For a long time, researchers have found links between high sodium intake and higher blood pressure, and between higher blood pressure and increased risk of stroke. At the same time, critics, including Gary Taubes, have argued that the data do not support the idea that most people should reduce their salt intake.
New evidence suggests the critics were wrong. Four different studies support the idea that high amounts of salt intake are generally bad.
One study recently appeared in BMJ Open. “The UK initiated a nationwide salt reduction programme in 2003/2004. The programme has been successful and resulted in a 15% reduction in population salt intake by 2011,” write the authors. That might not seem like much but the reduction was in the salt in processed foods, which for most people is most of their salt intake. The more processed food you ate — and the more extreme your salt intake — the greater the reduction.
The main finding of the study was that over the same period, there was a large and steady decrease in both blood pressure and strokes in the UK. Mortality from stroke and ischemic heart disease (IHD) went down by 40%!
There were small changes in other environmental variables over the same period: people ate slightly more fruit and vegetables, weighed slightly more, smoked somewhat less, and so on. Maybe these other changes were what led one critic to dismiss the results in a New York Times article:
Dr. Niels Graudal, a senior consultant in the department of internal medicine at Copenhagen University Hospital, said that connecting the two events “is meaningless.”
“This paper describes two independent incidents,” he added. “That these incidents should be in any way connected is absolutely unlikely.”
Consistent with the low quality of science journalism in the New York Times, it seems the reporter, Nicholas Balakar, did not ask Dr. Graudel any hard questions — for example, for an alternative explanation of the decline.
The BMJ study did a poor job of determining the a priori likelihood of such a big decline. It could have looked at year by year changes in stroke and IHD mortality before 2003, for example. Was stroke and IHD mortality rising or falling? It could have looked at changes in stroke and IHD mortality in similar countries without a salt reduction program over the same period. Such comparisons would have helped a lot. But the BMJ study did report the results of other nationwide salt reduction programs.
Japan, in the late 1960s, carried out a government-led campaign to reduce salt intake. Over the following decade, salt intake was reduced, particularly in northern areas from 18 to 14 g/day. Paralleling this reduction in salt intake, there were falls in BP and an 80% reduction in stroke mortality in spite of large increases in fat intake, cigarette smoking, alcohol consumption and obesity which occurred during that period. Finland, in the late 1970s, initiated a systematic approach to reducing salt intake through mass media-campaigns, co-operation with the food industry and implementing salt labelling legislation. This led to a significant reduction in the average salt intake of the Finnish population from ≈14 g/day in 1972 to less than 9 g/day in 2002. The reduction in salt intake was accompanied by a fall of over 10 mm Hg in systolic and diastolic BP and a decrease of 75–80% in stroke and IHD mortality.
Again, reductions in salt intake happened just before huge decreases in stroke and IHD mortality. It is the triple repetition of an unlikely event (big reductions in mortality) and the experimental aspect (something was specifically changed) that convince me. The critics are not going to come up with a plausible alternative explanation of all three cases (UK, Japan, Finland) any time soon.
A paper co-authored by the same Dr. Graudal who dismissed the new findings found that high sodium intake was associated with increased mortality. It also found that low sodium intake was associated with increased mortality. This is why a reduction in the salt in processed food makes so much sense: 1. It’s easy. You don’t have to do anything. 2. It reduces salt intake the most in people who eat the most salt — exactly where it is likely to be the most beneficial.
Just to be clear, this data also says that if you don’t eat a lot of salt, there is no good reason to reduce your salt intake (unless you have high blood pressure).