The Evidence on Salt

Thanks go to BradF for some pointers to the evidence and debate over salt’s effect on health. He directed me to Marion Nestle of the blog Food Politics who links to a dozen or so articles and commentaries on the issue. Disclosure: I have not read those articles and commentaries.

I have read the 5 Feb. 2009 NY Times opinion piece in which Michael Alderman argues that a randomized controlled trial is necessary to determine salt’s effect on health. (Alderman is also the author of a 3 Feb. 2010 JAMA commentary that makes the same points and cites the academic literature.) In his NY Times column, Alderman writes,

The best available evidence on how salt consumption affects our health comes from observational studies. … Nine such studies … have had mixed results. In four of them, reduced dietary salt was associated with an increased incidence of death and disability from heart attacks and strokes. In one that focused on obese people, more salt was associated with increased cardiovascular mortality. And in the remaining four, no association between salt and health was seen.

People who advocate curtailing salt consumption typically prefer to discuss two other observational studies from Finland and Japan, where salt consumption is generally higher than in the United States. In both of these, more salt was associated with more cardiovascular problems.

But observational studies do not demonstrate causality. …

Nevertheless, the research on salt intake can help identify questions to address in randomized clinical trials, the most rigorous kind of medical research.

Since I haven’t read it, I’m not going to comment on the evidence pertaining to a salt-health connection. I will make two points about Alderman’s view. First, I agree that observational studies are helpful in identifying hypotheses and issues to be explored in future studies, including randomized trials. So, even when observational studies don’t provide conclusive evidence, they can make scientific contributions.

I disagree with Alderman’s statement that observational studies don’t demonstrate causality. That’s what most people seem to think, but it is far too broad. As I’ve been writing about for weeks, research designs that exploit randomness, whether purposeful (as in a randomized trial) or not (as in natural experiments) permit causal inference. Observational studies that exploit random factors that effect treatment but not potential outcomes do demonstrate causality. They differ from randomized trials only in degree.

One possible advantage of cities imposing reduced salt regulations on restaurants is that it provides the type of randomness one can exploit to infer the causal effect of salt on health. I’m not necessarily advocating such regulations. Nor am I saying there should not be a randomized trial on salt. I’m just saying that we may be able to learn a few things even without a randomized trial. In particular, I’m saying, screaming, don’t give up on observational studies. Only some of them are uninformative about causal effects. Properly designed ones can reveal more than Alderman, and others, may think.

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