The following is a guest post about that piece by Kevin Lomangino, one of our story reviewers on HealthNewsReview.org. He is an independent medical journalist and editor who is currently Editor-in-Chief of Clinical Nutrition Insight, a monthly evidence-based newsletter which reviews the scientific literature on nutrition for physicians and dietitians. He tweets as @Klomangino.
The statistics Ms. Brody reveals are breathtaking: some half a million deaths, and possibly as many as 850,000, could be prevented over the next decade from small reductions in sodium intake. Up to 1.2 million lives could be saved by an instantaneous drop in sodium intake to 1500 mg/d, she says.
There’s just one problem with this rosy forecast that Brody neglects to mention: There has never been a study that’s definitively proven that we can save any lives – let alone a million of them – by reducing our intake of sodium from current levels. All of the figures Brody cites are derived from observational studies that cannot prove cause and effect, or else from clinical trials of blood pressure drugs that assume a similar benefit for sodium restriction (even though the effects on health outcomes may well be different).
Brody, seemingly aware of this problem, claims that one of the projections she mentions was based on a “randomized, controlled clinical trial of sodium reduction among men and women followed for 10 to 15 years.” But that description too is misleading. The study she was referring to was actually an observational follow-up study of patients who participated in two shorter trials that were originally designed to look at blood pressure. Information about cardiovascular events, collected 10-15 years later, was available for only 77% of the participants, and these data were no longer protected by randomization. Glossing over these details, Brody leaves her readers with an incomplete picture of the study’s strengths and weaknesses.
And Finland’s experience with sodium restriction hardly “proves” that sodium restriction will save American lives, as Brody optimistically contends. Sodium intake in Finland may have come down since the 1970s, but it was higher than in the United States to begin with. And many other aspects of the Finnish diet and lifestyle have surely changed since the 70s. Attributing all of the cardiovascular improvements to salt restriction once again confuses association with causation.
Perhaps the biggest shortcoming of Brody’s piece is that it completely ignores conflicting evidence. No one but the study authors themselves are quoted regarding the study’s implications. (And one of their comments is identical to a quote appearing in an article on the UCSF website – akin to a news release, suggesting that Brody may not have spoken with this author as the story implies.) There is not one word that suggests that anyone might disagree with the researchers’ assumptions or their analysis.
In fact, observational studies have reported both positive and inverse associations, as well as no relationship, between sodium intake and heart outcomes. Blood pressure is beneficially affected but other markers, such as insulin sensitivity, may go in the wrong direction. In one recent study, lower intake of sodium was associated with higher cardiovascular mortality. (Michael Alderman, MD, a former president of the American Society of Hypertension as well as a consultant to the salt industry, provides a skeptic’s view of the evidence on sodium in this JAMA commentary.)
Most experts are confident that the blood pressure effects of sodium restriction will outweigh any potential adverse effects. That’s a reasonable interpretation of the evidence. And I don’t think the evidence needs to be iron clad for us to demand less sodium in our food supply. (It’s very difficult for people who want to avoid sodium to do so with the amounts that are currently added.) However, we are continually learning that observational studies can be confounded in unexpected ways, and that markers of disease don’t always tell the whole story when it comes to outcomes.
In other words, it’s harder to save lives than we think, so claims that any intervention can save millions demand very strong evidence to back them up.
The definitive test of sodium restriction would be a long-term, randomized controlled trial enrolling thousands of participants and tracking their outcomes over time. Researchers have long said that this kind of diet study is simply too expensive and too difficult to perform. Perhaps they’re right, but with fresh enthusiasm for dietary interventions in the wake of the huge Mediterranean diet study from Spain, I’m hopeful that one day we’ll have better data to inform health policy.
In the meantime, how about a moratorium on pie-in-the-sky projections for unproven health interventions? There’s little chance they’ll prove accurate, and they erode the public’s confidence in health recommendations when repeatedly shown to be wrong.
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