Questions & Answers on Dietary Salt and Health
Salt Institute June 9, 2005 Webcast
(l to r) Drs. David A. McCarron, Suzanne Oparil, Alexander G. Logan & Michael H. Alderman
(References cited)

When blood pressure is reduced, is there lower incidence of cardiovascular events like heart attacks and strokes?

Transcript of answers

Dr. Alderman:   That's a very difficult and interesting question. Blood pressure is clearly an intermediate variable, in other words, in a general way, lowering blood pressure is associated with improved cardiovascular outcomes, but it also matters how you lower blood pressure and, for example, when we use different antihypertensive drugs, we find that the effects on blood pressure may be the same, but the effects on ultimate outcomes sometimes differ. In fact, there have been antihypertensive drugs that have adverse effects, despite the fact that they lower blood pressure and have been removed from the market place. But I think there are other analogies, other than drug analogies, that are important here. For example, 50 years ago, we had the notion that if women restricted weight gain during pregnancy they would reduce the likelihood of eclampsia, which was a real hazard and is a real hazard during pregnancy, but what was found after more extensive study was that a calorically-restricted and a sodium-restricted diet during pregnancy actually increase fetal wastage. So we no longer recommend a low calorie weight-restricted pregnancy. I think the story is more analogous really because of the notion of the relationship of salt to cardiovascular outcomes.

Dr. Alderman:   Well, I think a low sodium diet and a diuretic therapy are very different kinds of interventions. In one case you're altering the dietary intake, not only of salt, but a variety of other compounds to accomplish one objective, the reduction of salt intake. A diuretic works in part by increasing sodium loss and reducing blood volume, but diuretics have a variety of other effects as well. So I think the analogy might be fixing or dealing with the problem of a leaky roof either by fixing the roof or by digging a hole in the floor. They're very, very different strategies and it doesn't seem to be sensible to expect that the outcomes would be the same.

Dr. Alderman:   Well that's an important question. I closed my remarks by saying we needed to do the right kinds of studies to address this question before we ask the entire United States population to alter their sodium intake. I think the ultimate and ideal test is a clinical trial, a randomized prospective study in which participants are randomly allocated either to a low sodium diet or a regular sodium diet and we look for health outcomes; strokes, heart attacks and all-cause mortality. That study of course can be done. TOHP II, in which I think about a 1000 patients were randomized to different sodium intakes, they were able to sustain that for about 3 years. I think if you can do it with a 1000 people you can probably do it with three or four or five thousand that would be required to answer the question. But I think there's another point that's got to be considered first, and a randomized clinical trial is only justified when there's good observational data that's consistent and robust to suggest that a low sodium diet would be beneficial. I think that you would agree that the available data does not meet that test, and so I think so far we're not ready for that randomized clinical trial, which would be difficult and expensive.

Dr. Logan:   The weight of the evidence suggests that there is a very modest effect of salt restriction on blood pressure. There is one pivotal study that most of the recommendations have stemmed from and that is the DASH-Sodium study. When we looked at the data, if you have individuals on a high quality diet the evidence of a reduction in blood pressure with salt restriction to current recommended levels is modest and similar to what we noted in our meta-analysis. If on the other hand you restrict your analysis to the individuals consuming a low quality diet, then there is a dramatic reduction in blood pressure and this effect can be mitigated, principally by changing the quality from a low to a high quality diet.

Dr. Logan:   The background diet of the reported trials was poorly documented, so therefore we don't know exactly what individuals were consuming when they went on a salt restricted diet. There is compelling evidence; however, from the DASH-Sodium study and from other trials to show that a low sodium diet will increase your salt sensitivity and a high quality diet will reduce salt sensitivity, and indeed on a very high potassium diet you can almost abolish completely salt sensitivity.

Dr. Logan:   We found that the effects of dietary salt restriction in the short-term studies, those under 30 days, were greater than the effects of salt restriction in long-term studies.

Dr. Logan:   When making health policy, one doesn't look at single variable changes, but rather, at the totality of effect and that is what's missing at the present time in the salt literature and the policy recommendations that have flowed from that. There has to be health outcome studies to know what the net effect of dietary salt restriction will be before we can make it public health policy.

Dr. Logan:   We have taken a more conservative approach with regard to recommending dietary salt restriction as a Public Policy Statement. We are convinced that in individuals who are salt sensitive should have their salt restricted in order to lower blood pressure in the hope that this will improve their survival and quality of life. We do not recommend in Canada across the board reduction in dietary salt restriction.

Dr. Oparil:   Yes, I think we are learning that. I think we're a little bit schizophrenic in the levels of evidence that we insist on with respect to drugs or pharmacologic treatment of disease. We're very strict and if there's any problem with a drug, it gets dropped very quickly. On the other hand, lifestyle modifications are regarded as universally beneficial to various extents, and very few people think about possible adverse effects of changes in diet, changes in activity. I think Dr. Alderman has opened everyone's eyes and hopefully other people will take this seriously and extend this inquiry further.

Dr. Oparil:   That is an excellent question and actually we have done some work with that. We found that if you feed a hypertensive rat a very high salt diet, the blood pressure goes up dramatically, particularly in male animals. However, if you accompany the high salt with a very high calcium diet, there is a null effect. Also, in other strains of rat, there is no effect of the high salt diet whatsoever. The kidney can handle a huge amount of salt, suggesting a number of things. Number one, the response to salt is substantially genetically determined. Number 2, the response to salt is, in a major way, influenced by the diet. We studied calcium. The same thing applies to potassium and perhaps magnesium.

Dr. Oparil:   Well I think we are falling behind on rigorous assessment of what you can achieve with lifestyle modification, particularly in this case, salt reduction. We had used blood pressure as a surrogate measure of cardiovascular health for a long time and showed benefit from antihypertensive drugs for a long time, but more recently, recommendations such as the JNC 7, which is a really an arm of the National Heart, Lung, and Blood Institute, have recommended for antihypertensive treatment, or at least for initial antihypertensive treatment, only classes of drugs that have shown benefit with respect to cardiovascular events and mortality, and so several classes have been left out. I think the pharmacologic area is ahead of the nonpharmacologic area and we need more research.

Dr. Oparil:   The HRT area is a very complex area also. Initially, based on animal studies and based on limited human studies and in vitro evidence from cells, we recommended very strongly that hormones, in particular estrogen, should be used for cardiovascular disease protection. Now, based on randomized control trials in older women, we say that hormones should definitely not be used for cardiovascular disease protection because they may do harm. This is a little bit similar to the salt controversy and I think the questions are still open.

Dr. McCarron:  That's obviously a critical issue for a country facing escalating health care costs. A little over a year ago a colleague and I, Dr. Robert Heaney from Nebraska at Creighton University, published a paper in the American Journal of Hypertension that looked at this issue using results of randomized trials largely supported by the US government.It had health care cost figures for various conditions and it went beyond the cardiovascular disease issues we’ve talked about and included cancer mortality, issues of weight control, outcomes of pregnancy, and kidney stones. If you just look at the cardiovascular benefits based upon the government's data, you could project that some place in the neighborhood of 14-18 billion dollars would be saved the first year that the DASH diet as introduced into just 30% of the US adult population. That is a substantial savings.

Dr. McCarron:  Yes. That's a good question because salt sensitivity has been one of those definitions that has sort of wandered across the medical literature and it is very difficult to nail down who is salt sensitive and who is not. There is not a test that you can do like you can for other medical conditions, and I think we now understand with all the information we have from the DASH trials and the work that went into developing the DASH studies over the past 20 years that if salt sensitivity is really a surrogate marker for a poor quality diet, well then, depending on what an individual is eating in any given period of time they may be salt sensitive or eating a lousy diet, and you could take those same individuals back in a month later, a couple of weeks later, and study them for salt sensitivity. And if they had for some reason over the last couple of days substantially improved the quality of their diet, they wouldn't be salt sensitive and that is in fact what we see in the medical literature It has been very difficult to pin down who is salt sensitive and who is not. We have some general categories, African American, older, overweight but many of these people are the same ones who have a poor quality diet.

Dr. McCarron:  That raises another, I think, valuable question or observation. As we've heard, we've tended to have different standards for drug therapy in this country than we have for lifestyle therapies in terms of blood pressure regulation. Doctors no longer take one medication and prescribe it to everybody with hypertension. Our guidelines tell you not to do that; they say if you have a certain population or individual, you may consider this drug category or that drug category. What turns out the same is that this is probably true for dietary recommendations. This is a newer idea, in fact I think it's fair to say it was introduced in the paper just published in the American Journal of Hypertension. We looked at the third and fourth NHANES, National Health and Nutrition Examination Surveys, done in the late ‘80s and ‘90s and then again in the late ‘90s and early 2000. You could literally separate people into systolic hypertension, diastolic hypertension, or some mixture of systolic and diastolic, based upon their dietary pattern and consistent with the finding from the original DASH Study and the second DASH-Sodium study. It turns out, from the government database, systolic hypertension in the population is associated with a mineral poor or poor quality diet, and in fact those individuals actually were eating lower salt rather than normal or higher salt intake. So one could argue the best thing to do for people, individuals who have systolic hypertension, which is the bulk of the population and is where the greatest risk is, is to really focus on improving their quality of diet. That paper pointed out that if you have diastolic hypertension you tend to be overweight, and it's really an issue of calories and probably exercise and possibly alcohol intake.

Dr. McCarron:  Again a valuable question to raise, you know, is salt sensitivity really something that is imbedded in us or is it really a manifestation of environment, and I would say specifically our nutritional environment, that an individual or the population lives in. It's my view now that salt sensitivity is not an immutable trait that's found in certain human beings, but is really a reflection of the nutritional milieu that each of us and then collectively as a country we live in and we tend to have poorer quality diets than we did years ago. That means there are not enough minerals in a diet: calcium, potassium, magnesium probably even manganese and iron, and probably a little too much fat and not enough fiber. If we focus on getting those elements back into the diet, salt sensitivity will essentially disappear from the map.

Dr. McCarron:  The new US Dietary Guidelines issued in 2005, in terms of sodium recommendations, suggested that the entire US population should now move down the 2300 mg/day, even if your blood pressure's normal, even if you have no health problems. It further recommended that those individuals who have high blood pressure or who are at risk of having high blood pressure, which is a vague term, should consume a diet of 1500 mg/day. To reflect on that based upon the presentations today, we have no evidence, no evidence that a 1500 mg reduction in sodium intake will improve cardiovascular events in people with high blood pressure or who are at risk of having high blood pressure. And further, we have no evidence that for the entire population 2300 mg is in fact safe and efficacious in terms of doing what we expect it to do, which would be to reduce cardiovascular events. That data does not exist, as you've seen today. So I think the more important thing for all of us to focus on is another recommendation in the Dietary Guidelines, which is the DASH Diet. The DASH Diet is mentioned numerous times in the 2005 Guidelines. It's there for the promotion of bone health, it's there for the promotion of blood pressure improvements, it's there for the promotion of cancer reduction, in the area of say colon cancer, and it's emerging in general as a possible diet that will improve weight control in this country. And from what you've heard today it would be far better to tell individuals who are interested in improving their blood pressure control to focus on fruits and vegetables and low fat dairy foods than it would ever be to focus on sodium restriction.

Dr. McCarron:  Well that's one of those questions that we would hope the answer was yes, because that's what we've been telling people to do for actually quite a while, several decades. Unfortunately, there is no science to support the notion or the concept or the idea that eating foods that are lower in their sodium content actually results in individuals consuming, overall, a lower sodium diet.