Iocaine powder is, as we know, the deadliest poison on earth. Odorless, tasteless, it dissolves instantly in liquid. Is salt the same? Not quite. The tale of dietary salt, however, has much in common with the Battle of Wits. It is a tale of death and destruction, dastardly deceptions—and a canny solution.
Public health authorities like the CDC insist “Most people eat too much salt,” meaning amounts that will cause strokes and heart attacks. This is based on data showing greater salt intake increases blood pressure (slightly), and lowering it reduces blood pressure (slightly). But what do the data say about strokes and heart attacks? The most recent Cochrane review, a trusted source, is a decade old and therefore misses some studies. But it concludes that older trials saw few if any cardiac outcomes, and therefore the data were too sparse to resolve the question.
They note one fascinating exception, however, a study of Taiwanese veterans in retirement homes, in which kitchen staff used a potassium-based salt substitute for half the residents, and seemed to reduce deaths significantly. In public health terms this is a bit of a cheat, since the veterans didn’t make or choose their own food, but it’s still interesting. (Wonder what the depression rate was in the low salt group).
Evidence before 2014 is therefore inconclusive. In 2019 a review examined a dozen new trials and with more than 7,000 participants found no impact on strokes or heart problems. But in 2022 an updated review reported on an incredible 32,000 participants, and a large effect on deaths and strokes. What changed? The China study.
Starting in 2014 a Chinese and Australian research group began randomizing rural villagers in China—nearly 21,000 across 600 villages—to either use a reduced-salt substitute for cooking and seasoning, or continue with regular salt. They followed the villagers for 5 years and in 2021 reported their findings: an average 3-point drop in blood pressure, along with 14% less strokes and 12% lower mortality. This, hallelujah, was what salt-reduction advocates had been saying all along.
Before we all dull our diets, however, it’s worth figuring out how this applies to everyone else. Notably, the only way to get into the China study was to be over 60 with high blood pressure, and nearly three quarters of participants had experienced a prior stroke. That’s critical, since the top risk factor for having a stroke is having had a stroke, making the study cohort a very high risk group. How high? Participants had strokes at a rate of 3% per year.
That is humongous. Which makes them perfect targets for preventing strokes. But compare that to an American gentleman of average build and blood pressure in his mid-50s. The American College of Cardiology suggests via an online calculator that my (ahem, his) risk is 7% over 10 years, or about 0.7% per year. But the ACC calculator is known to overestimate. Plus I fudged the cholesterol, putting in a doozy to be conservative (I don’t follow or care about cholesterol). But even assuming 7%, roughly half of which would be stroke risk, that would mean 0.35% per year.1 The study rate was 10 times higher.
When the study rate is 10 times higher, the potential for benefit is ten times lower.2 Quick math: The relative reduction of 14% in strokes for villagers translated to an absolute of about 0.5% (see footnote for calculations),3 meaning 1 in every 200 people who ate less salt avoided a stroke because of it. For me, presuming my body reacts to salt just like a hypertensive Chinese villager, that means a 1 in 2000 chance I’ll avoid a stroke by eating less salt for 5 years.
And that’s generous, since we estimated conservatively at each step.4
What’s it all mean? Iocaine powder doesn’t affect everyone the same way. If per chance you spent years building a tolerance, for instance, you might do just fine with some deadly iocaine in your wine. But if not, you might drop dead mid-cackle.
So yes, there’s a possibility salt reduction may help you avoid a stroke. But I encourage you to calculate your own risk, do some back-of-the-envelope work, and decide for yourself. For me, I’m headed to the prosciutteria.
And of course, you can eat… as you wish.
Typically about half of overall cardiac risk is stroke risk, so if we start with 7% over 10 years and call that 0.7% per year, then the per-year stroke risk would be half of 0.7%, or about 0.35%.
The potential for experiencing a benefit from anything preventive—a pill, a surgery, a diet—is always dependent upon how often the problem occurs. There’s no point giving medicine to prevent heart attacks in 5-year olds, because they don’t have heart attacks. But 75-year olds, particularly those with heart problems, can definitely benefit from medicine to prevent heart attacks. For 45-year olds it gets more mathematical. If a medicine cuts the heart attack rate in half, then a 5-year old will go from 0% to half of 0%. Not much help. The 75-year old, assuming a very high baseline heart attack risk of 20%, would go to 10% (a 1 in 10 chance it will help, pretty good). And the 45-year old might have a 1% risk that drops to 0.5% with the medicine. That’s a 1 in 200 chance it will help. Point is, the chance of benefiting is directly dependent on the baseline chance of experiencing the problem.
The absolute numbers were 33.65 strokes per 1,000 person-years in the control (regular salt) group, and 29.14 in the salt reduction group, which means the difference is 4.51 per 1,000 person-years. 4.51 per 1,000 person-years, is roughly equivalent to 0.45 per 100 person-years, in other words a 0.45% difference per year. I’ve rounded this up to 0.5%.
There are many other reasons to think we’ve been generous. One issue to consider is that hypertension is a contributor to stroke risk, but it’s not the only factor. For people like me, who do not have high blood pressure, if I have a stroke there’s a good chance the precipitating factor won’t be my blood pressure. Therefore reducing my BP won’t help prevent it from occurring. But everyone in the China study was hypertensive. There’s also the thorny issue of biological, geographic, and cultural differences. Chinese people have among the highest lifetime stroke risks in the world which, again, makes them a perfect target for stroke prevention studies. But also means the results translate less well, and we should expect anything that works in such a setting to be less effective in other settings.
As always, David.
Another great article, Dr. Newman. Thank you!