Home > modeling, statistics > Salt it up, baby!

Salt it up, baby!

May 15, 2013

An article in yesterday’s Science Times explained that limiting the salt in your diet doesn’t actually improve health, and could in fact be bad for you. That’s a huge turn-around for a public health rule that has run very deep.

How can this kind of thing happen?

Well, first of all epidemiologists use crazy models to make predictions on things, and in this case what happened was they saw a correlation between high blood pressure and high salt intake, and they saw a separate correlation between high blood pressure and death, and so they linked the two.

Trouble is, while very low salt intake might lower blood pressure a little bit, it also for what ever reason makes people die a wee bit more often.

As this Scientific American article explains, that “little bit” is actually really small:

Over the long-term, low-salt diets, compared to normal diets, decreased systolic blood pressure (the top number in the blood pressure ratio) in healthy people by 1.1 millimeters of mercury (mmHg) and diastolic blood pressure (the bottom number) by 0.6 mmHg. That is like going from 120/80 to 119/79. The review concluded that “intensive interventions, unsuited to primary care or population prevention programs, provide only minimal reductions in blood pressure during long-term trials.” A 2003 Cochrane review of 57 shorter-term trials similarly concluded that “there is little evidence for long-term benefit from reducing salt intake.”

Moreover, some people react to changing their salt intake with higher, and some with lower blood pressure. Turns out it’s complicated.

I’m a skeptic, especially when it comes to epidemiology. None of this surprises me, and I don’t think it’s the last bombshell we’ll be hearing. But this meta-analysis also might have flaws, so hold your breath for the next pronouncement.

One last thing – they keep saying that it’s too expensive to do this kind of study right, but I’m thinking that by now they might realize the real cost of not doing it right is a loss of the public’s trust in medical research.

Categories: modeling, statistics
  1. nikos2evangelos
    May 15, 2013 at 7:41 am

    Here’s the only finding that matters: Beer doesn’t kill you.

    • May 15, 2013 at 9:46 am

      Amen! As Ben Franklin said, it’s proof that God loves us and wants us to be happy.

  2. May 15, 2013 at 7:52 am

    One last thing – they keep saying that it’s too expensive to do this kind of study right, but I’m thinking that by now they might realize the real cost of not doing it right is a loss of the public’s trust in medical research.

    Well, epidemiologists have to eat too, and no researcher forgets J.B. Conant’s famous maxim (really warning) that one must “publish or perish”.

    Nevertheless, you’re quite right. The yo-yo of biomedical research has gotten ridiculous. IN fact the salt story has been whipsawing the public for decades now. And the research on dietary fat is at least as bad: Science and Nova both did excellent take-downs of the scientific conventional wisdom, pointing out how weak the evidence is for low fat diets and the uncomfortable fact that many humans appear to actually have a “high-fat” metabolism that is perfectly normal.

    Whence all this nonsense? As I recall from the Science article on fat, the story begins with too much enthusiasm for public health legislation in the ’60s and ’70s, fanned by scientifically ignorant do-gooders who were fueled by scientists a bit too eager for the limelight and too arrogant to heed good practices, a public so desperate for eternal youth to be very gullible, and a media too ignorant and too profit-hungry to either care about the truth or detect scientific bullshit. In short, an intellectual cesspool that met the sodium cake of greed and vanity and created … well, you get the picture!

    • Higby
      May 15, 2013 at 9:38 am

      Excellent story, excellent comment — “whence all this nonsense?” — I was asking the same question.

      My other question is, what are the health effects of literally pouring salt all over everything at every meal? Do effects change over the life-span? What about effects on people that have trouble controlling their blood-pressure?

      • May 15, 2013 at 9:54 am

        When you get to large bulk quantities, I suspect you start causing problems of a different sort. You can overdo just about anything, even drinking water. (Yes, people have died from drinking too much water.)

        The real problem is that metabolic processes are collections of complex feed-back loops. The translation of genes into proteins and the protein functions themselves, are all regulated by complicated chemical processes that are interdependent. And being chemical, we can only really understand what we can detect; so, what we don’t know is usually sufficient to kill any theory.

  3. TDHawkes
    May 15, 2013 at 8:24 am

    The problem with many of these kinds of studies is simply that scientists are looking for the purple squirrel of variables that will explain everything related to their question. In the case of blood pressure, if salt is the purple squirrel variable affecting blood pressure, controlling it will control the blood pressure, but we know nothing in biology is like that.

  4. Ion
    May 15, 2013 at 8:45 am

    I am blown away by preventative double mastectomies. Talk about faith in epidemiology. Did we learn nothing from the appendectomy scourge? We don’t even remove tonsils anymore, and movie stars are having their breasts cut out

  5. May 15, 2013 at 9:43 am

    Judging from the tone of the comments, maybe I should rush out and buy some fast food stocks. How long before we learn about the benefits of more sugar?

    • May 15, 2013 at 9:50 am

      Those of us old enough to remeber Woody Allen’s movie Sleeper have wondered this for years:

    • mathematrucker
      May 15, 2013 at 10:02 am

      or petrochemicals, or sugar mixed with petrochemicals?

      “The ocean will take care of this on its own if it was left alone and left out there. It’s natural. It’s as natural as the ocean water is.” -Rush Limbaugh, May 3, 2010

  6. George Cooley
    May 15, 2013 at 11:20 am

    My understanding is that this study was limited to the effect of lowering daily sodium intake from 2,300 mg to 1,500 mg. The benefits of keeping your sodium low at 2,300 mg have still not been questioned (average daily comsumption is 3,400 mg).

    • pjm
      May 15, 2013 at 10:06 pm

      People on low carb diets typically have to increase their sodium levels in order not to feel crappy. Guess what the blood pressure improves substantially and rapidly despite this.

  7. rcyran
    May 15, 2013 at 12:10 pm

    One problem is that some people with hypertension are salt sensitive, some are not. And dose usually matters in medicine.

    But it’s an easier message to say “salt is bad for your heart” than “more than 2,500mg of salt daily is bad for your heart” or god forbid “more than 2,500mg of salt is bad for the heart of some people”. Anyway, since compliance is a big problem in healthcare, health authorities often opt for the easier to remember message. So you get “DON’T EAT SALT”

    Simplistic, and as we just found out, probably wrong.

    Anyway, this result makes everything more interesting – still seems pretty clear that salt matters for some people. But this finding makes it a lot harder to get the message across.

    • FogOfWar
      May 15, 2013 at 5:23 pm

      Agree–from what I understand salt is completely healthy for the vast majority of the population and for most people cutting back on salt has no beneficial health impacts at all. Plus, salt makes your food taste better and there ain’t nothing wrong with that!

      This is one of the area’s where Bloomberg (the mayor, not the media company) crossed the line for me. There were a bunch of “cut the salt” ads on the subway, but it’s just not medically or statistically supported.

      -Fow

  8. pjm
    May 15, 2013 at 10:04 pm

    Gary Taubes has been writing about the poor quality of evidence on both salt and dietary fat for 15 years. He is worth reading. One of his collaborators Peter Attia MD blogs prolifically on these topics (as do many others, Chris Masterjohn, Denise Minger). Both Gary and Peter have training as engineers and know enough about stats to be able to asses weak evidence (e.g. observational studies). More to the point, it is not just that there is weak evidence behind medical conventional wisdom, there exist fairly robust alternative hypothesis that have effectively been shut out.

  9. Pav
    May 16, 2013 at 2:01 am

    Worth reading Dr Mercola’s 2011 article on debunking the salt myth: http://articles.mercola.com/sites/articles/archive/2011/09/20/salt-myth.aspx

  10. Higby
    May 16, 2013 at 11:22 am

    Thinking more about this — where are the studies tracking INCREASED salt intake?

    Without these studies, the restriction studies only tell half the story.

  11. Savanarola
    May 16, 2013 at 2:52 pm

    Frankly, the problem seems to me to lie in the bell curves used to measure whether something is abnormal. On any given bell curve with a normal distribution, 80% of those you look at will have a normal blood pressure under that curve. The other 20% have a “normal” for them blood pressure that is not under the bell. Those people are STILL normal – and they are statistically significant. But when you get lab results back, often the flag numbers that fall outside of the 80% part of the bell. That number might be off for you – or not. It’s off for 80% of the population, in essence, which might or might not include you.

    In the medical profession in general, I see way too much slavish adherence to a number on a sheet of paper and very little to listening to the patient and looking at symptoms. I find that these niceties about how a statistical norm is built seem to be lost on most doctors I’ve ever talked to. I find myself saying things like “Yes, it’s low. And so is my mother’s. And so was her mother’s. If you’d like – you can ask her. She’s 98 and she’s picking me up from this appointment on her motorcycle.”

    You get weird results from that. Some people need a lot of salt to keep going. There are recognized differences in synthesis of aldosterone, for example. We just really don’t know why some people crave salt, some have salt sensitive hypertension and some don’t, etc. And yet they make a blanket pronouncement. Goofy.

  12. mathematrucker
    May 17, 2013 at 12:27 am

    Dr. Fereydoon Batmanghelidj (http://en.wikipedia.org/wiki/Fereydoon_Batmanghelidj) firmly believed that dehydration is the root of all medical illness and hence water, taken with relatively large amounts of salt, can cure just about anything under the sun.

    Several years ago I was approached by a fanatical follower of his who had devoted himself to preaching the gospel of water and salt. He said the regimen saved him from some life-threatening health problems he had in his fifties. He did seem unusually vibrant and energetic for a sixty-something, and he definitely wasn’t a dumb person, so I couldn’t help but wonder if maybe he was on to something.

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