Discussion
Reducing salt intake in China has the potential to substantially reduce CVD incidence and mortality in men and women of all ages across the country. As demonstrated in various parts of China, a reduction by 1 g/day in a year would be easily achievable.5 21 22 Should this very modest reduction be sustained, some 9M CVD events could be prevented by 2030; and if we account for the more prolonged effects of salt on blood pressure, this figure might increase to 10M. Achieving WHO's interim target of 30% reduction by 2025 and the Chinese government’s recommendation of bringing salt intake down to ≤5 g/day by 2030, representing average salt reductions of 3.2 g/day and 6 g/day, respectively, could prevent approximately 1.5 and 2 times more CVD events and deaths.
Our results are concordant with those of a previous simulation estimating the health impact of a 15% salt reduction in 23 low-income and middle-income countries (including China).24 Furthermore, when we accounted for the more prolonged effect of salt reduction on blood pressure, we obtained estimates of reductions in blood pressure and relative risk of CVD that correspond to what has been observed in countries with successful salt reduction programmes and 24-hour urinary sodium data, namely the UK and Finland.19 25
However, our estimates of CVD events and deaths prevented are greater than those of a previous projection by Wang et al.26 This discrepancy could be explained by Wang et al’s use of different salt reduction scenarios, notably affecting individuals with a high salt intake only (as opposed to the entire population); salt intake estimates derived from dietary assessment methods (which are notoriously unreliable in settings such as China, where most of the salt consumed comes from the discretionary salt27); smaller CVD risk reductions following SBP change based on a recalibration of the US-based Framingham Heart Study risk function28 (while we derived ours from a pooled analysis of 1.38 million participants in North America, Western Europe and Asia Pacific20); accounting for lag time (while we did not, due to the uncertainty around the time it takes for sustained reductions in SBP to reduce CVD risk) and obsolete CVD incidence rates (dating back to 1991–2009).29 A more in-depth discussion around CVD relative risks and incidence can be found in the online supplemental materials.
Our study has multiple strengths. First, we made use of the most up-to-date and robust data available. Baseline salt intake and the effect of salt reduction on blood pressure were derived from 24-hour urinary sodium excretions, that is, the most accurate method to assess salt intake.6 13 19 27 Salt intake and blood pressure data were extracted from the published baseline data of three large-scale cluster randomised controlled trials of over 5000 adult participants from six different provinces throughout China (Qinghai, Hebei, Heilongjiang, Sichuan, Jiangxi and Hunan), where stringent protocols were followed to ensure data collection quality.6 The parameter estimate of the relationship between salt reduction and SBP was drawn from a meta-regression of modest salt reduction trials lasting at least 4 weeks,13 so as to exclude trials bearing no relevance to public health (ie, trials of very short duration that consist of acute salt loading followed by severe salt restrictions, for example, from 20 g/day to less than 1 g/day of salt for only a few days). Nevertheless, it is still unlikely that salt reduction has exerted its maximal effect within 4–5 weeks (the average duration of the randomised trials included in the meta-regression)30 31 and we, therefore, for the first time to our knowledge, conducted additional analyses using parameter estimates from a population study to approximate the more prolonged effect of salt reduction on blood pressure over several years. Second, we modelled salt reductions that are highly relevant to policy-making, as they align with key national and international salt reduction targets, both in the extent of salt reduction and in time frame. Moreover, we modelled them as gradual percentage reductions, which is more likely to reflect the reality of salt reduction than a linear or an ‘overnight’ reduction. As existing models did not provide enough flexibility to suit the structure and parameterisation needed to meet our modelling objectives,9–12 a de novo model was built. Though not formally piloted, calibrated and validated, the model’s results were concordant with those of other major modelling studies24 as well as with empirical data from countries with successful salt reduction programmes.19 25 32
Due to scarce data, we were unable to model all health gains that would be expected from salt reduction. First, we did not account for the reduction in the risk for recurrent CVD events (ie, secondary prevention) and for diseases other than CVD, such as chronic kidney disease and gastric cancer, the rates of which are either increasing or already very high in China.33 Second, it has been suggested that higher salt intake levels were associated with a greater increase in blood pressure as one gets older.34 This means that in addition to lowering blood pressure immediately, salt reduction could also attenuate the rise in blood pressure associated with ageing. Taking this into account would have captured the full effect of salt reduction on blood pressure; however, there was insufficient data to quantify the association between salt intake and the rise in blood pressure with age. Third, although it has been shown that salt reduction in childhood leads to falls in blood pressure that could prevent hypertension and CVD in later life,35 we did not have sufficient data to include children in our model. Fourth, the proportion of haemorrhagic strokes is significantly higher in countries like China, and raised blood pressure is a stronger predictor of haemorrhagic than ischaemic strokes.36 37 The inclusion of Prospective Studies Collaboration participants (90% of whom from Europe, North America or Australia) in the pooling of relative risks of SBP change on CVD risk may have led to an underestimation of the potential impact of salt and SBP reduction on stroke incidence in China. It is, therefore, probable that we have underestimated the full potential of reducing salt intake in China.
The Chinese government’s action plan ‘Healthy China 2030’ includes nutritional recommendations to reduce the intake of salt, sugar and oil.23 This modelling study shows that salt reduction alone could bring enormous health benefits to the entire population of China. It is important to note that our estimates rely on salt reductions to not only be achieved, but also sustained over time, which may be a great challenge given the fast-changing dietary patterns seen in China given its rapid urbanisation. Most notably, the consumption of processed and out-of-home foods has increased in recent years and this trend is expected to continue.8 Processed foods in China have also been found to be saltier than in other countries.38 To anticipate this, it would be necessary to implement a strategy based on setting incremental salt targets for all manufactured foods in order to decrease salt content over the whole range of products from the food industry, as pioneered in the UK and successfully adopted by many countries, for example, Australia and South Africa.39 Nevertheless, most (70%–75%) of the salt consumed in China still comes from the salt added by the consumer during cooking.8 40 Health education can effectively lead to behaviour change, as shown in the trial in Northern China of a school-based programme, which was successful in reducing salt intake in both schoolchildren and their families.21 A scale-up study of this school-based programme is currently ongoing in other parts of China, with the aim of nationwide implementation if proved to be effective. Other trials, on low-sodium high-potassium salt substitutes, health education to home cooks and restaurant interventions are ongoing or have recently been completed, some of which have already shown promising results.41
The evidence for the substantial benefits of salt reduction in China is consistent and compelling. Achieving and sustaining population salt reduction in China could prevent millions of unnecessary cardiovascular events and deaths. Given the sheer size of the Chinese population, this would also bring major benefits to global health.