Is there a safe level for adding sodium to food versus is it safe to reduce dietary sodium intake?

Carefully reviewed extensive evidence supports the lowering of dietary sodium for cardiovascular health [1]. This is reflected in dietary guidance worldwide [1]. For example, the WHO recommends that adults limit their sodium intake to less than 2 g/day (equivalent to 5 g/day of salt) [1]. Despite this, there continues to be a controversial argument that it may be unsafe to lower dietary sodium. In this editorial, we briefly discuss the basis of this narrative and propose a more scientifically sound perspective that questions the safety of adding sodium to foods at levels that substantively increase dietary sodium. The international observational Intersalt study showed that higher sodium intake was associated with higher blood pressure and increased blood pressure with age. Intersalt and other studies have identified several populations that consume very little salt and natural foods that have very little sodium [2]. Hunter-gatherer populations that only eat natural foods without added sodium generally consume less than 700 mg sodium/day with vegetarian diets containing less than 500 mg sodium/day [1,2]. Most hunter gather diets contained 100–700 mg sodium/day [1,2]. Hypertension is rare and increases in blood pressure with age do not occur in populations with diets under 1000 mg sodium/day. Atherosclerotic and hypertensive cardiovascular diseases did not occur in hunter-gatherer populations [1,2]. Current diets contain vast amounts of sodium, which is added during the commercial and home processing of foods [1]. Commercially added sodium is the dominant dietary source in industrialized countries, whereas sodium added at home remains an important source in the few countries that have not undergone nutritional transition [3]. Countries undergoing nutrition transition usually have both home and commercial sources and the highest average dietary sodium intake (e.g. China) [3]. A dose–response relationship between sodium intake and blood pressure has been confirmed in multiple randomized controlled trials [4]. Randomized controlled trials have shown approximately linear increases in blood pressure as dietary sodium increases above 800 mg/day [1,4]. The trials did not define a lower threshold at which dietary sodium intake did not increase blood pressure. A meta-analysis of randomized controlled trials with posttrial follow-up reported a 26% reduction in cardiovascular disease with a modest average decrease in dietary sodium (3642–2690 mg/day) and a linear association between cardiovascular disease and dietary sodium within the limits of the trial data (2300–4100 mg/day) [1,5]. Similarly, a meta-analysis of cohort studies that assessed dietary sodium using the recommended standard of multiple nonconsecutive 24 h urine collections found a linear association between dietary sodium and cardiovascular disease within the range of the study data (1846–5230 mg/day) [1,6]. These studies did not define a lower threshold at which dietary sodium did not increase cardiovascular disease or mortality. Conversely, low-quality observational data have shown increased cardiovascular disease with dietary sodium below 3000 mg/day [7,8]. A substantial proportion of the studies include assessing dietary sodium with spot urine samples and estimating equations that have been shown to produce false associations with health outcomes, and hence have been strongly recommended not to be used by national and international scientific organizations [9]. Studies have also included populations that include people with diseases where reverse causality (people who are ill are likely to eat less sodium) as well as indication bias (people with hypertension and cardiovascular disease may be prescribed lower sodium diets) could be expected to cause an inverse association between dietary sodium and cardiovascular disease. For some controversial and widely cited studies, particularly the Prospective Urban Rural Epidemiology (PURE) study, access to data for external independent verification has been repeatedly declined [7,8,10]. For decades, controversial low-quality research results have been highly promoted by the food and salt industry, scientists conducting studies, and several scientists and clinicians with financial conflicts of interest, including overt funding from the salt and food industries and membership in their advisory boards [7,8]. The salt and food industries have also questioned the validity of studies demonstrating the benefits of lowering sodium intake [11]. Low-quality studies have results that are not reliable or reproducible, and most are incompatible with higher quality, more rigorous research. Nevertheless, low-quality studies continue to be conducted with investigators defending the methods because they are ‘feasible’ (i.e. ‘easy to do’) [12]. Extensive misinformation (false and misleading statements), based in part on low-quality research, is generated by the food sector and often financially conflicted scientists and clinicians [7,8,10]. In the past, mainstream scientific organizations have often disregarded the balance of evidence provided by extensive and repeated reviews by highly regarded unbiased scientific organizations [1,8]. Controversial low-quality research has been extensively presented and highlighted at meetings and in lay and scientific publications, creating controversy. This has created the common current perception that the scientific community needs to be concerned about the harm from reducing dietary sodium and performing randomized controlled trials with the null hypothesis being that lowering dietary sodium causes cardiovascular disease [13]. Cardiovascular disease is the leading cause of death worldwide [14]. Approximately 30% of hypertension cases are attributed to high dietary sodium [1]. Currently, increased blood pressure affects more than one-quarter of adults and is the leading global risk factor for death [1,14]. Increases in dietary sodium cause hypertension and cardiovascular disease, without a defined lower limit of sodium in the diet that causes harm. The most scientifically valid null hypothesis, therefore, is that adding sodium to food causes increases in blood pressure and increases in cardiovascular disease. In the absence of clinical trial data to support the safety of adding sodium to food, efforts to reduce dietary sodium must be markedly increased. ACKNOWLEDGEMENTS Conflicts of interest N.R.C.C. reports personal fees from Resolve to Save Lives (RTSL), the Pan American Health Organization, and the World Bank outside the submitted work; and is an unpaid member of the World Action on Salt, Sugar, and Health and an unpaid consultant on dietary sodium and hypertension control to numerous governmental and nongovernmental organizations. N.R.C.C. was on the Medical Advisory Board of Switch Health (2022–2023) and was a one-time reviewer of a joint Novartis Canada Alberta Health Services collaborative project to treat dyslipidaemia. All honoraria from Switch Health and Novartis Canada were donated to the University of Calgary to support a community cardiovascular disease prevention recognition award. F.J.H. is an unpaid member of Action on Salt and World Action on Salt, Sugar and Health (WASSH). G.A.M. is the unpaid Chair of Action on Salt, Action on Sugar, WASSH and Blood Pressure UK. R.M.M. reports no conflicts of interest.

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