronic kidney disease, diabetes, cardiovascular disease, and cancer, results showed an increasing risk of all-cause premature mortality rose with increasing frequency of adding salt to foods.
The adjusted hazard ratios, compared with those who never or rarely added salt, were 1.02 (95% CI, 0.99-1.06) for those who added salt sometimes, 1.07 (95% CI, 1.02-1.11) for those who usually added salt, and 1.28 (95% CI, 1.20-1.35) for those who always added salt.
The researchers also estimated the lower survival time caused by the high frequency of adding salt to foods. At age 50, women who always added salt to foods had an average 1.50 fewer years of life expectancy, and men who always added salt had an average 2.28 fewer years of life expectancy, as compared with their counterparts who never/rarely added salt to foods.
For cause-specific premature mortality, results showed that higher frequency of adding salt to foods was significantly associated with a higher risk of cardiovascular mortality and cancer mortality, but not for dementia mortality or respiratory mortality. For the subtypes of cardiovascular mortality, adding salt to foods was significantly associated with higher risk of stroke mortality but not coronary heart disease mortality.
Other analyses suggested that the association of adding salt to foods with an increased risk of premature mortality appeared to be attenuated with increasing intake of food high in potassium (fruits and vegetables).
The authors point out that the amounts of discretionary sodium intake (the salt used at the table or in home cooking) have been largely overlooked in previous studies, even though adding salt to foods accounts for a considerable proportion of total sodium intake (6%-20%) in Western diets.
“Our findings also support the notion that even a modest reduction in sodium intake is likely to result in substantial health benefits, especially when it is achieved in the general population,” they conclude.
Conflicting information from different studies
But the current findings seem to directly contradict those from another recent study by Messerli and colleagues showing higher sodium intake correlates with improved life expectancy.
Addressing these contradictory results, Dr. Qi commented: “The study of Messerli et al. is based on an ecological design, in which the analysis is performed on country average sodium intake, rather than at the individual level. This type of ecological study has several major limitations, such as the lack of individuals’ sodium intake, uncontrolled confounding, and the cross-sectional nature. Typically, ecological studies are not considered useful for testing hypothesis in epidemiological studies.”
Dr. Qi noted that, in contrast, his current study analyzes individuals’ exposure, and has a prospective design. “Our findings are supported by previous large-scale observational studies and clinical trials which show the high intake of sodium may adversely affect chronic diseases such as cardiovascular disease and hypertension.” =
Lead author of the ecological study, Franz Messerli, MD, Bern (Switzerland) University Hospital, however, was not convinced by the findings from Dr. Qi’s study.
“The difference in 24-hour sodium intake between those who never/rarely added salt and those who always did is a minuscule 0.17 g. It is highly unlikely that such negligible quantity has any impact on blood pressure, not to mention cardiovascular mortality or life expectancy,” he commented in an interview.
He also pointed out that, in Dr. Qi’s study, people who added salt more frequently also consumed more red meat and processed meat, as well as less fish and less fruit and vegetables. “I would suggest that the bad habit of adding salt at the table is simply a powerful marker for an unhealthy diet.”
“There is no question that an excessive salt intake is harmful in hypertensive patients and increases the risk of stroke. But 0.17 g is not going to make any difference,” Dr. Messerli added.