The problem
In almost all countries worldwide, most people eat too much salt. This is a problem because eating too much salt can cause high blood pressure, which can lead to health problems such as heart disease and stroke. To reduce the amount of salt eaten, governments in many countries have developed national salt reduction initiatives.
The review question
Our aim was to examine whether national salt reduction initiatives have been effective in reducing the amount of salt consumed in those populations.
Study characteristics
We searched research papers and government reports and had direct communication with individuals working in salt reduction in their respective countries. The evidence is current as of 5 January 2015, when we last searched electronic databases. Initiatives in 15 countries met the inclusion criteria. Ten of these countries provided sufficient data for quantitative analysis, gathered from studies that included 64,798 participants. Initiatives ranged from one activity (e.g. in Japan, which at the time of writing had a public information campaign) to many activities (e.g. in the United Kingdom, which provided five activities including on-package nutrition information, restrictions on marketing to children and food product reformulation). Of the 15 countries that met inclusion criteria, seven provided information about funding source, of which six reported non-industry funding. The other eight countries did not report a funding source for one or more data point(s).
Key results and quality of the evidence
Five of the 10 countries included in the quantitative analysis (China, Finland, France, Ireland and England) showed a decrease in salt intake after the intervention. Two of the 10 countries (Canada, Switzerland) showed an increase in salt intake after the intervention, however, in both countries the only data available were from several years prior to the intervention starting. Because the initiatives were very different, we cannot present an overall finding of whether these types of initiatives work.
When we focused on the subset of seven countries whose salt reduction initiatives included multiple components and were not focused solely on educating the public, we found that more than half (four of seven) showed a decrease in salt intake from pre-intervention to post-intervention.
When we examined the nine initiatives that analysed men and women separately, we found that amongst men, more than half (five of nine) showed a decrease in salt intake after the intervention. Amongst women, the pattern of findings was less clear, with three of nine interventions showing a decrease, two showing an increase and four showing no change in salt intake.
Low-bias study designs, such as randomised controlled trials, typically are not suitable for evaluating complex initiatives such as these; therefore, we rated all of the studies included in this review as having low methodological quality. Large nationally representative samples of the population and careful measurement of dietary sodium intake were strengths of several studies. However, because of study design limitations, the trustworthiness of study results is not clear.
Overall, our results show that national government initiatives have the potential to achieve population-wide reductions in salt intake, especially amongst men, and particularly if they employ more than one strategy and include structural activities such as food product reformulation (i.e. food companies putting less salt in food products). The wide variation of results across the studies we found presents a challenge in interpreting the current evidence and this warrants more research to help us understand this.
Population-level interventions in government jurisdictions for dietary sodium reduction have the potential to result in population-wide reductions in salt intake from pre-intervention to post-intervention, particularly if they are multi-component (more than one intervention activity) and incorporate intervention activities of a structural nature (e.g. food product reformulation), and particularly amongst men. Heterogeneity across studies was significant, reflecting different contexts (population and setting) and initiative characteristics. Implementation of future initiatives should embed more effective means of evaluation to help us better understand the variation in the effects.
Excess dietary sodium consumption is a risk factor for high blood pressure, stroke and cardiovascular disease. Currently, dietary sodium consumption in almost every country is too high. Excess sodium intake is associated with high blood pressure, which is common and costly and accounts for significant burden of disease. A large number of jurisdictions worldwide have implemented population-level dietary sodium reduction initiatives. No systematic review has examined the impact of these initiatives.
• To assess the impact of population-level interventions for dietary sodium reduction in government jurisdictions worldwide.
• To assess the differential impact of those initiatives by social and economic indicators.
We searched the following electronic databases from their start date to 5 January 2015: the Cochrane Central Register of Controlled Trials (CENTRAL); Cochrane Public Health Group Specialised Register; MEDLINE; MEDLINE In Process & Other Non-Indexed Citations; EMBASE; Effective Public Health Practice Project Database; Web of Science; Trials Register of Promoting Health Interventions (TRoPHI) databases; and Latin American Caribbean Health Sciences Literature (LILACS). We also searched grey literature, other national sources and references of included studies.
This review was conducted in parallel with a comprehensive review of national sodium reduction efforts under way worldwide (Trieu 2015), through which we gained additional information directly from country contacts.
We imposed no restrictions on language or publication status.
We included population-level initiatives (i.e. interventions that target whole populations, in this case, government jurisdictions, worldwide) for dietary sodium reduction, with at least one pre-intervention data point and at least one post-intervention data point of comparable jurisdiction. We included populations of all ages and the following types of study designs: cluster-randomised, controlled pre-post, interrupted time series and uncontrolled pre-post. We contacted study authors at different points in the review to ask for missing information.
Two review authors extracted data, and two review authors assessed risk of bias for each included initiative.
We analysed the impact of initiatives by using estimates of sodium consumption from dietary surveys or urine samples. All estimates were converted to a common metric: salt intake in grams per day. We analysed impact by computing the mean change in salt intake (grams per day) from pre-intervention to post-intervention.
We reviewed a total of 881 full-text documents. From these, we identified 15 national initiatives, including more than 260,000 people, that met the inclusion criteria. None of the initiatives were provided in lower-middle-income or low-income countries. All initiatives except one used an uncontrolled pre-post study design.
Because of high levels of study heterogeneity (I2 > 90%), we focused on individual initiatives rather than on pooled results.
Ten initiatives provided sufficient data for quantitative analysis of impact (64,798 participants). As required by the Grades of Recommendation, Assessment, Development and Evaluation (GRADE) method, we graded the evidence as very low due to the risk of bias of the included studies, as well as variation in the direction and size of effect across the studies. Five of these showed mean decreases in average daily salt intake per person from pre-intervention to post-intervention, ranging from 1.15 grams/day less (Finland) to 0.35 grams/day less (Ireland). Two initiatives showed mean increase in salt intake from pre-intervention to post-intervention: Canada (1.66) and Switzerland (0.80 grams/day more per person); however in both countries the pre-intervention data point was from several years prior to the initiation of the intervention. The remaining initiatives did not show a statistically significant mean change.
Seven of the 10 initiatives were multi-component and incorporated intervention activities of a structural nature (e.g. food product reformulation, food procurement policy in specific settings). Of those seven initiatives, four showed a statistically significant mean decrease in salt intake from pre-intervention to post-intervention, ranging from Finland to Ireland (see above), and one showed a statistically significant mean increase in salt intake from pre-intervention to post-intervention (Switzerland; see above).
Nine initiatives permitted quantitative analysis of differential impact by sex (men and women separately). For women, three initiatives (China, Finland, France) showed a statistically significant mean decrease, four (Austria, Netherlands, Switzerland, United Kingdom) showed no significant change and two (Canada, United States) showed a statistically significant mean increase in salt intake from pre-intervention to post-intervention. For men, five initiatives (Austria, China, Finland, France, United Kingdom) showed a statistically significant mean decrease, three (Netherlands, Switzerland, United States) showed no significant change and one (Canada) showed a statistically significant mean increase in salt intake from pre-intervention to post-intervention.
Information was insufficient to indicate whether a differential change in mean salt intake occurred from pre-intervention to post-intervention by other axes of equity included in the PROGRESS framework (e.g. education, place of residence).
We identified no adverse effects of these initiatives.
The number of initiatives was insufficient to permit other subgroup analyses, including stratification by intervention type, economic status of country and duration (or start year) of the initiative.
Many studies had methodological strengths, including large, nationally representative samples of the population and rigorous measurement of dietary sodium intake. However, all studies were scored as having high risk of bias, reflecting the observational nature of the research and the use of an uncontrolled study design. The quality of evidence for the main outcome was low. We could perform a sensitivity analysis only for impact.