The global cardiovascular burden of excess salt intake
Despite adverse impact of excess salt intake on population health, reliable global salt intake data were not available, with limitations of coverage, time era, representativeness, comparability and potential heterogeneity by age and sex. The overall purpose of the work presented in this thesis was to provide, for the first time, the best estimates of the national (187 countries), regional (21 regions) and global burden of cardiovascular disease attributable to higher than optimal salt intakes in 1990 and 2010 by systematically retrieving the best available evidence.
First, I reviewed and critically appraised the scientific evidence on the link between excess salt intake and health outcomes, including its relationship with high blood pressure, gastric cancer, osteoporosis, and renal disease. After that, existing controversies about the impact of excess salt intake on health were discussed; to do so, a recent publication that questioned the health benefits of population salt reduction was critically evaluated. In the next part, various terminologies for defining the exposure to salt and their strengths and limitations were discussed. In the following chapter, using a multi‐disciplinary approach, an optimal level for salt intake was suggested. Systematic review and analysis of 24‐hour urinary sodium excretion and dietary surveys worldwide were the bases of the next chapter. Data from 247 surveys conducted from 1980 onwards including 143 using 24‐h urinary sodium and 104 diet‐based surveys, representing 66 countries and 74.1% of the world population were used. Using a Bayesian Hierarchical method, national, regional, and global average sodium intakes and their associated uncertainties in 1990 and 2010, were estimated. Mean global salt intake in 2010 was 10 g/d, twice the WHO recommendation of 5 g/d. Substantial heterogeneity was seen by country and sex. Mean intakes of adult men and women in 97% and 95% of countries were >5g/d. Highest intakes were seen in Kazakhstan (15.2 g/d), Mauritius (14.2) and Uzbekistan (14.0); and lowest in Kenya (3.8), Malawi (3.8) and Rwanda (4.0). Little variation was seen by age. From 1990 to 2010, global salt intake increased by 315 mg/d; it increased by >250 mg/d in 83 countries, and decreased by >250 mg/d in only 15 countries. The chapter after that was dedicated to estimation of national, regional and global impacts on mortality due to excess salt intake. First, the relationship of salt intake with blood pressure was quantified by reviewing the literature and performing new meta‐analyses of sodium reduction randomized trials. After that, changes in national, regional, and global levels of blood pressure due to reduction in salt intake ‐ including a hypothetical situation in which salt intake was reduced to the optimal level ‐ were estimated. Finally, by taking into account the mortality associated with different blood pressure levels, the reductions in national, regional, and global cardiovascular mortality that would be gained by reducing populations salt intake levels were estimated. 1.38 million (95% CI 0.9, 1.5M) cardiovascular deaths were attributable to excess salt in 2010, 45% due to coronary heart disease, 46% to stroke, and 9% other cardiovascular disease. 55% of these deaths were in men. More than 80% of deaths were in low and mid‐income countries. Among the top 30 most populous nations, highest mortality due to excess salt intake was seen in Ukraine (1,163 deaths per million adult population), Russia (1,006), and China (505); highest proportional mortality in Thailand (18.5% of all CVD deaths attributable to excess salt), China (17.6%), and Korea (17.4%). The final two chapters focused on salt reduction in Iran, where a national salt reduction plan was proposed and discussed.