The Shaker's a Heartbreaker
Abstract & Commentary
By Barbara A. Phillips, MD, MSPH, Professor of Medicine, University of Kentucky; Director, Sleep Disorders Center, Samaritan Hospital, Lexington. Dr. Phillips is a retained consultant for Cephalon and Ventus, and serves on the speakers bureaus for Cephalon and Boehringer Ingelheim.
Synopsis: High salt intake is associated with increased risk of stroke and heart disease.
Source: Strazzullo P, et al. Salt intake, stroke, and cardiovascular disease: Meta-analysis of prospective studies. BMJ 2009;339:b4567.
To test the hypothesis that high salt intake is associated with an increased risk of cardiovascular disease, these authors undertook a systematic review and meta-analysis of prospective studies published over a period of time spanning more than four decades (1966-2008).
To be included in this meta-analysis, a study had to be an original article, be a prospective population study, assess salt intake as baseline exposure, determine either stroke or total cardiovascular disease prospectively as the outcome, follow participants for at least 3 years, include an adult population, and indicate the number of participants exposed and the rate or number of events in different categories of salt intake.
The investigators analyzed 19 independent cohort samples from 13 different studies, which included a total of 177,025 participants and more than 10,000 vascular events (5346 strokes and 5161 other cardiovascular disease events). The range of follow-up was 3.5-19 years. The authors calculated the relative risk of higher vs lower salt intake by comparing the adverse cardiovascular event rate in the two categories with a difference in average salt intake closest to 100 mmol of sodium or about 6 g of salt a day. Subgroup or meta-regression analyses were used to identify associations between risk of cardiovascular disease and relevant study characteristics, such as the age and sex of participants, year of publication, duration of follow-up, method of assessment of sodium intake, difference in sodium level, and control for baseline blood pressure. The effect of salt intake on risk of stroke did not change much with the exclusion of any one study, including the largest one,1 which accounted for about 40% of all participants in the meta-analysis.
Higher salt intake was associated with about a 25% greater risk of stroke (relative risk [RR], 1.23; 95% confidence interval [CI], 1.06-1.43; P = 0.007) and a 14% increase in cardiovascular disease (RR, 1.14; CI, 0.99-1.32; P = 0.07). There was no association between mean age of study participants and effect of sodium intake on the risk of stroke or cardiovascular disease. There was a suggestion that men with high salt intake might not be at as high a risk for cardiovascular disease as are women with high salt intake, but this finding is based on only three studies and it was not robust. High salt intake was a significant risk factor for stroke in both men and women. The association between salt intake and stroke or cardiovascular disease was more robust when based on food-frequency questionnaires than when based on 24-hour urine collection. Adjustment for blood pressure and for body mass index (BMI) did not change the findings much. The larger the difference in sodium intake reported in each study and the longer the period of follow-up, the stronger the relationship between salt intake and stroke, but duration of follow-up and difference in intake was not statistically significantly associated with an increased risk of other cardiovascular disease. The authors note, "The habitual salt intake in most Western countries is close to 10 g a day (and much higher in many Eastern European and Asian countries), and we calculated that the average difference between higher and lower salt intake across the study cohorts included in our meta-analysis was 5 g a day. Given this approach, we believe that, despite the inherent inaccuracies, the results of our meta-analysis are applicable to real life conditions. A reduction of 5 g (about one teaspoon) of salt would bring consumption close to the WHO recommended level."
Commentary
This study demonstrates a strong association between high salt intake and stroke and cardiovascular disease, independent of age. So, if high salt intake is associated with increased stroke and other cardiovascular disease, is it possible to demonstrate reduction in these bad outcomes by reducing sodium intake? Certainly, reduced salt intake has been demonstrated to be associated with reduced blood pressure, an "upstream" cause of stroke and cardiovascular disease.2 Because of the association of high sodium diets with hypertension, and the prominent role of high blood pressure in promoting cardiovascular diseases, the World Health Organization has suggested that a worldwide reduction in salt intake could significantly reduce the incidence of cardiovascular disease. The implications of this study, according to the editors, are striking: "A difference of 5 g a day in habitual salt intake is associated with a 23% difference in the rate of stroke and 17% difference in the rate of total cardiovascular disease. Each year a 5 g reduction in daily salt intake at the population level could avert some 1.25 million deaths from stroke and almost 3 million deaths from cardiovascular disease worldwide."
While writing this on an airplane, I munched a gourmet turkey remoulade sandwich that I picked up at an airport kiosk. I chose it because the nutrition label (now conveniently included on every package of prepared food sold in this country) said it contained "only" 690 calories. My plan was to eat half of it, with a net caloric intake of "only" about 350 calories. With this new insight about sodium, I glanced again at the nutrition information, and gasped in alarm. My sandwich contained 1490 mg (about 1.5 g) of sodium, 62% of the recommended daily value. Yikes! My seat mate's teeny weeny package of peanuts contained 100 mg of sodium (I ate 3 of those packages on the connecting flight). This prompted me to Google "salt content of foods," and I found the USDA publication4 lengthy, but enlightening (cornmeal, 1860 mg/cup; catsup, 27 mg/packet; who knew?). But internet searches are not usually necessary. It's right there on the package.
We are increasingly focused on caloric restriction as a way of managing weight and health outcomes. This study, which shows that high sodium intake is associated with increased risk of stroke and heart disease, independent of BMI, sounds a loud warning that our review of nutrition labels needs to include sodium content too.
References
1. Umesawa M, et al; JACC Study Group. Relations between dietary sodium and potassium intakes and mortality from cardiovascular disease: The Japan Collaborative Cohort Study for Evaluation of Cancer Risks. Am J Clin Nutr 2008;88:195-202.
2. He FJ, MacGregor GA. A comprehensive review on salt and health and current experience of worldwide salt reduction programmes. J Hum Hypertens 2009;23:363-384.
3. World Health Organization. Reducing salt intake in populations: Report of a WHO forum and technical meeting. Geneva, Switzerland: 2007:1-60.
4. United States Department of Agriculture. USDA National Nutrient Database for Standard Reference, Release 20. Available at: www.nal.usda.gov/fnic/foodcomp/Data/SR20/nutrlist/sr20a307.pdf. Accessed Dec. 11, 2009.
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