Another week, another roiling debate about nutrition. In the immortal words of Iago the parrot, I'm going to have a heart attack and die from that surprise.
Actually, heart attacks are directly germane to this topic; strokes even more so. The particular goal of guidelines addressing salt (or sodium) intake is to prevent ambient high blood pressure, a major contributor to cardiovascular disease and the leading cause of stroke. There are numerous other health effects of sodium intake as well, including an influence on bone density, but blood pressure tends to grab the spotlight.
And spotlight it is at the moment. Recent studies have reached almost shockingly divergent conclusions about the pros and cons of sodium restriction. Compounding matters, the studies in question appeared in the very same issue of the New England Journal of Medicine, published on August 14.
Two articles, by the same large, international group of researchers called the "PURE investigators," standing for "Prospective Urban Rural Epidemiology" study, challenged the current emphasis on restricting sodium. Or at least, that's what the related headlines say. One of these studies looked at variation in sodium excretion in urine and its association with blood pressure; the other looked at the same measure and its association with all-cause mortality and cardiovascular disease.
For both of these studies, the authors used a database of morning urine specimens from over 100,000 people in 18 countries to estimate 24-hour sodium and potassium excretion, and from those estimated values, to extrapolate daily intake of sodium and potassium. We may leave the methods at that, other than noting that as estimates are predicated on estimates, the error bars get pretty wide, pretty fast.
As noted, the inevitably hyperbolic headlines attached to these studies suggest they found that we should abandon salt restriction, and pour it on. But here are what the authors concluded in their own words. In the first of the studies, they stated:
In this study, the association of estimated intake of sodium and potassium, as determined from measurements of excretion of these cations, with blood pressure was nonlinear and was most pronounced in persons consuming high-sodium diets, persons with hypertension, and older persons.
If you think that's a long way from "pour it on," well -- I agree. Essentially, the researchers found that excess sodium was most likely to raise blood pressure in older people, and those already prone to high blood pressure. And, high sodium intake was most important when sodium intake was... high. Well, alrighty then.
Moving on. The second study concluded with this:
In this study in which sodium intake was estimated on the basis of measured urinary excretion, an estimated sodium intake between 3 grams per day and 6 grams per day was associated with a lower risk of death and cardiovascular events than was either a higher or lower estimated level of intake.
Superficially, that translates to: we can eat too much salt, and we can eat too little. That we have long known, since sodium is an essential nutrient. Too little can result in a life-threatening condition called hyponatremia. The study may have raised questions about how much is too much, since the 3-gram threshold is higher than current recommendations, although not higher than prevailing intake. But we have to be careful not to over-interpret that isolated finding. What does it mean if your intake of sodium is lower than average for the population of which you are a member? It means you are different. That might be good, but it could readily be bad. Being "different" might mean not fitting in with prevailing norms for any number of reasons, from poor health to social isolation. A lower daily intake of salt could result merely from a lower daily intake of food. Where any of these factors is operative, they might account for variations in both blood pressure, and mortality, quite independently of sodium.
Wherever these first two studies left us, we couldn't stay there long, because the third study followed immediately after to shake things up some more. This one, by a different group of investigators, obtained data about sodium intake and cardiovascular death for over 70 percent of the global population of adults. What's good for the goose is good for the gander, so here is what these researchers concluded:
In this modeling study, 1.65 million deaths from cardiovascular causes that occurred in 2010 were attributed to sodium consumption above a reference level of 2.0 g per day.
They went on to note that excess sodium intake was responsible for one in ten of all deaths from cardiovascular disease around the globe. Associated headlines either indicated that our salt intake is, indeed, too high; or more bluntly, that too much salt is killing us.
A pretty confusing batch of papers to say the least, and that, too, has made headlines.
Here's where I think it all shakes out.
There is no doubt it's possible to consume too little sodium, and there is no doubt it's possible to consume too much.
Not everyone is equally sensitive to sodium excess, and in general, it matters more as we age, and to those of us prone to high blood pressure.
A lower intake of sodium than prevails in a given population might indicate other important differences in behavior, health, or social integration. The current studies account for these imperfectly.
The studies purportedly raising questions about the importance of sodium restriction are actually only challenging the optimal threshold, suggesting it should perhaps be 3 grams daily rather than the current World Health Organization recommendation of 2 grams daily.
Missing from all headlines is this important tidbit: more than twice as many adults have a sodium intake above 6 grams daily as have an intake below 3 grams daily; and nearly seven times as many have an intake above 4 grams, as have an intake below 3 grams.
This, in my view, leads to key point one: it is theoretically possible to consume too little sodium, but whether the relevant cut-point is set high or low, the vast majority of adults living in the real world consume too much. All three studies actually agreed on this point.
So, yes, I presume if you fill a house with water, it might cause drowning. But I'm not sure that theoretical concern is of great practical value when putting out a fire.
The second key point, certainly for those of us in the U.S., is this: more than 75% of the sodium we consume comes from processed foods. This figures in the manipulation of recipes to maximize our calorie intake.
The implications are rather clear. Any shift from a diet of more to less processed foods will result inevitably in a decrease in sodium intake. That shift is advisable because of the decisive health benefits associated with it, and regardless of the specific contributions of sodium reduction to that benefit. A typical American diet tends to be too high in sodium whether the higher or lower cut-point is invoked. But its more important liability is likely the fact that it is... a typical American diet, in which a third or more of calories routinely come from "junk." There is no debate about the value of eating food in place of junk.
As ever, competing headlines propagating confusion are partly a result of the legitimate nuances associated with the incremental advance of scientific understanding, and partly the machinations of media profiting from hyperbole and intrigue. But we can bypass the potential confusion altogether if we take it all with the proverbial grain of salt.
Whether the topic du jour is sodium, or fructose; wheat or meat; gluten or saturated fat- we are subject to the impasse of perpetual confusion if we fixate sequentially on each successive study of each particular nutrient. If instead we embrace what we reliably know about healthful eating in general, sodium intake will tend to fall in the sweet spot, along with the intake of all other nutrients. In other words, we could reliably defend ourselves against hyperbole and headlines, malnutrition and misinformation alike -- with wholesome foods, in sensible combinations.
Tune in next week when that news... will be exactly the same.
-fin
David L. Katz, MD, MPH, FACPM, FACP is the founding director of Yale University's Prevention Research Center, President of the American College of Lifestyle Medicine, and author of 'Disease Proof.' He has authored roughly 200 scientific papers and 15 books, including three editions of a nutrition textbook for health care professionals.