Cutting salt saves lives? Yes. No. Maybe.
It would be hard to argue that we should keep gobbling down 3,400 milligrams of sodium a day in the face of research suggesting that a gradual reduction in our salt intake could save close to a half-million American lives in 10 years alone. But what if proposals to cut back salt in Americans’ diets could not be shown to prevent any deaths at all? Would we still do it?
In the wake of a study published Monday in the American Heart Assn.'s journal Circulation that is not an idle question. That study sliced and diced the numbers three different ways in an effort to project realistically the impact of reduced sodium consumption on the nation’s population of 314 million. The new research drew upon a number of studies that have found powerful links between high salt intake and high blood pressure, between high blood pressure and cardiovascular disease, and between high salt intake and cardiovascular disease.
But it did not take into account a welter of studies--including several published recently in respected journals--that could find no evidence of health benefits from diets low in sodium.
This may be confusing to some: no health benefit--or upward of 500,000 lives saved over 10 years? Even those who are comfortable with squishy projections must admit it: These claims plainly conflict. And so has gone the salt teeter-totter for some four acrimonious decades.
A recent seven-year European study, published in the Journal of the American Medical Assn (JAMA) in May 2011, found that while subjects’ higher sodium intake nudged their systolic (but not diastolic) blood pressure upward, it did not increase their risk of hypertension or of cardiovascular complications. In fact, the research suggested the opposite: that subjects whose sodium intake was lowest were most likely to die of any cause during the study.
Two separate meta-analyses (studies that consolidate and distill the findings of existing clinical trials) have come to similar conclusions: The first of the Cochrane Review articles, published in 2011, found “no strong evidence” that in people without hypertension, reducing salt drove down deaths; the second, also published in 2011, concluded that 150 randomized control trials and 13 population studies had failed to produce “an obvious signal in favor of sodium reduction.” Focusing on the impact of low-salt diets on the hormone and lipid levels of people without hypertension, the second Cochrane study found no clear benefit.
In short, in the 40-year skirmish over salt and its relationship to hypertension and heart disease, Monday’s study saying hundreds of thousands of lives could be saved is just the latest salvo.
That dispute was described in detail in a fascinating recent article in the journal Health Affairs. Some health policy debates are easy to figure out: They pit strictly commercial interests (say, the makers of tobacco) against clear scientific evidence that points to a need for change. But the authors of the Health Affairs article write that in the salt debate, this dispute is largely over science--what kinds of research studies matter, what level of proof is needed, and how much faith we’re willing to put into “associations” between one thing--say, sodium consumption--and another--say, heart attacks and strokes.
“What is striking in the salt debate...is that the combatants cannot be neatly divided between the respected advocates of a mainstream position and a band of marginal dissidents,” wrote bioethicist Ronald Bayer and his colleagues at Columbia University’s Mailman School of Public Health. “Respected scientists have found themselves on opposite sides of the divide. References to ideological rigidity or corporate interference do not appear to us an adequate explanation” for the often bitter debate, they wrote.
The existence of conflicting research findings shouldn’t necessarily paralyze public health advocates, the authors concluded: Judgments and a society’s values must play a role in deciding whether to take action amid scientific uncertainty. At the same time, public health advocates would do well to own up to the scientific uncertainties of their position.
In the debate over salt, “the concealment of scientific uncertainty is a mistake that serves neither the ends of science nor of good policy,” the authors wrote.
Still, the American Heart Assn., and a long list of other respected groups, is pushing hard to reduce the sodium in Americans’ diets. In addition to working with food companies and restaurants to lower the sodium content of snack foods and meals--and with government agencies to set possible future limits, the American Heart Assn. is calling on individuals to watch their own salt intake: In early January, it launched a 21-day challenge to resolution-minded Americans to “change your salty ways.”