After Study Suggests Harm With Too-Low Sodium Intake, the American Heart Association Rebukes
Consuming less than 3 grams of sodium per day is associated with a greater risk of all-cause death or major cardiovascular events compared with more moderate intake in both hypertensive and normotensive individuals, an observational study of more than 130,000 participants has shown. In contrast, consuming 7 grams or more per day is tied to worse outcomes in hypertensive patients only.
The findings conflict with advice by the American Heart Association (AHA) to consume no more than 1.5 grams of sodium per day.
“The observation that high sodium intake is only associated with increased cardiovascular disease in individuals with hypertension raises questions whether public health policies targeted at reducing sodium in the entire population are appropriate,” lead author Andrew Mente, PhD (McMaster University, Hamilton, Canada), and colleagues write online May 20, 2016, ahead of print in the Lancet.
“Therefore, until new robust data emerge from large trials, it might be prudent to recommend reduction in sodium intake only in those with high sodium intake and with hypertension,” they argue, pointing out that such patients encompassed about 10% of the studied population.
But Daniel Jones, MD (University of Mississippi Medical Center, Jackson, MS), speaking to TCTMD on behalf of the AHA, which issued a public statement refuting the study, strongly disputed its results.
“This is a flawed study, and no health policy should be based on this study,” he said. It’s “difficult to do good studies, but the preponderance of the evidence is that most people eat too much sodium and that people’s general health will be improved by eating less sodium. This message that people should be concerned about eating too little sodium is just something that should not be taken seriously.”
Harm at High and Low Intake
Mente and colleagues looked at pooled data on 133,118 people (63,559 with hypertension and 69,559 with normal blood pressure) from 49 countries who participated in four large prospective studies: PURE, EPIDREAM, ONTARGET, and TRANSCEND. For all patients, a morning fasting urine sample was used to estimate 24-hour urinary sodium excretion as a measure of daily intake.
Mean estimated sodium excretion was roughly 5 grams/day in hypertensive patients and 4.8 grams/day in normotensive individuals. Systolic blood pressure was higher for each 1-gram increase in daily intake, with a larger effect observed in hypertensive versus normotensive participants (2.08 vs 1.22 mm Hg).
Overall, the primary composite outcome of death, MI, stroke, or heart failure occurred in 11% of hypertensive patients and 4% of the normotensive population during a medial of 4.2 years of follow-up.
The relationship between sodium intake and outcomes differed based on hypertension status. Among patients with hypertension, risk of the primary outcome was higher in those with excretion of at least 7 grams/day (HR 1.23; 95% CI 1.11-1.37) and those with excretion of less than 3 grams/day (HR 1.34; 95% CI 1.23-1.47) compared with excretion of 4 to 5 grams/day.
For normotensive individuals, on the other hand, risk was elevated only among those with excretion of less than 3 grams/day (HR 1.26; 95% CI 1.10-1.45). Adjustment for blood pressure did not change the findings in either the hypertensive or normotensive groups.
“Our findings replicate previous reports and extend these observations to populations based on baseline hypertension status,” the authors note. “Further, they suggest that although there is a limit below which sodium intake would be unsafe, the harm associated with high sodium consumption seems to be confined to those individuals with hypertension.”
Flaws in Existing Research
Jones, who is a past president of the AHA, said this and prior studies showing a relationship between low sodium intake and increased cardiovascular risk suffer from similar shortcomings, primarily inadequate evaluation of sodium intake.
In the current study, for instance, daily sodium intake was estimated using a single urine sample, noted Jones, who added that a better approach is to use multiple measurements over the course of 24 hours.
Eoin O’Brien, MD (University College Dublin, Ireland), addresses that argument in an accompanying editorial.
“The counter viewpoint is that, whereas the technique is unsuitable for estimation of 24-hour sodium excretion in individuals, it has been validated against 24-hour urine collections in both healthy individuals and those with hypertension, and serves, therefore, as a valid measure of mean population sodium intake in epidemiological studies of association,” O’Brien writes.
Jones responded to that assertion by saying “most scientists would disagree with that.”
Ideally, large, long-term randomized trials would be performed to definitively evaluate the effects of reducing sodium intake to various levels, Mente and colleagues say.
It will be claimed that such trials are not feasible “and that the benefits of a low-salt-for-all strategy outweigh any potentially harmful effects,” O’Brien notes in his editorial. “However, without defining precisely the latter risk, this approach is inherently flawed in that the assumption on which the premise is based is scientifically incorrect.”
He concludes by saying that “we must acknowledge that given the dependency of so many physiological systems on the sodium cation, it should come as no surprise that a low-salt-for-all policy would benefit some and disadvantage others. So rather than allowing contrary evidence to dispel the positive efforts that have been made to reduce the salt content of foods, we must now direct our efforts to formulating a policy that will benefit the majority in society without comprising the minority.”
Mente A, O’Donnell, Rangarajan S, et al. Associations of urinary sodium excretion with cardiovascular events in individuals with and without hypertension. Lancet. 2016;Epub ahead of print.
O’Brien E. Salt—too much or too little. Lancet. 2016;Epub ahead of print.
- Mente reports receiving a Research Early Career Award from the Hamilton Health Sciences Foundation.
- Jones and O’Brien report no relevant conflicts of interest.