Sodium and Blood Pressure: Don't Be Confused by Big Salt

High blood pressure is not the only harmful effect of too much salt—it’s also been tied to stomach cancer, kidney stones, bone loss, obesity, and direct damage to our kidneys, arteries, and heart. But, as I reviewed previously in my video, The Evidence That Salt Raises Blood Pressure, there is a consensus that dietary sodium plays a significant role in raising people’s blood pressure, a dispute that has now finally been resolved.

In Sodium Skeptics Try to Shake Up the Salt Debate, I discuss the unequivocal evidence that increased sodium intake is associated with increased blood pressure, which we know leads to increased risk of vascular diseases like strokes, aneurisms, and atherosclerosis. To quote the long-time editor-in-chief of the American Journal of Cardiology, “We all must decrease our salt intake!,” a sentiment echoed by many other authorities. So, how is the food industry going to keep the salt controversy alive? If salt leads to high blood pressure and high blood pressure leads to disease, doesn’t it follow that salt should lead to disease? I mean, if A leads to B, and B leads to C, then A should lead to C, right? The logic seems sound. Blood pressure is one of the best validated surrogate markers for cardiovascular disease, and, when countries have tried cutting down on salt, it seems to have worked.

Campaigns in England were able to successfully bring down salt consumption. Blood pressures dropped, as did rates of heart disease and stroke. They also successfully brought down cholesterol levels and smoking prevalence, though, and improved fruit and vegetable consumption. In Japan, however, they dropped salt intake while eating a worse diet and smoking more, yet still saw a large reduction in stroke mortality. Based on what they were able to achieve in Finland, one daily teaspoon of salt may mean between 25 to 50 percent more deaths from heart attacks and strokes.

Are there randomized controlled trials to show that? Researchers never randomized people into two groups—one low-sodium and one not—and followed them for 20 years to see if the differences in blood pressure translated into the expected consequences. But, for that matter, such a study has never been done on smoking either. Imagine randomizing a group of smokers to quit smoking or stay smoking for ten years to see who gets lung cancer. First, it’s hard to get people to quit, just like it’s hard to keep people on a low-salt diet. Second, would it be ethical to force people to smoke for a decade knowing from the totality of evidence that it’s likely to hurt them? That’s like the Tuskegee experiment. We can’t let the perfect be the enemy of the good.

We may never get a decade-long randomized trial, but, in 2007, we got something close. There have been randomized trials of sodium reduction, but they didn’t last long enough to provide enough data on clinical outcomes. For example, the famous TOHP trials randomized thousands into at least 18 months of salt reduction. What if you followed up with them 10 to 15 years after the study was over, figuring maybe some in the low-salt group stuck with it? Indeed, they found that when people cut sodium intake by 25 to 35 percent, they may end up with 25 percent lower risk of heart attacks, strokes, and other cardiovascular events.

This was considered the final nail in the coffin for salt, addressing the one remaining objection to universal salt reduction. It was the first study to show not only a reduction in blood pressure, but a reduction in hard end points—morbidity and mortality—by reducing dietary sodium intake. Case closed, 2007.

But, when billions of dollars are at stake, the case is never closed. One can just follow the press releases of the Salt Institute. For example, what about the Institute of Medicine report saying that salt reduction may cause harm in certain patients with decompensated congestive heart failure? An analysis of those studies has since been retracted out of concern that the data may have been falsified. It is certainly possible that those with serious heart failure, already severely salt-depleted by high dose salt-wasting drugs, may not benefit from further sodium restriction. However, for the great majority of the population, the message remains unchanged.

What about the new study published in the American Journal of Hypertension that found the amount of salt we are eating is just fine, suggesting a kind of u-shaped curve where too much sodium is bad, but too little could be bad, too?

Those biased less towards Big Salt and more towards Big Heart have noted that these studies have been widely misinterpreted, stirring unnecessary controversy and confusion. It basically comes down to three issues: measurement error, confounding, and reverse causality. All these data came from studies that were not designed to assess this relationship, and they tended to use invalid sodium estimates simply because it’s hard to do the multiple, 24-hour urine collections necessary to get a good measurement. And, in the United States, many of those eating less salt are simply eating less food—maybe because they’re so sick—so it’s no wonder they’d have higher mortality rates. So, compiling these studies together is viewed as kind of like garbage in, garbage out. But why would they do that? They claim to have no conflicts of interest. When confronted with evidence showing at least one of the co-authors received thousands of dollars from the Salt Institute, they replied they didn’t get more than $5,000 from them in the last 12 months, so, no conflict of interest!

If you instead look only at the trials in which they did the gold-standard, 24-hour urine collections in healthy people to avoid the reverse causation and controlled for confounders, the curve instead has a continuous decrease of cardiovascular disease (CVD) events like heart attacks and strokes as sodium levels get lower and lower. There was a 17 percent increase in risk of CVD for every gram of sodium a day. And, this is for people without high blood pressure. We’d expect the benefit to be even greater for the 78 million Americans with hypertension. Unfortunately, the media has widely misreported the findings and a false sense of controversy has been broadcast, confusing the public. But it’s not just the media. When editorials are published on the subject in some of the most prestigious medical journals in the world, you don’t expect them to be written by someone who got paid personal fees by Big Salt. Before she accepted money from the Salt Institute, the author was accepting money from the Tobacco Institute and was a frequent expert witness in defense of Philip Morris and other tobacco companies. So, if that’s who the New England Journal of Medicine chooses to editorialize about salt, you can see the extent of industry influence. The editor-in-chief of the American Journal of Hypertension himself worked for many years as a consultant to the Salt Institute.

This video is part of my extended, in-depth series on sodium, which includes:

Salt restriction is also important for kidney stones, as I discussed in How to Treat Kidney Stones with Diet, but aren’t low-salt diets tasteless? Only for a little while. See Changing Our Taste Buds.

For more on how industry influence can distort nutritional science, see:

In health,
Michael Greger, M.D.

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Don’t Be Confused by Big Salt

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