Cutting the average salt intake in half could prevent a substantial proportion of strokes and heart disease in most Western countries, a meta-analysis showed. A decrease of 5 grams of salt a day (about one teaspoon) was associated with a 23% lower stroke rate and up to 17% less total cardiovascular disease.
This kind of change at the population level “could avert some 1.25 million deaths from stroke and almost three million deaths from cardiovascular disease worldwide,” the researchers wrote.
Americans, like those in many Western countries, average about 10 g of daily salt intake; whereas the World Health Organization recommends only 5 g per day, and the U.S. Department of Agriculture recommends daily intake be limited to 5.8 g.
The American Medical Association and other organizations have called for a combined individual and societal approach to cutting sodium intake, but levels remain far above the target even in countries where regulations have been introduced, Strazzullo’s group noted.
Awareness of the need to shake the salt habit has improved in recent years but hasn’t translated into real reductions in salt intake, commented Deepak L. Bhatt, MD, MPH, of Brigham and Women’s Hospital in Boston.
Mixed messages about whether drastic changes are called for have made it all too easy for people to believe what is consistent with their current lifestyle, Bhatt said.
The food industry has also long opposed a tougher public health policy on salt, in part citing poor evidence for efficacy on hard endpoints at a population level. But the meta-analysis results should clearly address those doubts, Strazzullo and colleagues said.
Lawrence J. Appel, MD, MPH, of Johns Hopkins, agreed, calling the meta-analysis a “useful and welcome addition” to the perplexing and methodologically plagued literature on the direct effects of salt intake on hard endpoints.
“At a minimum, Strazzullo and colleagues’ analyses should dispel any residual belief that salt reduction might be harmful (a canard resulting from misinterpretation of studies, often with flawed analyses),” Appel said in an accompanying editorial.
The researchers systematically reviewed the literature and found 19 independent cohort samples from 13 prospective studies published from 1966 through 2008. These studies with a total of 177,025 participants and more than 11,000 vascular events were of generally good quality (average 15.5 on a 19-point scale).
Pooling the study results, the average 86-mmol (roughly 5 g) higher salt intake per day was associated with greater risk of stroke (relative risk 1.23, P=0.007). There was significant heterogeneity among individual studies, though, with nine suggesting a direct positive link with sodium intake (four significant) and three showing a nonsignificant inverse relationship.
For cardiovascular disease, the same roughly 5-g difference in salt intake produced a trend for higher risk (RR 1.14, P=0.07). Again, individual cohorts differed in outcome, with 10 showing a direct association and four showing a least an inverse trend.
Excluding the one study with a significant inverse link suggesting higher risk with lower salt — which the researchers called an outlier — made the overall association between salt and cardiovascular disease significant (RR 1.17 for an approximately 5-g difference, P=0.02).
A dose-response with higher risk for incrementally higher salt intake appeared for stroke (RR 1.06 per 50 mmol per day, 95% confidence interval 1.03 to 1.10). But the trend for a dose increase in cardiovascular disease risk was not significant (RR 1.19 per 50 mmol per day, 95% CI 0.69 to 2.07). But age, sex, and hypertension status did not appear to influence the risks substantially.
Longer duration of follow-up in the studies appeared to strengthen the relationship to salt for stroke, although not for total cardiovascular events.
“Possible explanations for this discrepancy are the higher mean age at occurrence of stroke, which would increase the chances of an event the longer the follow-up, and the closer relation of high blood pressure to stroke compared with other types of vascular events,” the researchers suggested.
They cautioned that the meta-analysis likely underestimated risk associated with higher salt intake because all component studies used only a single baseline salt intake measurement despite large day-to-day variations in salt consumption.
Strazzullo P, et al “Salt intake, stroke, and cardiovascular disease: meta-analysis of prospective studies” BMJ 2009; 339: b4567.