Here we go again. Claims about the low sodium diet being deadly. These types of papers resurface every few years and elicit multiple responses pointing out their flaws. Occasionally the response takes the form of a review paper discussing high quality studies. Such papers usually limit the studies they report on to sodium studies. The publication (1) to be reviewed today is one of the few that reviews both sodium and potassium. The authors reviewed the recent medical literature and found 52 publications that studied sodium, potassium, or both and the relationship to cardiovascular disease. They limited their reviews to randomized controlled trials (RCT). RCTs are considered the gold standard for clinical intervention studies.
Flawed Studies
This paper was done in response to a recent high profile paper (2) that was published in 2011 and rekindled a recurring debate about the safety of a low sodium diet. As is usual with these papers, the evidence was lacking in strength. The paper was observational and had the usual problems with design and methods. A major issue in this particular paper was the under-collection of 24 hour urine samples in the group purported to have the lowest sodium intake.
This debate about the safety of a low sodium diet started in the 1990s when some studies reported that consuming sodium at the lowest level recommended by the Institute of Medicine led to an increase in cardiovascular mortality. The authors of those studies reported that modest reductions in sodium could have a favorable effect, but that more intense reductions lead to more deaths. This is the often discussed U-curve or J-curve. These earlier studies have been discussed and reviewed extensively in the medical literature. There are multiple serious flaws with these studies.
One of the easiest flaws to understand occurs repeatedly in many of these papers. It is reverse causation. This flaw is present when the group of individuals with the lowest sodium intake has a higher percentage of people with serious health conditions, especially heart and kidney conditions. This means the low sodium group has more people who were far more likely to die than any of the other groups. They are on a low salt diet to try to improve their condition. In other words, their deadly condition is the cause of being on a low salt diet, not that the low salt diet is the cause of their deadly condition – i.e., reverse causation.
Other problems that commonly occur include the inaccuracy of spot urinary sodium representing sodium intake in those on medications for hypertension, under-collection of 24 hour urine collections, use of inappropriate statistical methods, and inclusion of inappropriate studies that were originally designed for a different purpose if the study is a meta-analysis.
To avoid such problems, the authors of the present publication excluded papers on several grounds. The paper was excluded if it did not include relevant data, such as systolic and diastolic blood pressure, or did not include indicators of kidney and vascular damage. A major reason to exclude a paper was if it was examining patients with heart failure.
Findings
The authors found 52 publications of randomized controlled trials (RCT) that met their criteria of a dietary sodium and/or potassium intervention, and a method to assess the actual dietary sodium and/or potassium intake. In 28 of the studies only sodium was modified in the diet. These studies showed that a decrease in sodium had a favorable effect on cardiovascular disease. They found 12 studies that modified potassium only. The authors found 12 studies that modified both sodium and potassium. Furthermore, in this last group they found 2 studies in which a strong relationship was shown with a dose response curve.
When they looked at grading the quality of the studies, the authors found many of the studies were of high quality, which they graded A. This grade meant that further research would be unlikely to change the confidence in the estimate of the effect found by the study.
Based on the grades of the studies, they used two levels to determine how strong a recommendation was. Level 1 meant that “most people in your situation would want the recommended course of action and only a small proportion would not.” Level 2 meant that “the majority of people in your situation would want the recommended course of action, but many would not.”
Recommendations For Healthy And Unhealthy
The authors found that the quality of the sodium studies led to a Level 1 recommendation to reduce salt to prevent cardiovascular disease. Even the lowest level of sodium intake was safe and helpful. They found that the potassium studies led to a Level 1 recommendation to increase potassium intake to prevent cardiovascular disease. Thus the recommendations of the IOM, AHA, and USDA concerning sodium and potassium in the diet were supported by the highest quality research. Levels of sodium below 1500 mg per day were safe and reduced cardiovascular disease. Levels of potassium above 4700 mg per day were safe and reduced cardiovascular disease.
The authors then concluded with some recommendations for doctors treating patients. They felt that there were strong recommendations to reduce sodium and increase potassium in the diet. There was strong evidence that these changes in diet would reduce blood pressure and all the sequelae of hypertension and cardiovascular disease.
However if someone already had damage to their heart or kidney, they needed extra special care. Those with severe heart failure requiring high dose medications would not benefit from salt restriction. Those with salt wasting tubulopathies (a kidney disease) would need close supervision of their salt intake. Patients with advanced kidney disease would need careful monitoring to prevent development of too much potassium in the blood. Patients with multiple advanced diseases complicating their medical care would need individualized dietary management.
A high potassium foods diet would not be likely to help these individuals with advanced organ damage. A high potassium foods diet is a preventive diet that will prevent cardiovascular disease. But it is not a therapeutic diet for those with advanced organ damage.
For links to tables of high potassium foods, click the tab at the top of the page, labelled “Links To Food Potassium Tables.”
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1. Role of dietary salt and potassium intake in cardiovascular health and disease: a review of the evidence. Aaron KJ, Sanders PW. Mayo Clin Proc. 2013 Sep;88(9):987-95. doi: 10.1016/j.mayocp.2013.06.005.
2. Fatal and nonfatal outcomes, incidence of hypertension, and blood pressure changes in relation to urinary sodium excretion. Stolarz-Skrzypek K, Kuznetsova T, Thijs L, Tikhonoff V, Seidlerová J, Richart T, Jin Y, Olszanecka A, Malyutina S, Casiglia E, Filipovský J, Kawecka-Jaszcz K, Nikitin Y, Staessen JA; European Project on Genes in Hypertension (EPOGH) Investigators. JAMA. 2011 May 4;305(17):1777-85. doi: 10.1001/jama.2011.574.
