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- Alex Hutchinson (@sweatscience)
One of the health topics I’ve been curious about recently is the question of salt intake. It seems that everyone — governments, scientists, heart institutes — agrees that we eat way too much salt, and it’s leading to an epidemic of hypertension.
But there are skeptical voices — the much-maligned Gary Taubes is one, but there are many others (like my wife!) who argue that (a) sodium levels in the body are very tightly regulated, so any excess in a healthy individual is simply excreted out in the urine, and (b) dramatic reductions in salt produce only a minor blood pressure change of a few mmHg, and not in everyone, so blanket recommendations for the whole population to cut salt make little sense.
So I was interested to see a study from Walter Willett at Harvard in the current issue of the American Journal of Clinical Nutrition (full text here). He and a colleague reviewed studies between 1957 and 2003 that measured sodium excretion in urine — a very accurate way of determining salt intake that gets around the difficulties in figuring out exactly how much salt is in your food. They found two main things: (a) sodium intake averaged about 3,700 mg per person per day, which is way higher than the upper recommended limit of 2,300; and (b) it essentially hasn’t changed in the half-century studied.
Interestingly, these results agree almost exactly with similar reviews of studies from 33 different countries: salt intake is high, and it hasn’t changed in recent memory.
Now, here’s the question that Willett dances around in his discussion: If hypertension is a rising epidemic in the U.S. (and elsewhere), and if salt intake hasn’t changed in 50 years, how can salt possibly be the culprit? He points out that hypertension has “multiple etiologic factors,” and suggests that obesity may be a more important determinant.
The article is accompanied by a fairly strident editorial by David McCarron titled “Science trumps politics: urinary sodium data challenge US dietary sodium guidelines” (full text here). McCarron is a familiar figure in the salt wars, and is often maligned because he has consulted with the Salt Institute in the past. Lots of money sloshing around in this debate. What’s so interesting about this study is the involvement of Willett, a public health titan who normally lines up on the other side. Here’s what Willett had to say in a debate about New York City’s proposed salt rules on the New York Times website earlier this year:
The evidence is solid, based on many years of research, that reducing sodium intake from the high amounts now consumed in the U.S. will reduce the risks of these conditions [hypertension, stroke, heart disease].
His own study now tells us that the amounts now consumed in the U.S. are not at all “high,” historically speaking. Will this lead him to change his views? I’m not counting on it — but it’s at least encouraging that he’s interested in looking at the data.