Interview with Scientific American on the complexities of salt science
I complained to Scientific American about one of its blog posts about salt, which I viewed as rather one-sided. The result was a conversation with Michael Moyer that ended up in the form of a Q and A. I did not have a chance to review it before it was posted, so please see addendum at the end).
The Salt Wars Rage On: A Chat with Nutrition Professor Marion Nestle
A researcher explains why there may never be a good study on whether excess dietary salt causes hypertension and heart disease
By Michael Moyer | Thursday, July 14, 2011
Is salt bad for us? In just the past few months researchers have published seemingly contradictory studies showing that excess sodium in the diet leads to heart disease, reduces your blood pressure, or has no effect at all. We called Scientific American advisory board member Marion Nestle, a professor of Nutrition, Food Studies, and Public Health at New York University and the author of Food Politics, to help parse the latest thinking regarding salt and heart health.leads to heart disease, reduces your blood pressure, or has no effect at all. We called Scientific American advisory board member Marion Nestle, a professor of Nutrition, Food Studies, and Public Health at New York University and the author of Food Politics, to help parse the latest thinking regarding salt and heart health.
[An edited transcript of the interview follows.]
I understand this area is controversial.
Hugely.
Could you take us through some of the controversy?
If you talk to any kidney specialist or anybody working on hypertension they will tell you that the first thing they do is try to lower the amount of salt their patients are eating because it helps with blood pressure control. But if you do a clinical trial where you try to put large amounts of people on a low-salt diet, you just don’t see much difference between the people who say they eat a lot of salt and the people who say they don’t eat a lot of salt. In clinical trials the relationship doesn’t show up.
Why not?
Two reasons: One that it’s impossible to put a population of people on a low-salt diet. Roughly 80 percent of the salt in the American food supply is in foods before people eat them—either in processed food or in restaurant food. Because so much salt is added to the food supply and because so many people eat out, it’s impossible to find a population of people who are eating a low-salt diet. They basically don’t exist.
In the one comparative epidemiological study they did some years ago—the Intersalt study—they managed to find two populations of people in remote areas of the jungle someplace who weren’t eating a lot of processed foods and who weren’t eating in restaurants. They were on a low-salt diet, and they never developed hypertension.
So in that trial did they put one group on a high-salt diet and put one on a low-salt diet?
No, no, no. It wasn’t a comparative trial. They just looked at the amount of salt that populations were eating and the amount of hypertension that they had. Only in these two populations were there very low rates of hypertension. With everybody else the salt intake was so high that they couldn’t see any difference between high and higher.
So except for people living in the jungle somewhere, there aren’t any populations on Earth that are eating a low amount of salt?
Not anymore. Maybe we used to be, but not anymore. We have a global food supply, so it’s impossible to do a really careful study.
What’s the other issue?
Not everybody responds to a low-salt diet. There’s a proportion of people in the population who are sensitive to salt—if you lower their intake of salt, then their blood pressure goes down. There’s another (probably larger) percentage of the population who doesn’t respond. They are people who can eat as much salt as they want and still their blood pressure is low.
So you have this curious anomaly where whenever you do a clinical trial you get these complicated, difficult-to-interpret results that don’t show much of an effect. But everybody who works with patients who have hypertension think they do better [on a low-salt diet]. And every committee, body, and group that has ever in my lifetime considered whether salt has anything to do with hypertension says, “yes,” and has recommended salt reduction as a public health measure. That’s the curious situation that we are in.
There’s one other wrinkle and has to do with people’s taste for salt. Campbell’s soup, for example, just announced yesterday that can’t sell low-sodium soups and so they’re adding salt back. And part of the reason they can’t sell it is that if you’re on a high-salt diet, food that isn’t salty tastes terrible to you. And if you’re on a low-salt diet it takes three to six weeks to get accustomed to being on a low-salt diet and then everything you eat tastes salty. And so the more salt in the food supply the more salt people need to bring the flavor you associate with salt. That complicates things, too.
So from a public health standpoint, if you want to deal with the percentage of the population that seems to be extremely responsive to a low-salt diet what you want to do is get the sodium level in the food supply as low as you can. And that makes the people who sell salty food go nuts. And it makes the people who like salty foods go nuts. They think the food tastes bland. And so there are different stakeholders in this system who have very different views and that accounts for the level of passion, I think, in a situation where the science is murky.
Couldn’t you just make the case that people should eat fewer processed foods?
Well what about restaurants? I’m a food professional. I eat out professionally.
Well chefs need to make their food taste good—otherwise people won’t go to their restaurant.
No, they need to make the food taste good by their standard. And chefs, because they’re dealing with a great deal of salt in their food, tend to raise the sodium level. It just goes up and up and up and up. As they get more used to a certain level of salt taste it no longer tastes salty to them and they have to raise it. So the pressure is to raise the salt in the food supply. And reducing it is very difficult.
So you advocate regulation to limit the amount of salt in restaurants?
Yeah, I do. Certainly for processed food. I think everybody would be healthier if they ate less salt. You can always add salt if you don’t think it’s salty enough, whereas I can’t take it away if it’s presented to me. And that’s the dilemma. And the ferocity of the arguments gets into the whole question of personal responsibility and “nanny state” and all of these other enormous debates that really don’t get at the public health question. And the public health question is hard to resolve because the science is really difficult to do.
Couldn’t you imagine a study where you look at sodium levels in urine, which is a direct measure of salt intake, and correlate that with hypertension?
Yeah they’ve done that, and they don’t see any difference in hypertension rates. The reason is that the baseline [level of salt intake] is so high that it doesn’t make any difference. To suggest that people get down to 1,500 mg a day—the recommended level—would be really really hard, and that level may be too high. And it’s unclear that that’s the right level because you can’t do a really decent dose response, and because people vary so much.
Will there ever be a good study?
I don’t know!
Is it possible that this represents the limits of science? It’s black hole event horizons and salt intake?
It may be. It very well may be. Or the science that we have is completely adequate and we already have the answer. I was once at a sodium meeting at which there were a bunch of statisticians. And I left with the statisticians and they said that “anyone who thinks that salt has anything to do with hypertension is delusional.” And that was on the basis on the clinical trials that show so little. And yet every single committee that has dealt with this question says, “We really need to lower the sodium in the food supply.” Now either every single committee that has ever dealt with this issue is delusional, which I find hard to believe—I mean they can’t all be making this up—there must be a clinical or rational basis for the unanimity of these decisions.
But that’s the thing—these committees should be able to point to the evidence that supports their recommendations. But they seem to rely so much on anecdote and individual experience.
Or on some clinical trials that everybody argues about. Everybody argues about every clinical trial no matter what the conclusion. So I find the whole thing completely fascinating. I don’t think anybody can underestimate the difficulty of doing nutritional research. Because people aren’t eating just sodium. They’re eating sodium in food. And it may be that high-sodium diets are a marker for some other things in the food supply or it may be that the physiological differences are so profound that you just don’t get clean results. That human variation is so great. I don’t know the answer to that. I just know it works for me. That’s anecdotal. With an “n” of one.
Addendum
If I had been given the opportunity to review this before it was posted, I would have edited it carefully. Yes, this is the way I talk but I don’t think what I said reads clearly in print. In reading over this piece, I think it may give the wrong impression of my views on how expert committees decide on salt recommendations. The piece may give the impression that committees make dietary recommendations basied on anecdotal evidence, not science. That’s not true. They base their recommendations on their interpretation of the experimental and clinical evidence, including that from clinical trials.
For example, the Advisory Committee for the 2010 Dietary Guidelines concluded that “a strong body of evidence has documented that in adults, as sodium intake decreases, so does blood pressure.” Was this committee delusional in viewing the evidence as strong? I don’t think so.
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Comments
I think your wonderful unedited responses most accurately reflect your true stance. (They say you should never go back and change your responses to a multiple choice quiz if you’re only guessing)
Sure, you’d like a choreographed “do-over” on your interview responses, just like you would on the preponderance of studies that fail to support your cherished anecdotal “salt is killing us all” opinion.
The statisticians are correct – opinionated academics tend to become “delusional”.
When you see a caustic, snarky response that belittles Dr. Nestle and/or NYU, and it’s posted by Suzanne or Suzanne_G, know it’s Doc Mudd playing games.
Sincerely,
Suzanne Garrett
I’m only one person, not a population, but I eat zero processed food and rarely eat out–mostly because I now find even chef-prepared fresh and local food too salty (just as you described). My blood pressure went down when I lost 45 lbs, but it’s gone down more since I reduced salt use at home. The same thing happened with sweetness many years ago when I quit drinking soda. I can’t stand any store bought beverages–even juice is just way too sweet. Water for me–and straight from the tap. It’s nice to know I have so much in common with the residents of the remotest jungles!
Note to Campbell Soup Co: I make great soup, a whole big pot with a tsp of salt–so my serving has less than 1/4 tsp. Your soup is just nasty and that is why you need all the salt to make it taste like anything at all.
Thanks for posting this, warts and all. I think it was a great discussion and goes a long way toward explaining the discrepancies in the scientific literature.
@Suzanne. Sorry you’ve been ID thefted by the troll, but thanks for letting us know.
I’ve noticed that my perception of salt changed once I stopped eating processed foods. I have always been a big fan of salt, but I’m far more aware of the presence and flavor of salt particularly in restaurant-produced items and seem to desire less overall.
Darya Pino, of the blog Summer Tomato did a very interesting presentation on “Salt – How Bad is it Really?”
You can view it here:
http://tinyurl.com/salt-summer-tomato
@Anthro, Thank you! This situation is a little embarrassing, but the thought that Dr. Nestle would connect me with the comments caused me to speak up. Am with you on the nasty that is Campbell’s!
Right Suzanne. Who ever started that childish imposter-poster prank nonsense, anyway? It has really loused up our delightful little echo chamber around here — corrodes our cult’s shiny faux talking points. Glad that dreadful Mudd character has finally been blocked, along with all the other insufferable non-believers!
Dr. Nestle would never suspect your intentions, Suzanne, what with your head so far up her colon – shoulders deep at all times! We all know you would never stray from the party line, would never think for yourself. God bless you and all you bring to our amazing forum!!
Excessive salt over the years has made me fat and boring. I blame the government.
Do not despair, girls. Just claim post-menopausal dementia when being confronted for your slatternly biased opinions and utter ignorance of science.
Works for me every time!
[...] Read interview by Michael Moyer from Scientific American here. Read addendum from Dr. Marion Nestle blog here [...]
Dr. Nestle,
What are your thoughts on Gary Taubes’s claim that hypertension is more so the result of metabolic syndrome, and not the result of excess salt consumption?
Marion,
The original Scientific American blog about ending the war on salt had a big mistake. They said that Dahl fed rats the human equivalent of 500 gm of salt/day. That is ridiculous, and wrong. I looked at Dahl’s paper, and he fed rats a diet with 8% salt that caused hypertension. The human equivalent would be, very roughly, 50 gm salt/day. That’s extremely salty, but an amount a human might possibly eat.
So the author misrepresented the research to make it sound ridiculous.
I read somewhere that people had fed rats a diet of 1% salt by weight and that also raised blood pressure, but I haven’t found the articles. Do you know of citations for such research?
I’m curious about the research in animals because of all the confusion and contradictions in epidemiological studies. And other animals besides rats, too There was a study on chimps that found that salt added to their diets – within the range of human diets according to the abstract – raised their blood pressure.
Warning! This comment may be offensive and too “earthy” for some readers, so if that is you, stop reading right now!
As a retired chef I am here to report a popular saying in the commercial kitchen: “when the soup is salty, you know the chef is horny”.
‘Nuff said.
I don’t know whether there’s a real controversy about salt – a solid scientific consensus – or just a controversy manufactured by the food industry and people’s liking for salt.
This editorial: http://www.newscientist.com/article/dn18835-theres-no-doubt-about-the-health-dangers-of-salt.html says it’s a only a manufactured controversy, a matter of the food industry opposing limits on salt that are important for public health.
Then I saw your interview and you say there’s a huge controversy and the science is murky.
And the FDA is apparently planning to set limits on the salt content of processed foods, reducing it slowly over the next decade. The scientists there don’t seem to think the science is weak.
I had hypertension. It went away when I stopped eating grains and sugars and lost around 80 lbs. My salt intake has not changed. In fact, since discovering the wonderful flavor of grey celtic sea salt, I probably eat more salt than ever! Taubes was right, at least in my case.
Linda,
Being overweight and metabolic syndrome are known to contribute to hypertension.
My husband is tall and thin. People are amazed that he is hypertensive, as it has nothing to do with metabolic syndrome. He is salt sensitive, to the tune of 20 points each, systolic and diastolic. Lowering the salt in restaurant food would give us a lot more options for eating out. His kind of hypertension may never show up for the statisticians in a large population study, but it certainly exists. We’ve experimented with it and learned what works for us. Thanks for engaging in the debate.
Recent research points out that the balance of Potassium and Sodium plays an important part in heart health. Link to an article in Archives of Internal Medicine:
Sodium and Potassium Intake and Mortality Among US Adults: Prospective Data From the Third National Health and Nutrition Examination Survey
http://archinte.ama-assn.org/cgi/content/short/171/13/1183
A discussion of the article also makes note of the ever present sodium in processed foods.:
http://podblog.blogs.hopkinsmedicine.org/2011/07/15/sodium-potassium-and-death/
This reminds me of when all cholesterol was bad, before we learned the roles of all types of cholesterol. It is important to understand all the synergies before blanket policies are made.
Marion, I sincerely wish you could do something about having our screen names hijacked by this worthless troll. He, of course, knows nothing of playing by the rules.
Is this why many blogs have the poster type in the “funny” words in the box?
———
Mudd, not only are you arrogant and annoying, but you are also a coward–not that morality concerns you. Man up and stop hiding behind the skirts of women who out-manned you long ago.
http://archinte.ama-assn.org/cgi/content/short/171/13/1183
That study also showed a drastic association of all-cause mortality (not just cardiovascular disease) with total sodium intake.
This study is certainly no argument against restricting sodium in processed foods. The FDA is apparently considering lowering the allowable sodium content in foods, slowly.
It seems like an excellent idea to me. People get used to heavily salted, and if the salt content is slowly decreased, they can enjoy food just as much. I don’t even see why the food industry would object.
The food industry loves adding salt because it’s a very cheap way to make tasty food. It probably covers up for a lot of problems like tasteless canned food. But if everyone by law has to decrease the amount of salt, low salt food isn’t a competitive disadvantage.
I read that 55% of white people with hypertension are salt sensitive, and 30% of the white people in general are salt sensitive. Black people are more salt sensitive than white people. I don’t know about other races.
I’ve read this elsewhere in the news – looks like there’s at least some mechanisms behind why only just certain group are more sensitive to salt x bp:
> http://www.jci.org/articles/view/43124
> JCI – Rac1 GTPase in rodent kidneys is essential for salt-sensitive hypertension…
There seems to be a kind of salt-sensitive hypertension that develops slowly, over decades, and is somewhat irreversible. See http://www.nature.com/ijo/journal/v32/n6s/full/ijo2008205a.html and http://www.med.mun.ca/getdoc/96bf37c1-1ab3-49d1-af24-3293264153e1/BVV_SaltloodPressure.aspx
Thus, if the number of people who are salt-sensitive is estimated by the number of people whose blood pressure goes down a certain amount on a low-salt diet, this might underestimate how many people are salt sensitive.
I’ve read that the rise in blood pressure with age is thought to be mostly due to salt. So if almost everyone’s blood pressure goes up with age, this may mean that almost everyone has this kind of long-term salt sensitivity.
An interesting study on rats who developed hypertension after being fed a high salt diet for about a year: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2136278/pdf/71.pdf
These were regular rats, not a special salt-sensitive strain.
Some of them were fed a diet that looks similar in saltiness to a human high-salt diet: 2.8% salt in their dry food.
Some of the rats were, rather horribly, fed diets of 8% and 9.8% salt, and the author describes how their kidneys were destroyed and they developed edema and died.
Does it really matter what percentage of people might be salt-sensitive? Whatever the number, it is a huge number of people and is, therefore, a public health issue. As Marion points out, it is easy enough to add salt to one’s food, but not the opposite.
No one is saying that salt should be banned, just lowered in processed and restaurant foods. It is pretty clear that the palate adjusts to less salt whether or not one is salt-sensitive.
Some people don’t think salt causes hypertension, or only in a small number of people. And that a high-salt diet is healthy if you don’t have hypertension.
I’ve been trying to find out if it’s a real scientific controversy or resistance that comes from the salt industry and from people’s liking for salt. So far it looks more like the latter – the pro-salt studies don’t look convincing and seem to be outweighed by a lot of animal studies and epidemiological studies that do show a salt-hypertension link.
[...] More at Food Politics [...]
The thing that bothers me about these studies/articles is that they never specify what kind of salt they are talking about. There is a big difference between iodized salt, msg and celtic sea salt (just to name a few). Iodized salt has been processed, bleached and many times has anti-caking agents added. MSG is also a processed salt and has many negative side affects. And celtic sea salt is pure salt that is unprocessed and full of vital minerals. For all we know, the people in these studies were eating MSG in all of their foods.
These scientists can’t be saying that each of these salts has the same affect on the body. Or are they? So, my question is: which salt are they asking us to eat less of?
Is it possible that anyone is going to address the fact that low-salt alternatives exist for those who want them? Just taking Campbell’s, they have a “Healthy Request” line. Marvelous. Just what the doctor ordered.
Why do I need the FDA to decide for me when I want to do business with Campbell’s and purchase their “normal” soups so my kids will eat vegetables and have an inexpensive lunch? They removed the salt in a stealth way and made the soup unpalatable. If I want low-salt soup for my children, I will by their excellent “Healthy Request.”
Why, exactly, would Dr. Nestle, and the FDA and most of you imply that somehow the salt needs to be “controlled”?
If you want to raise awareness for public health issues, keep beating the “maybe-salt” drum for hypertension. You might be very, very, right. But instead of bugging manufacturers, get the science figured out!! Isn’t that a legitimate function for government, rather than minding my soup needs?
Same thing for Chef Boyardee. The manufacturers are reading the signs and know the government is coming for them.