It’s a mantra that we’ve heard for years—cut back on salt! But a new study dashes that advice, demonstrating that it’s not just too much salt that’s bad for our hearts but apparently also too little.
Not all of this surprised me, because many practitioners of natural medicine have long held the view that advice on salt intake should be individualized—and not simply be “less is best” for everyone. So I called the study’s lead author, Martin J. O’Donnell, MB, PhD, an associate clinical professor of medicine at McMaster University in Canada, to learn more about the research.
Dr. O’Donnell told me that his study—published in November 2011 in Journal of the American Medical Association—is the first large study to report potential heart health risks for both low and high salt intake in a single study.
Dr. O’Donnell and his colleagues examined data from nearly 29,000 men and women (all age 55 or over) from 40 countries who either had heart disease or were at increased risk for it because of prior history and co-morbidity factors such as diabetes. They looked at how much sodium was excreted in their morning fasting urine (so it wasn’t self-reported salt intake) at the start of the study. The participants were not aware that their salt consumption was being measured, nor were they asked to raise or lower their intake—they just ate the amount that they normally ate.
What the researchers found was that, over four years, those who consumed higher-than-average amounts of salt and those who consumed lower-than-average amounts of salt experienced more heart problems (including deaths) than those with an average intake.
It’s worth noting that the “average” salt consumption among participants in this study—estimated between 4,000 mg and 6,000 mg per day—is much higher than the recommended upper limits of 1,500 mg per day (the advice of the American Heart Association) or 2,300 mg per day (the recommended dietary guideline from the US Department of Agriculture). And yet in this study it was the people who consumed this much salt who had the least number of heart problems.
The study showed that those with the highest and lowest amounts of sodium excretion had the highest risk. For example, those who consumed more than 8,000 mg daily were at a 50%-to-70% higher risk of suffering a cardiovascular event over the four-year period, compared with the “average” salt group. And, not quite as alarming but still of concern, among those whose daily salt intake was between 2,000 mg and 3,000 mg, the likelihood of dying from a cardiac event related to congestive heart failure rose by 20%, compared with the “average” salt group.
Now, it’s important to note that patients who ate the least amount of salt might have been doing so because they were at very high risk for disease and their doctors had insisted that they cut back severely on salt. In other words, their risk uptick might not be due to their salt intake (or their salt intake alone), but also due to poor health. “We did some analysis to address this issue, but we can’t exclude this possibility—larger clinical trials will be the only way to truly answer this question,” said Dr. O’Donnell.
The mystery is—how could consuming a low amount of salt increase cardiovascular risk? There may be several potential reasons for this, Dr. O’Donnell said, but the most prominent hypothesis is that lower salt intake activates the body’s renin-angiotensin system, which results in narrowed blood vessels, which of course makes it harder for blood to flow to and from the heart.
This study—and others that have recently shown that similarly negative health effects may be associated with a low-salt diet—caught the attention of the government. Congress has already put on hold its plans to legislate lower sodium levels for school lunches, requesting more information from the US Department of Agriculture before putting it to vote. Dr. O’Donnell said that there is “an urgent need to establish a safe range for sodium intake.”
In terms of how much salt you should eat, talk to your doctor about your particular risk factors—and remember that the jury is still out.
Source: Martin J. O’Donnell, MB, PhD, associate clinical professor of medicine, McMaster University, Hamilton, Ontario, Canada.