Might Reducing Salt Intake Promote ?Cardiovascular Disease?

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According to a prospective observational (PURE) study that followed over a hundred thousand people 35-70 years old in 17 different countries for an average of 3.7 years, those whose salt intake was below 3,000 mg of sodium per day (or <7.5g of salt) were 30% more likely to die from cardiovascular disease (CVD) compared to those whose estimated sodium intake was between 4,000 and 6,000 mg/day (or 10 to 15g of salt/d) (1). According to the authors of this study and the media hype around it, the study seriously undermined current guidelines to reduce dietary sodium intake from the current levels of about 3,500 to 6,000 mg to the currently recommended levels of 1,500 to 2,300 mg of sodium per day. The authors of this study suggested that attempts to reduce salt intake may actually increase the risk of having a heart attack or a stroke, as well as dying from CVD. The 10,810 subjects with the least sodium in their urine (suggesting a dietary intake of <3,000 mg per day) differed from those excreting more sodium. They were older, more likely to have diabetes, more likely to have a personal history of CVD, had a highest LDL-C level, and were taking far more blood pressure medications than those consuming more salt.

According to Dr. O’Donnell, the subjects excreting the least sodium in their urine were also excreting the least potassium too. The latter observation seems odd since the subjects excreting the least sodium and potassium were also reportedly consuming significantly more fruits and vegetables. Certainly inaccurate data and confounding from other variables may well explain why lower sodium in the urine was associated with more CVD. The important question from a public health perspective is should efforts to reduce the amount of salt added to food be reduced and dietary guidelines recommending reducing salt intake be abandoned?

English Experience Shows Reducing Salt Cuts CVD Mortality

A study looking at the impact of a public health measure to get people in England to reduce their salt intake certainly undermines the concerns that cutting dietary salt intake may somehow increase CVD.

Dr. He looked at what happened to average blood pressure (BP) and CVD mortality in England from 2003 to 2011 as public health efforts to cut salt intake were ramped up. Most of the reduction resulted from pressuring the commercial food industry to cut back on added salt. The result of these efforts was that salt intake in England was reduced by 15%. That’s from 9.5 to 8.1 g per day, on average, which translates to a shift from 3,800 mg of sodium to 3,240 mg. Dr. He and colleagues also observed an average decline in BP of 3.0/1.4 mmHg over that 8-year period. Factor analysis showed that most of this reduction in BP was attributable to the modest reduction in sodium intake. More importantly, deaths from stroke fell 42% and deaths from coronary artery disease dropped by 40% from 2003 to 2011. The authors concluded: "The reduction in salt intake is likely to be an important contributor to to the falls in BP in England from 2003 to 2011 (2). It should be noted the reduction in sodium intake occurred within the range (3,000 to 6,999 mg sodium/day) in which the PURE researchers claimed would have little or no impact on CVD mortality. Data from the DASH Sodium Trial clearly show that reducing dietary sodium from the level now the norm in England (to about 1500 mg/day) would likely lower average BP even more. Dr. Oparil noted that elevated BP accounts for about 9.4 million deaths from CVD worldwide. The data from the two PURE studies also showed increasing BP with increasing salt intake. Studies of human populations that add very little or no salt to their food show that almost everyone's BP remains in the normal range (<120/80 mmHg) at least into their 60s. By contrast, most Americans have seen their BP rise into the either the prehypertension or hypertensive range by the time they are in their 40s. By age 65, the majority of Americans have hypertension.

Bottom Line: The experience from England seriously undermines Dr. Oparil's editorial claim that the PURE study results “... call into question the feasibility and usefulness dietary sodium as a population-based strategy for reducing blood pressure.” This makes me wonder why none of the science writers and journalists who were so quick to parrot Dr. Oparil's opinions ever thought to ask why she believed the questionable data and opinions from the PURE researchers were not largely refuted by the real-life public health efforts ongoing in England.

By James J. Kenney, PhD, FACN

1. References:
   O'Donnell, M, Mente A, Ranjarajan S, et al. Urinary sodium and potassium excretion , mortality, and cardiovascular events. N Engl J Med 2014;371:612-23
   He FJ, Pombo-Rodrigues S, MacGregor GA. Salt reduction in England from 2003 to 2011: its relationship to blood pressure, stroke and ischemic heart disease mortality. BMJ Open 2014;4:e004549. doi:10.1136/bmjopen-2013-004549
   Oparil S. Low sodium intake - cardiovascular health benefit or risk? N Engl J Med 14;371:677-9