Is Reducing Dietary Sodium Helpful in Reducing Blood Pressure and Cardiovascular Disease Risk? An Argument Generated From the PURE Study.

Abstract

To the Editor: The bound of healthy dietary sodium and the risk rising in high dietary sodium have generated much controversy. The current standing is that higher prevalence of cardiovascular disease (CVD) is associated with higher sodium intake.1-3 Two studies published in the New England Journal of Medicine (NEJM) have also confirmed this perception.4, 5 Mozaffarian and colleagues suggested that 1.65 million deaths from cardiovascular causes that occurred in 2010 were attributed to sodium consumption above a reference level of 2.0 g/d.4 However, the PURE study published in NEJM argued that increased CVD mortality and morbidity were observed in the patients whose urinary sodium excretion was <3.0 g/d.6 At the same time, these studies confirmed the association of lower dietary potassium with both increased blood pressure and adverse health outcomes5, 6 The unusual results of the Prospective Urban Rural Epidemiological (PURE) study6 also attracted the attention of Chinese doctors, especially the results that a sodium intake <3 g/d leads to an increase of 0.1 mm Hg/g in diastolic blood pressure. We have been reviewing and summarizing the Chinese data from 1980 until now. Large-scale, typical, or influential reports were selected from the reference data from the China National Knowledge Infrastructure and PubMed database (Table). Most of the trials showed that dietary salt or sodium excretion was positively correlated with blood pressure or CVD risk, regardless of the region, rural or urban. The range of high salt was 6 g/d to 10 g/d in the Chinese participants. Meanwhile, some data showed that different ethnic groups have different tolerance to high salt. Although the data were based more on epidemiological survey and questionnaires and less on intervention observations, more on point urine samples and less on urine samples collected over a 24-hour period, and more on single centers and less on multiple centers, the results still suggest that higher salt intake (>6 g/d) results in higher prevalence of hypertension and CVD. Some academic associations have also questioned the results of the PURE study. The American Heart Association criticized the PURE publication in NEJM and indicated that the publication has methodological weaknesses, such as reverse causality, wherein people eat less salt because of their illness rather than lowering salt consumption to avoid illness (American Heart Association, http://newsroom.heart.org/news/excessive-sodium-consumption-has-dire-impact-on-global-health-new-study-finds). Studies based on weak methodologies are likely to continue to generate controversy, and the science will only be advanced by carefully conducting rigorous research (Canadian Institute for Health Research and Heart and Stroke Foundation Chair in Hypertension Prevention and Control: http://www.hypertensiontalk.com/science-of-salt-weekly/). What's more, the World Hypertension League (WHL) and many other organizations have confirmed and expanded the associations between excess dietary sodium and human diseases. Many of these studies have utilized more rigorous scientific methods than the PURE study but are published in journals with lower scientific impact and publicity than NEJM. WHL is committed to the regular reviews of the literature, the setting of minimum standards for research methods, and the regular updates to dietary recommendations. China has the largest number of hypertensive patients and the Chinese people have very high sodium consumption. Chinese doctors, researchers, patients, and medical insurance policy makers should have consistent voice in sodium management. We should evaluate any study with a balanced view. The PURE study has declared limitations in validated method, probability-sampling approach, and observational studies. We suggest further studies and deliberation before setting up the guides for healthy dietary sodium intake.