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Why the American Heart Association needs to acknowledge the lack of scientific consensus around restricting salt

REUTERS/Ilya Naymushin
Although restricting sodium may help lower the risk of heart disease and stroke in people with existing high blood pressure, it does not seem to do the same for others.

In a recent CardioBrief column, reporter Larry Husten takes the American Heart Association to task for its out-of-hand rebuke of a study published last Friday in the Lancet.

“Once again,” writes Husten, “the American Heart Association has taken a strong stance against science.”

The Lancet study investigated a controversial topic in medicine: the relationship between dietary sodium (a component of salt) and heart disease. Its findings suggest that although restricting sodium may help lower the risk of heart disease and stroke in people with existing high blood pressure, it does not seem to do the same for others.

In fact, the study found an association between low-sodium diets and an increased risk of heart disease and stroke.

This finding conflicts with the AHA recommendation that all adults reduce their sodium consumption to no more than than 1,500 milligrams (mg) daily to protect against heart disease and stroke. Sodium levels in the typical American diet currently average around 3,400 mg daily, according to the Centers for Disease Control and Prevention. (The AHA is not alone in recommending sodium restrictions, but other organizations are less severe about it. The latest edition of the U.S. Dietary Guidelines for Americans, for example, recommends restricting sodium intake to no more than 2,300 mg daily.)

Another side to the story

Husten, who specializes in writing about heart disease and related topics, isn’t saying, however, that the AHA’s position on salt is necessarily wrong, only that the organization isn’t acknowledging that the issue remains unresolved:

First it’s important to admit that the AHA has a legitimate point: the Lancet study is by no means perfect and it certainly doesn’t “prove” the case against salt restriction. The study, I think everyone would agree, has all the usual limitations of an observational study. It can detect an association or link but it is unable to demonstrate cause and effect. The study is further weakened by its reliance on a morning urine collection to calculate sodium excretion and then estimate dietary sodium. The authors claim that this technique has been validated in previous studies, but it is important to acknowledge that this is an imperfect measure at best.

The key point here is that the authors of the Lancet study make no claim that their study is definitive. Instead they point out that the study was performed in the first place in response to earlier, less definitive studies hinting at possible harms associated with severe salt restriction.

The AHA is quick to find limitations in the Lancet study but it doesn’t acknowledge that its own evidence supporting salt restriction is itself profoundly imperfect. In fact, the entire case for salt restriction is based on the observation that salt raises blood pressure and that increases in blood pressure can lead to cardiovascular disease. Now it is absolutely true that high blood pressure can cause cardiovascular disease. This is about as well established a fact as can be found in the messy world of medicine. But it does not necessarily follow that any method to lower blood pressure will have a beneficial effect.

We now know that blood pressure lowering drugs, for instance, should not be chosen solely on the basis of their ability to lower blood pressure. Before gaining approval a new blood pressure drug needs to demonstrate not only that it can lower blood pressure but that it is also safe. The reason for this is that there have been several instances in which blood pressure drugs have failed because they were shown to be unsafe, despite their ability to lower blood pressure. The only way to prove these drugs are safe is through randomized controlled trials.

The main difference between drugs and dietary interventions is that we know much less about nutrition. It’s (relatively) easy to test a drug in a blinded and randomized controlled trial. But when it comes to nutrition it’s almost impossible to perform large, long-term, randomized, well-controlled, and well-designed trials of free-living human beings. The fact is no one really knows for sure the long term effects of large scale interventions involving an essential nutrient like sodium.

Why it matters

As Husten also emphasizes, the AHA should, of course, be allowed to make its case for a low-salt diet, “but they should first of all admit that this is the opinion of their own experts and that there are may other reputable scientists who disagree.”

Acknowledging that this issue remains unresolved is extremely important, he adds:

It’s hard to overstate the potential dangers of getting something like this wrong. As I’ve noted several times in the past, the American Heart Association should be especially careful in this regard, since it’s already been down this road before, with disastrous consequences. Back in the 1980s the AHA developed enormously influential guidelines on cholesterol and diet. These guidelines helped spark the campaign against dietary fat and had the catastrophic consequence of pushing people to consume more carbohydrates, including sugar, instead of fat and protein.  We will probably never know the full extent of the damage, but many have speculated that this may have contributed to the obesity and diabetes epidemics. Let’s make sure this doesn’t happen again with salt.

You can read Husten’s entire column at CardioBrief

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