Reducing Population Salt Intake—Time for Global Action

In this issue of The Journal of Clinical Hypertension, the World Hypertension League (WHL) and the International Society of Hypertension (ISH) in conjunction with several other organizations have published a Dietary Salt Factsheet1 that provides a clear and concise description of the strong evidence for a universal reduction in population salt intake and an urgent need for global action. This is complementary to the policy statement2 published by WHL and ISH in a previous issue, which provides a roadmap for national governments, nongovernmental organizations, and the food industry to take concerted action to reduce salt consumption in all countries around the world.

Raised blood pressure (BP) throughout its range is the single biggest cause of death, accounting for 9.4 million deaths per year worldwide.3 There is compelling evidence that dietary salt intake is the major cause of raised BP, which, in turn, increases the risk of cardiovascular disease (CVD; including strokes, heart attacks, and heart failure) and kidney disease.4 The evidence that relates salt to BP comes from various types of research, including epidemiological, migration, intervention, treatment, animal, and genetic studies.4 More recent studies provide further strong support for population‐wide reduction in salt intake. Updated meta‐analyses of randomized salt reduction trials demonstrate that a longer‐term modest reduction in salt intake, as currently recommended, results in significant and, from a population viewpoint, important falls in BP in both hypertensive and normotensive individuals, men and women, in all age groups, and in all ethnic groups, although there is a variation in the extent of the fall in BP among different groups.5, 6 Importantly, there is a dose‐response relationship and, within the range of 12 g/d to 3 g/d, the greater the reduction in salt intake, the greater the fall in BP. A reduction in salt intake through its effect on BP would reduce CVD. Indeed, both prospective cohort studies and outcome trials have shown that lower salt intake is associated with reduced risk of CVD.6, 7, 8

A few recent cohort studies have reported a U‐shaped association, with both a lower or higher salt intake being associated with higher CVD or all‐cause mortality.9, 10 These studies have created substantial controversy, particularly as they were publicized by the salt industry's public relations body, the Salt Institute. However, there are severe methodological flaws with these studies. Two recent papers from the Science Advisory of the American Heart Association (AHA),11, 12 along with several other papers,13 have provided detailed analysis of the methodological issues in cohort studies, eg, reverse causality, residual confounding, and errors in salt assessment, particularly the inherent problem of estimating individuals' usual salt intake when it varies hugely from day to day. As a result of the methodological problems, these studies cannot be used to inform public health policy on salt.

Six publications in patients with severe heart failure taking multiple drug treatments claimed that randomized trials showed that salt reduction had no benefits or increased mortality or re‐hospitalization. All of these papers were from the same group of researchers and the integrity of their data has now been seriously undermined with a recent meta‐analysis of these studies being retracted from the journal Heart after an investigation by BMJ's publishing ethics committee,14 and another meta‐analysis by Taylor and colleagues15 has also been withdrawn from The Cochrane Library because of the inclusion of the trial in heart failure as well as other methodological problems.

The totality of the evidence for population‐wide reduction in salt intake is strong.4 Indeed, salt reduction is one of the most cost‐effective measures to prevent CVD in both developed and developing countries.16, 17, 18 At the 2011 United Nations High‐level Meeting on Non‐communicable Diseases (NCDs), salt reduction was recommended as one of the top three priority actions to reduce premature mortality from NCDs by 25% by 2025.19, 20 The World Health Organization (WHO), in its recent guideline, recommends a 30% reduction in salt intake by 2025 with an eventual target of 5 g/d for all adults worldwide and lower levels for children based on calorie intake.21 Following from this, member states at the 66th World Health Assembly formally adopted these WHO salt targets as part of an omnibus resolution to tackle NCDs.22 The question now is not whether to reduce salt intake but how to reduce salt in the population to meet the WHO's target?

So far, there are only three countries (ie, Japan, Finland, and the United Kingdom [UK]) that have successfully reduced salt intake. In the late 1960s, Japan carried out a government‐led campaign to reduce the amount of salt used by households as it was realized that the high rate of stroke mortality in Japan was directly related to the high salt intake in the population. Over the following decade, salt intake was reduced, particularly in northern areas, from 18 g/d to 14 g/d. Paralleling this reduction in salt intake, there were falls in BP and an 80% reduction in stroke mortality23 in spite of large increases in fat intake, cigarette smoking, alcohol consumption, and obesity during that period. In the late 1970s, Finland initiated a systematic approach to reducing salt intake through mass media campaigns, co‐operation with the food industry, and implementing salt labeling legislation.24, 25, 26 This led to a significant reduction in the average salt intake in the Finnish population24, 26 from approximately 14 g/d in 1972 to <9 g/d in 2002.24 The reduction in salt intake was accompanied by a fall of more than 10 mm Hg in systolic and diastolic BP and a decrease of 75% to 80% in both stroke and ischemic heart disease (IHD) mortality.24 Although these results were attributable to several factors, the reduction in salt intake was likely to have played a major role, particularly in the fall of BP, as both body mass index and alcohol consumption increased during that time.

More recently, the UK, through Consensus Action on Salt and Health (CASH),27 a nongovernmental organization, and the Food Standards Agency, a quasi‐government organization, has successfully developed and implemented a program of voluntary salt reduction in collaboration with the food industry.28 The main components of the UK's salt reduction program is summarized in Figure 1.28 The key element is the rigorous setting of progressively lower salt targets for more than 80 categories of foods, with a clear timeframe and independent monitoring program. Significant progress has been made since the start of the salt reduction program in 2003/2004. The salt content in many food products has been reduced by 20% to 40%.28, 29 These reductions have been made slowly, and there have been no loss of sales to the food industry, and the public are largely unaware of these reductions. The average salt intake as measured by 24‐hour urinary sodium excretion in a random sample of the adult population has been reduced by 15%, ie, from 9.5 g/d in 2003 to 8.1 g/d, in 2011.30 This was accompanied by a significant fall in population BP and mortality from stroke and IHD (Figure 2).31 The National Institute for Health and Care Excellence estimated that the salt reduction program has resulted in approximately £1.5bn healthcare savings in the UK, at a cost of only approximately £5m per year.16, 32

Figure 1.

An action framework of reducing salt intake in the population — The United Kingdom model. CASH indicates Consensus Action on Salt and Health.

Figure 2.

Changes in salt intake as measured by 24‐hour urinary sodium (UNa) excretion, blood pressure (BP), stroke, and ischemic heart disease (IHD) mortality in England from 2003 to 2011. *P<.05 and ***P<.001 for trend.

The UK salt reduction program has been carried out on a voluntary basis, but this has been underpinned by sustained media pressure, direct pressure on the government, and ministers, particularly the public health ministers, so that they would maintain a strong stance with the food industry. Regulatory/legislative approaches are likely to be more effective than voluntary approaches. However, in many countries, the process of legislation is complicated and this may lead to severe delays in action as demonstrated by the pace of tobacco legislation (eg, taxation and banning smoking in all workplaces) coming into force.33

Countries therefore need to consider their own political processes to determine whether a regulatory/legislative or voluntary approach is more appropriate. Recently, South Africa started a similar program based on the UK model, but the salt targets are regulated34 and the same global food companies preferred a regulatory system rather than a voluntary system as it gave them a level playing field. For many other countries, the best way to proceed is to start with a voluntary salt reduction policy with the threat of regulation/legislation and, at the same time, enact the legislation process.

The evidence for a reduction in population salt intake is strong. From a public health perspective, reducing salt intake is one of the easiest strategies to carry out, as it does not require major conscious changes in the eating habits of individuals, but it does require the cooperation of the food industry as, in most developed countries, approximately 80% of salt consumed is hidden in processed, canteen, restaurant, and fast foods.35 To achieve a reduction in population salt intake, the food industry needs to make a gradual and sustained reduction in the amount of salt added to all foods. The UK's salt reduction model could be adapted by many countries with appropriate local modifications. Several countries such as the United States, Canada, and Australia are following the UK's lead and setting their own targets. The major challenge now is to spread this to all other countries. In most developing countries where the majority of salt in the diet is added by consumers, a public health campaign plays a major role. World Action on Salt and Health (WASH),36 a similar group to CASH with more than 500 members in 98 countries, is encouraging action groups to be formed in each country. With the important support from the WHL and ISH, all countries should therefore adopt a coherent and workable strategy to reduce salt intake. In view of the enormous benefits of salt reduction on public health, it would be negligent for any government not to take action now.