Controversy About Salt Intake

Summarized by Robert W. Griffith, MD
April 10, 2006

Introduction

The recommended limit for sodium intake is 2300 mg daily, in order to prevent or delay the onset of high blood pressure, and to help treat high blood pressure. African Americans, people with hypertension, and older persons should limit their intake to 1500 mg daily. These recommendations come from the 2005 guidelines published by the US Department of Health and Human Services. Now researchers from the Albert Einstein College of Medicine have reported the results of an analysis of salt intake in relation to cardiovascular disease and mortality that questions the value of these restrictions. It was published in the American Journal of Medicine, and is summarized below.

What was done

The second National Health and Nutrition Examination Survey (NHANES II) provided the data for this analysis. Baseline exams were made in 1976 to 1980 on over 20,000 enrollees aged 30 to 74. Those with a previous history of heart disease or stroke, or who were taking a low-salt diet for medical reasons, were excluded.

At baseline, nutrient habits were recorded by expert interviewers, based on 24-hour recall by the participants. A full physical exam was done, and demographic and lifestyle data were collected. Follow-up ranged from 0.5 to16.8 years (average 13.7 years). All deaths were counted and coded according to the International Classification (9th revision).

What was found

The average age of the participants was 48; half of them had a normal BMI (below 25), 35% were overweight, and 16% qualified as being obese. Half were male, a third were smokers, and 30% were receiving treatment for high blood pressure. There were 1343 deaths during the follow-up period; 541 of them were due to cardiovascular disease. Of the 541 cardiovascular deaths, 79 were due to stroke and 282 to coronary artery disease.

When the participants were divided according to their daily sodium intake (more or less than 2300 mg), those with the lower sodium intake were more likely to be older, have diabetes, less education, and be less active; they were also slightly heavier, and drank less alcohol. Those participants who consumed more sodium were more likely to be male, white, smoke cigarettes, and have a higher calorie and potassium intake.

There was a negative association between sodium intake and cardiovascular mortality - that is to say, those who consumed less sodium were more likely to die from cardiovascular disease. For each 1000 mg reduction in salt intake, the risk for cardiovascular mortality rose by 1%.

Those consuming less than 2300 mg salt daily were 1.4 times as likely to die from cardiovascular disease as those consuming more sodium, while their overall mortality was 1.3 times higher. The only group of participants who failed to show this association were those below 55 years of age, non-whites, and the obese.

What these results may mean

Not surprisingly, this report has been challenged by several experts. One (Professor David Katz) thinks that the findings merely reflect the effective use of drugs by older people to compensate for the effects of a high salt intake e.g. raised blood pressure. He believes that participants with chronic diseases like hypertension were working hard to cut their salt intake, and "it should come as no surprise that the sickest people have the highest mortality rate".

This type of study has limitations. First, the method of salt intake determination was based on a single self-assessment at baseline, and there's no data on intake over the follow-up period. Second, it was a cohort study, using retrospective analysis; this is inferior to a prospective controlled study for providing evidence of cause-and-effect. Third, the comparative groups formed for analysis - those eating more and those eating less than 2300 mg sodium daily - were clearly different in some respects: age, education, diabetes, activity, and so on. These limitations may make the findings a little wobbly, but they don't destroy them. As Professor Alice Lichtenstein has said, "Observational studies like this one are important for helping us frame the right question to ask in clinical trials, but they don't provide answers on their own." Or, as Katz says,". . . when first reported, it's the papers that ultimately prove to be wrong that sound most exciting."

Perhaps the best interpretation for most of us is to realize that not everyone needs to be on a low-salt diet. Eating plenty of fruits, vegetables, whole grains, and fish should not lead to a high sodium intake.

Source

• Sodium intake and mortality in the NHANESII follow-up study. HW. Cohen, SM. Hailpern, J. Fang, MH. Alderman, Am J Med, 2006, vol. 119, pp. 275.e7--275.e14

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