How Does Salt Intake Relate to Mortality?

Intake of salt is a biological necessity, inextricably woven into physiologic systems. However, excessive salt intake is associated with high blood pressure. Hypertension is linked to increased cardiovascular morbidity and mortality, and it is estimated that excessive salt intake causes approximately 5 million deaths per year worldwide. Reducing salt intake lowers blood pressure, but processed foods contain “hidden” salt, which makes dietary control of salt difficult. This problem is compounded by growing inequalities in food systems, which present another hurdle to sustaining individual dietary control of salt intake.

Of the 87 risk factors included in the Global Burden of Diseases, Injuries, and Risk Factors Study 2019, high systolic blood pressure was identified as the leading risk factor for disease burden at the global level and for its effect on human health. A range of strategies, including primary care management and reduction in sodium intake, are known to reduce the burden of this critical risk factor. Two questions remain unanswered: What is the relationship between mortality and adding salt to foods?, and how much does a reduction in salt intake influence people’s health?

Cardiovascular Disease and Death

Because dietary sodium intake has been identified as a risk factor for cardiovascular disease and premature death, high sodium intake can be expected to curtail life span. A study tested this hypothesis by analyzing the relationship between sodium intake and life expectancy and survival in 181 countries. Sodium intake correlated positively with life expectancy and inversely with all-cause mortality worldwide and in high-income countries, which argues against dietary sodium intake curtailing life span or a being risk factor for premature death. These results help fuel a scientific debate about sodium intake, life expectancy, and mortality. The debate requires interpreting composite data of positive linear, J-shaped, or inverse linear correlations, which underscores the uncertainty regarding this issue.

In a prospective study of 501,379 participants from the UK Biobank, researchers found that higher frequency of adding salt to foods was significantly associated with a higher risk of premature mortality and lower life expectancy independently of diet, lifestyle, socioeconomic level, and preexisting diseases. They found that the positive association appeared to be attenuated with increasing intake of high-potassium foods (vegetables and fruits).

In addition, the researchers made the following observations:

• For cause-specific premature mortality, they found that higher frequency of adding salt to foods was significantly associated with a higher risk of cardiovascular disease mortality and cancer mortality (P-trend < .001 and P-trend < .001, respectively).

• Always adding salt to foods was associated with the lower life expectancy at the age of 50 years by 1.50 (95% CI, 0.72 – 2.30) and 2.28 (95% CI, 1.66 – 2.90) years for women and men, respectively, compared with participants who never or rarely added salt to foods.

The researchers noted that adding salt to foods (usually at the table) is common and is directly related to an individual’s long-term preference for salty foods and habitual salt intake. Indeed, in the Western diet, adding salt at the table accounts for 6% to 20% of total salt intake. In addition, commonly used table salt contains 97% to 99% sodium chloride, minimizing the potential confounding effects of other dietary factors, including potassium. Therefore, adding salt to foods provides a way to evaluate the association between habitual sodium intake and mortality — something that is relevant, given that it has been estimated that in 2010, a total of 1.65 million deaths from cardiovascular causes were attributable to consumption of more than 2.0 g of sodium per day.

Salt Sensitivity

Current evidence supports a recommendation for moderate sodium intake in the general population (3–5 g/day). Persons with hypertension should consume salt at the lower end of that range. Some dietary guidelines recommend consuming less than 2300 mg dietary sodium per day for persons aged 14 years or older and less for persons aged 2–13 years. Although low sodium intake (<2.0 g/day) has been achieved in short-term clinical trials, sustained low sodium intake has not been achieved in any of the longer-term clinical trials (duration >6 months).

The controversy continues as to the relationship between low sodium intake and blood pressure or cardiovascular diseases. Most studies show that both in individuals with hypertension and those without, blood pressure is reduced by consuming less sodium. However, it is not necessarily lowered by reducing sodium intake (<3–5 g/day). With a sodium-rich diet, most normotensive individuals experienced a minimal change in mean arterial pressure; for many individuals with hypertension, the values increased by about 4 mm Hg. In addition, among individuals with hypertension who are “salt sensitive,” arterial pressure can increase by >10 mm Hg in response to high sodium intake.

The Effect of Potassium

Replacing some of sodium chloride in regular salt with potassium chloride may mitigate some of salt’s harmful cardiovascular effects. Indeed, salt substitutes that have reduced sodium levels and increased potassium levels have been shown to lower blood pressure. In one trial, researchers enrolled over 20,000 persons from 600 villages in rural China and compared the use of regular salt (100% sodium chloride) with the use of a salt substitute (75% sodium chloride and 25% potassium chloride by mass). The participants were at high risk for stroke, cardiovascular events, and death. The mean duration of follow-up was 4.74 years. The results were surprising. The rate of stroke was lower with the salt substitute than with regular salt (29.14 events vs 33.65 events per 1000 person-years; rate ratio, 0.86; 95% CI, 0.77 – 0.96; P = .006), as were the rates of major cardiovascular events and death from any cause. The rate of serious adverse events attributed to hyperkalemia was not significantly higher with the salt substitute than with regular salt.

Although there is an ongoing debate about the extent of salt’s effects on the cardiovascular system, there is no doubt that in most places in the world, people are consuming more salt than the body needs.

A lot depends upon the kind of diet consumed by a particular population. Processed food is rarely used in rural areas, such as those involved in the above-mentioned trial, with dietary sodium chloride being added while preparing food at home. This is a determining factor with regard to cardiovascular outcomes, but it cannot be generalized to other social-environmental settings. In much of the world, commercial food preservation introduces a lot of sodium chloride into the diet, and most salt intake could not be fully attributed to the use of salt substitutes. Indeed, by comparing the sodium content of cereal-based products currently sold on the Italian market with the respective benchmarks proposed by the World Health Organization, researchers found that for most items, the sodium content is much higher than the benchmarks, especially with flatbreads, leavened breads, and crackers/savory biscuits. This shows that there is work to be done to achieve the World Health Organization/United Nations objective of a 30% global reduction in sodium intake by 2025.

This article was translated from Univadis Italy.

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