Ask Dr. Hallberg: What's the truth about low-salt diets?

Take the results of a new study on sodium with a grain of salt. That's what the head of the Centers for Disease Control and Prevention is saying about a study on sodium and cardiovascular disease in a major medical journal.

The European study, published in the current issue of the Journal of the Medical Association, found that low-salt diets don't prevent high blood pressure and may even increase the risk of heart disease. Dr. Peter Briss, a medical director at the CDC, said the study is flawed and that its findings contradict much of what's already known about salt and heart disease.

Dr. Jon Hallberg, regular medical analyst for MPR's All Things Considered, and host Tom Crann discussed the study on Thursday. Hallberg is a physician in family medicine at the University of Minnesota and director of the Mill City Clinic in Minneapolis.

Tom Crann: Dr. Jon Hallberg is here now to tell us more about that study and the criticism surrounding it, as well, even minor controversy here, Jon, would you say?

Dr. Jon Hallberg: Maybe not a minor controversy, but a major one. This is pretty unusual. I mean this study has the most boring sounding title in JAMA. It's called "Fatal and Nonfatal Outcomes: Incidents of Hypertension and Blood Pressure Changes in Relation to Urinary Sodium Excretion." Who knew that that would lead to all of this controversy?

Crann: So first we're talking here about sodium salt. Tell us (how it affects) the body and why it isn't good for people who have heart disease.

Hallberg: When we're talking about salt, first of all, it's sodium chloride. It's not potassium chloride. We really are focusing on sodium. The kidneys have this wonderful way of story of handling sodium. Sodium in equals sodium out.

But when people have high blood pressure, heart disease, congestive heart failure, things like that, things don't work quite the way they're designed to. And so you end up keeping more fluid or water on board, and then that makes just much more work for the heart. Blood pressure goes up, fluid can back up into the lungs, into the legs. People have seen people like that. So it can be a real problem if people have underlying cardiovascular disease.

Crann: Now this study that's published in JAMA, what really did it say or find?

Hallberg: Well, what it found is quite extraordinary ... One would think that if you take healthy people and they have too much salt in their diet, that they're going to develop high blood pressure, and high blood pressure in turn would cause heart disease. But what they found was just the opposite.

If you take healthy people that do not have high blood pressure or heart disease and they have higher salt in their diet, they had less high blood pressure and heart disease as a result. People who consumed the least had the most cardiac death, actually, and heart disease.

Crann: Why did the Centers for Disease Control actually come out and publicly criticize it? That's a fairly rare move, isn't it?

Hallberg: That's a really rare move, and I think it's because this is just so unusual. This flies in complete face of everything that we've been saying ... We think from a population standpoint, that the less salt we consume will make us heart healthier, that we'll have less heart disease. And this is basically saying just the opposite.

Crann: In any of the criticisms of the study, are they saying it was badly done or there's a problem with the methodology?

Hallberg: Well, sure. That's what the U.S. officials are saying. I actually listened to a podcast by the authors of this study, and they are staunchly defending the way that it was done. I mean it's not a bad study.

They looked at about 3,600 people over eight years. So it was an observational study. They did not put people into categories. It's not a randomized control trial, which is sort of the gold standard. You simply can't do that. You can't take people for eight years and strictly limit the amount of salt that they get in their diet. That's just never going to happen. So I think it was as good a study as could be done with a variety of people, but does that apply to a U.S. population? That's probably a bigger question.

Crann: What are you telling people in the clinic who maybe have read this headline -- it was in the New York Times and other papers -- who have hypertension or heart disease?

Hallberg: Well, if that's the case, then nothing changes. In fact, the authors of this study are very clear about that, that if you have high blood pressure or if you have heart disease or congestive heart failure, nothing changes. Too much salt is a bad thing. There's no question about that. This was simply looking at healthy people. And for years, this has been sort of the mantra, I mean for the last 15, 20 years since I've been in medical school, that over time, exposing yourself to too much salt will lead potentially to high blood pressure and ultimately heart disease.

Crann: Does this study, contrary to a lot of accumulated wisdom, does this mean now that this is a new field of research that's going to be wide open, we'll see more studies about this?

Hallberg: Well, it's very interesting. I'm sensing that there's a very sort of U.S. versus Europe thing going on with this.

Crann: The study, we should say, was done in Belgium.

Hallberg: That's right.

Crann: And why is that?

Hallberg: Well, the Europeans by and large take a much more relaxed approach to health care and lifestyle. And the Americans, we tend to be much more uptight about that. The guidelines that are being proposed would be that average U.S. citizens in the next 10 years or so would consume less than 1.5 to 2 grams of salt a day. And that is going to be really, really hard to accomplish.

Crann: That's very low, isn't it?

Hallberg: Very low. And these authors of this study are saying that's just not going to be worth it. The work that's going to be done will be done at very high levels. Yes, there will some studies done to look at this in more detail, but it's probably not going to affect what we do at a daily clinical level. I don't think that's going to affect us at this point.

(Interview edited and transcribed by MPR reporter Madeleine Baran)

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