This week, the New England Journal of Medicine published three papers about the effects of salt consumption on health. Their apparently contradictory findings have served to further fuel an unwarranted debate about the harms, or otherwise, of excessive dietary salt.
One of the papers was from the Gates Foundation-funded Global Burden of Disease program, while the other two derived from the Prospective Urban and Rural Epidemiological, or PURE, study - a multi-country research project that included about 100,000 people around the world.
Both involved large data sets but one suggested serious adverse consequences from salt consumption while the other claimed the opposite.
The toll of salt
The headline finding by the Global Burden of Disease researchers was that excess salt consumption is causing 1.65 million premature deaths each year. That would be accompanied by at least as many non-fatal strokes and heart attacks.
This is the shocking toll from a product shaken onto food by unwitting consumers, and poured into the food supply by an industry intent on filling its coffers.
The PURE study reported anticipated effects of salt on blood pressure and showed a clear adverse effect of high salt intake on health.
But the authors also reported that moderate levels of salt intake, which they set at well above current World Health Organisation goals, were harmful. They suggested current targets for salt are too low and may cause heart disease and stroke.
This last set of conclusions is almost certainly flawed.
The large numbers and many countries included in PURE are impressive but the data weren’t collected with the intent of working out the effects of salt on ill health.
The study’s measure of salt intake was poor and its design is the wrong one for answering this type of question. The PURE researchers have pushed their data beyond its limits and the whole thing has become decidedly impure.
For exactly these same reasons, prior studies of the same type as PURE have variously concluded that salt causes disease, that salt protects against disease, and that salt has either beneficial or adverse effects on disease.
Clearly, this type of cohort study is incredibly prone to confounding and can easily get the wrong answer.
Consider this, for instance: many people who deliberately eat a low-salt diet have often switched down from a high-salt diet because they developed a health problem. The confusion this causes makes it look as if eating less salt is causing the problem, when it’s actually exactly the opposite.
This issue of “reverse causation” is very well known from the obesity, alcohol, and blood pressure fields and is notoriously hard to deal with. Even if you do complex analyses to try and control for it. Indeed, the research literature is littered with examples where these types of studies got the wrong answer.
Getting disproportionate attention
The problem is that a large piece of research like the PURE study gets a lot of coverage because it’s big and especially because it questions established wisdom. But big and confounded doesn’t add anything useful.
There have been other recent unusual claims about salt. It was suggested at the World Congress of Cardiology in Melbourne earlier this year, for instance, that normal salt consumption for humans is about ten grams a day – but a diet without added salt provides only about a gram of salt a day.
Current salt consumption is ten grams a day principally because this is how much salt consumers and industry add to the food supply. The same argument applied to tobacco would conclude that it’s normal for 1.3 billion people in the world to smoke.
It’s also been claimed that because salt is required to generate the “action potential” (the way nerves work), the very high levels of salt consumption observed in society today are somehow required. But there’s no evidence the nervous system requires high levels of dietary salt consumption. Indeed, the nervous system of populations consuming much lower levels of salt work perfectly normally.
Policy based on evidence
We’re now in the unfortunate situation where the debate around salt has become such a good way of interesting the media, and such a good way of promoting careers, that the quality of the data used in it is being forgotten. This is bad for science and bad for population health.
As always, the best approximation of truth comes from a systematic, objective and balanced overview of all the applicable data. The World Health Organization takes this type of approach, and recently recommended salt reduction as part of its non-communicable disease prevention program. It left the maximum recommended salt intake level at five grams a day.
These new data from PURE provide no reasons to change this recommendation. And the Global Burden of Disease data highlight the need for governments and the food industry to redouble their efforts to get more salt out of our diets.
Any suggestion that national or international guidelines should be change on the basis of the PURE data is absolutely contrary to global best practice. It’s universally accepted that guidelines should be based on the best unconfounded evidence, in particular, overviews of randomised trials.
Randomised trials of salt reduction show clear benefits on blood pressure, the leading cause of premature death in the world. This is the best evidence we have to date and it’s the evidence that should drive health policy.