SY 09-1 LOWERING SALT INTAKE AND CARDIOVASCULAR RISK REDUCTION: WHAT IS THE EVIDENCE?

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Abstract

The evidence. Salt (i.e. sodium chloride) is causally related to blood pressure (BP). The higher the salt intake, the higher the BP, an effect seen since birth. A small and sustained reduction in salt intake causes a fall in BP. The evidence from controlled studies, small and large, short and long, all agree on the following: (1) salt intake is one of the major determinants of BP in populations and individuals; (2) a reduction in salt intake causes a dose-dependent reduction in BP – the lower the salt the lower the BP; (3) the effect is seen in both sexes, in people of all ages and ethnic groups, and with all starting BPs. High BP causes strokes and heart attacks and a reduction in BP is associated with their reduction. A reduction in salt intake reduces BP, stroke and other cardiovascular events, including chronic kidney disease, by as much as 23% (i.e. 1.25 M deaths worldwide). The effect is related to the size of the fall in BP, the bigger the fall in BP the greater the benefits. It is therefore conceivable that a moderate reduction in salt intake in a population would help reduce stroke, heart attacks and vascular kidney disease through BP reduction.

The preventive imperative. In a population, there is a log-linear (exponential) relationship between the levels of BP (that are normally distributed) and the risk of developing a cardiovascular event, especially stroke. Whilst the relative risk of having a stroke is the highest in the upper level of BP (i.e. the hypertensives), the attributable events in the population (absolute risk) are fewer that those that would be attributable to the moderate relative risk of moderate levels of BP amongst the majority (i.e. the normotensives). Therefore, a shift in the entire BP distribution, even of a moderate amount, would avert a greater number of events than just the ‘treatment’ of those in the extreme end of the BP distribution. Population salt reduction strategies aim at exactly this.

The economic imperative. All countries need to satisfy stringent cost-effectiveness criteria within a general climate of ageing populations, escalating healthcare demands and recently reduced financial resources. Economic modelling studies have assessed the health effects and financial cost of reducing population salt intake. Despite methodological differences, they all demonstrate that a reduction in salt intake is cost saving. These economic savings would be achieved with either voluntary or mandatory reductions in the salt content of processed foods. Health benefits would be up to 20 times greater with government legislation on salt limits in processed foods. Cost savings are also estimated for a 15% reduction in salt intake in low- and middle-income countries, which would avert 13.8 M deaths over 10 years at an initial cost of less than $0.40 (US) per person per year.

The political imperative. Over the following 20 years both scientific evidence and public health initiatives have led to renewed recommendations from the WHO in 2007 and 2012 not to exceed a population average salt intake of 5 g per day. A significant step toward global policy action was the 2011 United Nations high-level meeting on non-communicable diseases (NCDs), which set a target for population salt reduction as a priority to reduce premature CVD mortality by 2025. Revised WHO guidelines now recommend a 30% reduction of salt intake by 2025 and a final maximum target of 5 g per day. The latter target was then adopted by the 66th World Health Assembly through its resolution in 2013.

The controversy. This important shift in public health has not occurred without obstinate opposition from organizations concerned primarily with the profits deriving from population high salt intake and less with public health benefits. The food and beverage industry has been particularly obstructive regarding public health actions, either directly or through its public relations organizations. Its strategies have included mass media campaigns, biasing research findings, co-opting policy makers and health professionals, lobbying politicians and public officials, encouraging voters to oppose public health regulation. Key components of this denial strategy are misinformation (with “pseudo” controversies) and the peddling of numerous rather well-worn myths. In general, poor science has been used to create uncertainty and to support inaction.

Policy options. A number of policy options for the implementation of national programmes globally are now available and population salt reduction is underway in many countries worldwide. In industrialized countries, the majority of the salt in the diet comes from that added to food during the manufacturing process (up to 75%), with only 15–20% due to consumer's choice (adding salt to food at the table or when cooking), the reminder naturally occurring in food. Hence, to achieve a population reduction in salt intake, a reformulation of the salt content of food is necessary in addition to the reduction of discretionary use.

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