Science staggers on

During my morning commute, NPR spent a full hour reporting that Osama bin Laden is still dead. I guess that means it's time to move on to another subject and this one is for sure interesting. It seems a couple of good sized, medium term prospective cohort studies in Europe find that dietary sodium consumption is inversely associated with heart disease and cardiac mortality. In other words, according to these observations, people who eat more salt are less likely to get heart disease or to die from it.

Increased sodium intake did have a modest association with increasing systolic, but not diastolic, blood pressure. The authors hypothesize that restricting dietary sodium sufficiently to reduce blood pressure is associated with other adverse effects on the body's regulatory systems that overcome any benefit from reduced blood pressure. The suggestion is that maybe people shouldn't worry about sodium intake and just do what their body wants them to do when it comes to eating salt.

As I always say, I'm not a real doctor and I'm not giving out advice. However, I should point out that 100% of the subjects were white Europeans and that Africans have long been known to be more salt sensitive, so even if this is absolutely correct, it might not apply to everybody.

With that out of the way, here's the main point I want to make. The arguments of a large public health benefit from reducing salt in the typical diet were based on the observed association of sodium intake with blood pressure; multiplying that by the observed association between blood pressure and disease; and extrapolating to the general population. There are some unexamined assumptions in there, and this research suggests pretty strongly that the assumptions are incorrect.

This doesn't mean that the biological science that led to a general recommendation to reduce salt intake was wrong. It does mean that people drew conclusions from what we did know that might have been unwarranted. (Just because of this one publication nobody should consider the issue settled. The authors review other studies and, while they show that there really hasn't been clear empirical support for an association between sodium intake and heart disease or mortality in the past, the picture has been murky.) Unfortunately, this is a common problem in public health.

At some point you have to decide that a recommendation is plausible enough to promote it publicly, because it would be irresponsible to allow continued injury to people that you truly believe is happening. But that point often comes before we have real certainty or clarity. That's why there is such a history of changing recommendations. Take antioxidant supplements! Whoops, never mind. Eat a low fat diet. Whoops! Avoid saturated fats and trans fats particularly, but vegetable oil is fine. Take post-menopausal hormone replacements. Whoops! You probably shouldn't.

And so on. This makes people cynical and distrustful of all these sorts of recommendations. Don't be. Sometimes they turn out wrong but the overall direction is progress. The mistakes eventually get fixed. It isn't true that everything causes cancer. Keep listening to the authorities because they're steadily getting it more and more right. That's the best we can hope for, but it's worth respecting.