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Should you abandon that low-salt diet? (uh oh, it’s the Lancet!)

Bos_gaurus

Russ Lyons sends along this news article by Ian Johnston, who writes:

The prestigious medical journal The Lancet has been attacked for publishing an academic paper that claimed eating too little salt could increase the chance of dying from a heart attack or stroke.

Johnston summarizes the study:

Researchers from the Population Health Research Institute in Canada, studied more than 130,000 people from 49 different countries on six continents and concluded people should consume salt “in moderation”, rather than trying to reduce it in accordance with government guidelines across the world. . . .

The paper compared the health of people who tests showed had consumed low levels of sodium (up to three grams a day), medium amounts (four or five grams) and high levels (seven grams or more).

“Those participants with four to five grams of sodium excretion had the lowest risk” of death or suffering a “major cardiovascular disease event”, the researchers reported.

Among people who had high blood pressure, eating high and low levels of salt “were both associated with increased risk”. And for people without high blood pressure, consuming less than three grams a day was “associated with a significantly increased risk” – 11 per cent – of death or a serious cardiovascular event.

But there are critics:

Professor Francesco Cappuccio, head of the World Health Organization’s Collaborating Centre for Nutrition, attacked both the methods used in the study and the journal for agreeing to publish it.

“It is with disbelief that we should read such bad science published in The Lancet,” he said.

Professor Cappuccio said the article contained “a re-publication of data” used in another paper.

“The flaws that were extensively noted in their previous accounts are maintained and criticisms ignored,” he said.

The measurement of salt intake used by the researchers, he said, was “flawed” because it was done by testing urine samples given in the morning and then “extrapolated to 24-hour excretion” using an “inadequate” equation.

Professor Cappuccio also said the participants were “almost exclusively from clinical trials of sick people that have a very high risk of dying and are taking several medications”.

Now I don’t know what to think. I really don’t. I haven’t looked at the paper or the criticisms. If the study really is so flawed, though, I can’t say I’m surprised that it was published in the Lancet, a journal that’s famous for producing headline-grabbing papers that are later refuted, such as that Iraq survey, or that article claiming that gun laws could reduce firearm deaths by 200% or whatever, or, most notoriously, that paper by Andrew Wakefield [no link needed]. The Lancet may well publish some high-quality work but they do seem to have a weakness for bold claims and publicity.

I suppose the Lancet will publish a letter by Cappuccio or someone else with the criticisms? Perhaps a reader can keep us up to date here.

P.S. I’m sure I eat too much salt for my own good. I have a big jar of pretzels just sitting here in my office!

24 Comments

  1. Keith O'Rourke says:

    What study are they talking about – after 5 minutes I found this http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(16)30467-6/fulltext

    In the link you give they apparently did not think it important to link to the original paper – just the name of one of the co-authors?

    I once gave advice on an early meta-analysis of salt reductions studies – maybe because I was polite given I knew some of them – they did not appreciate my advice to totally re-think their analysis as it made no sense. They published without any changes.

    From Lancet editorial “Although not from an RCT, these data are as robust as the data used to advocate reductions to low levels.”

    So likely – evidence of unknown but suspect quality versus evidence of unknown but suspect quality.

  2. Anoneuoid says:

    So much medical research takes this form: “Exposure to ubiquitous thing E is correlated with disease D”.

    People are not going to look back kindly on these years. “During the last half of the 20th and first half of the 21st century, access to cheap energy was largely squandered on wars and pseudoscientific activities. For example, a common practice was checking whether everything is still correlated with everything else, and then “spinning” whatever correlation was found into a narrative that supported whatever idea was most favored by the funding organization or was best for the researchers career.”

  3. Jack PQ says:

    The other obvious problem is endogeneity and omitted variable bias. This is not a case of control groups and treatment group. Something else could easily be driving the result.

  4. Jeff Walker says:

    In my limited reading of the epidemiology literature, it would seem that the vast majority of researchers (maybe not trained epidemiologists) believe that omitted variable bias in observational studies of causal effects (not prediction) is decreased (or that the coefficients approach the true value) by 1) including more covariates and 2) larger sample size. My little simulation of omitted variable bias showed that these beliefs are highly optimistic. https://scholar.google.com/scholar?cluster=1810947541250401712&hl=en&as_sdt=0,20&as_ylo=2012

  5. psyoskeptic says:

    It has to be the case that there are salt levels so low it causes harm. Those would necessarily be in with the < 3g group.

    • Yes, this is absolutely true, and is also associated with for example Water Intoxication https://en.wikipedia.org/wiki/Water_intoxication a condition that occurs typically when people have water drinking contests (a stupid idea that is naively assumed should be safe. There was an instance about 10 years back with a “hold your wee for a wii” contest out of a radio station in Sacramento, https://en.wikipedia.org/wiki/KDND#.22Hold_Your_Wee_for_a_Wii.22_contest ).

      • AP says:

        Water intoxication is a different problem though. It’s like blaming poverty on an excess amount of expensive things in stores. It occurs most often during endurance events, because people hydrate according to a theory and ignore their sense of thirst.

        • My understanding is that water intoxication is largely due to lack of electrolyte balance, so if you drank saline IV fluid all day long you’d be uncomfortable but at least FAR less at risk. So, it’s really not that “there are salt levels so low it causes harm” or “there are water levels so high they cause harm” but rather there are values of the dimensionless ratio R_s/R_w which cause harm, where R_s is rate of intake of salts and R_w is rate of intake of water.

          Since you can alter this ratio by changing either R_s or R_w it’s not really a different problem.

          • Keith O'Rourke says:

            The toxicologists I work with say the problem is fluid retention and something like drowning (water gets in, has to go somewhere and can’t get out).

            (Its an interesting case for them as it proves the “poison is in the dose”)

            • In the case where you’re in a contest to see who can drink water without urinating … all bets are off, conservation of mass obviously applies. But in the case where you do urinate freely, I think the difficulty is dealing with osmotic imbalances that force water into places it wouldn’t normally be in such high volumes. So for example you get swelling of the brain, potentially very high blood pressure, etc. In the presence of the appropriate salt concentration, the osmotic pressure imbalance isn’t so severe, so the fluid retention issue isn’t as big, and you just urinate it out before it can make your brain swell or the like. There’s also things like conductivity, I imagine it’s not good for the electrical activity of your heart to have too little salt concentration…

              anyway, based on general physical principles it’s probably two dimensionless ratios that control water intoxication, R_s/R_w as described before, which describes an intake imbalance and hence relates to a rate at which you’re driving your body towards an excess osmotic gradient, and M_w/M_b which is the mass of water as a fraction of the total body mass, which describes the time-integrated effect of the intake imbalance. It’s this second part which is accurately described as a “different problem” which truly is about having too much water (and applies significantly when you don’t urinate freely).

              • Corey says:

                Kidneys max out at about 1 L/hr.

              • That makes sense. So the quantity M_w/M_b is a function of (R_w-R_k)T/M_b where T is a duration over which you are intentionally drinking water. And R_k is related to R_s/R_w, where it’s easier for your kidneys to eliminate water when it’s isotonic with the body.

              • AP says:

                I think you are underestimating regulatory mechanisms in the body. One can not change the concentration of sodium in the blood by reducing sodium intake in a range from very low to very high (200mg/day to 7g/day). Salt intake is measured by 24h urine excretion. Blood concentration stays remarkably constant within those limits. I see the intake rate in your equations but not the excretion rate. I may be missing something. If you are assuming it constant, that’s a mistake. The change in sweat concentration is absolutely remarkable, at least it was for me. When sweating profusely, it’s the difference between swimming in a lake or in the ocean. For urine concentration, I defer to the literature.

              • AP: no doubt regulatory mechanisms are important, my main point is that altering the sodium concentration of fluid you drink alters the rate at which that water is forced into your cells by osmotic pressure (as anyone who has accidentally done a sinus wash while leaving the salt packet out knows… it’s quite painful to have distilled water forced into your capillaries by osmosis!)

                So, it’s not so much that you alter how much sodium there is in your body as the sodium in your fluid intake alters how dramatically the *water* is forced into your organs vs excreted in urine.

                Suffice it to say that I’m supposing drinking large quantities of water is less dangerous (still not risk free) if it’s IV fluid at isotonic salinity than if it’s distilled water, or sea water.

              • Martha (Smith) says:

                An additional complication: My understanding is that if the heart is not pumping up to a normal degree (i.e., heart failure — which is often not diagnosed until the symptoms are severe), then fluid accumulates in the lungs, lower limbs, perhaps other places — and is not adequately excreted by the kidneys if the heart is not pumping the fluids at an adequate rate.

    • It’s also got to be the case that there are salt intakes so high that they cause acute immediate harm. Like drinking a gallon of sea water for example. However, considering that people typically don’t TRY to eat a lot of salt, it may be less of an issue than the low consumption case where there ARE people trying to reduce salt intake.

      • Martha (Smith) says:

        “considering that people typically don’t TRY to eat a lot of salt”

        When I was a kid, it was fairly common to believe that you should consume a lot of salt when it was hot and you were sweating a lot — people would take salt pills, or just pour salt from a box into the palm of their hand, pop it into their mouth, and gulp it down.

        • AP says:

          Australian troops were issued salt tablets for missions in tropical environments. Not anymore. Athletes that are going to compete in a hot environments are sometimes put on a low sodium diet. The idea is if we get little of the thing, you also sweat out very little. The balance is the same, but you don’t need to worry about how much salt to take and avoid the nasty side effects (nausea and worse). When I was a kid, I was prevented from drinking for the duration of a training session (indoors, summer, no AC, brutal heat) because having a full stomach interfered with respiration. I am clearly dating myself to the bronze age here …

    • AP says:

      Extremely low sodium intake is recommended in heart failure and Meniere Syndrome. Sure it’s a cost benefit calculation but we are not dropping dead quite yet (I am on such a diet). There is a book, Salt matters — the killer nutrient, where the author makes an imperfect but reasonable case for extremely low intake, which motivates the Salt Skip program, an Australian health program aiming to eradicate high blood pressure. For instance, added salt was unavailable until somewhat recently, evolutionarily speaking. Several peoples still live on an extremely low sodium diet and they seem to be doing just fine. It looks like our body just stops excreting it through urination and sweat. Infant food can’t contain added salt by law in the US. Observational studies linking low sodium intake to increased mortality are suggestive but not definitive, because a major reason for the low sodium intake may be an ongoing disease, or a known predisposition for a disease, or a constrained diet for some other reason. Studies try to correct for this but reading the fine print shows assumptions and approximations, as detailed in other comments. Correlation is not causation. Extremes in everything seem more prone to counterintuitive effects. Remember the negative effects of non cigarette smoking, or of long distance running? Controlled studies exist but they use surrogate outcomes, like blood pressure. It’s hard to put people on high and low salt diets for 10 years to see if they die, both practically and ethically. Hyponatremia, the acute lack of sodium, can not be triggered by dietary changes alone. You need renal problems, use of diuretic drugs etc. I looked really hard for other effects of low sodium intake and I found one article, but the conflict of interest section listed a position in some national salt industry association!

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