Despite a number of studies questioning the usefulness of very low-salt diets in the last few years, most major medical organizations continue to recommend them. We would assume that they do so from a strong base of evidence.

But with respect to heart failure, there is a shockingly small amount of evidence.

Heart failure occurs when the heart can’t pump enough blood to the rest of the body. About 5.7 million people in the United States suffer from it. Chronic high blood pressure can force the heart to work too hard over time, weakening it and leading to heart failure. Treatment usually involves trying to strengthen the contractions of the heart muscle with drugs, or reducing the volume of blood with other drugs (like diuretics) or by restricting salt intake.

Recently, researchers searched for randomized controlled trials that evaluated the use of reduced sodium intake to treat heart failure. In all the literature, they found nine studies that involved 479 patients. One of them was published only in abstract form. None involved more than 100 patients. Over all, none were considered at low risk for bias.

There were no data that showed that salt restriction reduced mortality or cardiac disease; affected whether someone was admitted to the hospital; or influenced how long they had to stay if admitted. Of four outpatient studies, two showed no improvement in heart function, and two did.

This is a minuscule amount of data on which to base strong recommendations. An accompanying editorial written by Clyde Yancy, a professor of cardiology at Northwestern School of Medicine, noted that only 0.3 percent of the studies that looked at sodium restriction and heart failure were of sufficient quality to be included in this systematic review.

We need better research. Some of that may be on the way. The Geriatric Out of Hospital Randomized Meal Trial in Heart Failure randomly assigned 66 patients to home-delivered low-salt meals after hospital discharge to study how well they work. Its findings are pending.

Another study, still continuing, has randomly assigned 1,000 people to usual care or a very low sodium diet — 1,500 mg a day, what the American Heart Association recommends for most adults — to look at outcomes related to heart failure.

Until then, some will argue that there’s little harm from these recommendations, so why not continue them? One reason to stop them is that there’s a risk of emphasizing salt avoidance at the expense of other — potentially more useful — diet measures, when we really don’t know what’s best.

For example, the Agency for Healthcare Research and Quality examined the effects of both sodium and potassium intake on health. It reported this year that lower sodium intake “most likely reduces” blood pressure. But it also reported that increasing potassium intake most likely has the same effect, because consuming more potassium causes you to excrete more sodium in your urine. (Of course, excess potassium has its own risks — any major dietary changes should be discussed with a physician.)

Other systematic reviews find that an increased intake of fiber is associated with a lower risk of heart disease.

It’s important to note that we also lack a strong evidence base from randomized controlled trials where potassium and fiber are concerned. But recommendations for those diet changes don’t seem nearly as vocal or as strident as the push for salt reductions.

The larger point is that if all of these lack strong evidence, we should admit it and make our advice more equivalent and appropriately less confident.

Or we could push for more healthy holistic lifestyle changes in general, while acknowledging that, yes, people who consume large amounts of sodium might benefit from cutting back. We could advise people to exercise regularly (which has an enormous evidence base) and to eat more healthfully over all. Both the Dietary Approaches to Stop Hypertension diet and the Mediterranean diet have been associated with lower rates of heart failure. Such diets may affect more than just salt, but they certainly would benefit from more controlled trials to prove their worth.

In his editorial, Dr. Yancy writes bluntly that the evidence for sodium restriction is “vacuous, lacks depth, and in some cases lacks integrity.” He adds, “The first step is not a call for more trials but a retreat from an unbridled and potentially harmful insistence on rigorous sodium restriction in those with symptomatic heart failure.”

Unfortunately, this exhortation could apply to more than just this one instance. Whether it’s our consumption of fat, or meat, or sugar, or even salt, it seems the assurance with which we speak should match the quality of evidence behind our recommendations.

Aaron E. Carroll is a professor of pediatrics at Indiana University School of Medicine who blogs on health research and policy at The Incidental Economist and makes videos at Healthcare Triage. He is the author of “The Bad Food Bible: How and Why to Eat Sinfully.” @aaronecarroll

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