Salt, the villain of numerous public health campaigns, including one in San Diego County, isn't such a negative after all.

The sodium consumed with salt is widely supposed to raise blood pressure and increase the rate of cardiovascular disease. But that position, endorsed by the American Heart Association and the Centers for Disease Control, has been disputed in recent studies in peer-reviewed journals that fail to find health risks for most Americans in their salt consumption.

Now a study published today points to an unexpected benefit of salt: It fights infections.

Andrell Bower Andrell Bower

Sodium deposits build up where skin infections take place, increasing the activity of immune cells called macrophages that consume microbes, the study found. The phenomenon was noticed first in mice. When infected mice were fed a high-salt diet, the macrophages were activated, speeding healing of feet infected with Leishmania major.

Sodium also builds up in the skin of older people, suggesting the same mechanism is at work in humans, said Jens Titze, the study's senior author and a researcher at Vanderbilt University in Nashville, Tenn.

Titze says he agrees that excessive salt consumption has harmful effects. But the story is incomplete, and salt has hitherto unsuspected health benefits.

"That's the game-changer here," Titze said in a Monday interview. "It's a completely different science. The most important finding here is that tissues can accumulate massive amounts of sodium locally to boost immune response wherever needed. It's a biological principle."

In evolutionary terms, the ability to fight infection is critical to survival at any age, Titze said. But in the modern age of medicine and longevity, salt's harmful effects become more pronounced. Moreover, salt deposits in skin appear to play a role in autoimmune diseases.

The study was published in the journal Cell Metabolism.

That salt accumulates in the skin has been known for years, and scientists such as Titze have been trying to understand why.

Salt's properties as an anti-infective have long been advocated in popular medicine. A salt cure was key to Jane Goodall's acceptance in Africa when she began to do research on chimps in the 1960s. Her mother, Vanne, recommended a saline drip to cure a deep leg wound on a native African, thus winning acceptance.

"Every morning and every afternoon, the patient sat down with a large bowl of blood-warm salted water which he dripped very slowly over his sores," Goodall wrote in her book, In the Shadow of Man. "After three weeks the swelling had gone and the wounds were clean. Subsequently, it was only a matter of time before he was completely healed."

Another science dogma shaken

The study is the latest chapter in research exploring the role of salt in the human body, a story that grows more complicated as more research is conducted. And the same thing can be said of the scientific status of other dietary nutrients as fat and cholesterol, each of which was pointed out as a cause of poor health. But the original stance has been undermined by recent research.

In February, the U.S. government said it would drop its warning about consuming cholesterol. Likewise, the warnings to cut intake of dietary fat have been undercut by new evidence.

In short, the notion that consuming fat makes you fat, and consuming cholesterol produces high cholesterol is far too simplistic a concept for a complex thing like human metabolism.

Salt was fingered as a culprit decades ago, when governments and private health organizations decided that excessive salt intake caused high blood pressure, a known risk factor for cardiovascular disease. Some studies pointed to this association, however, they were observational studies, not scientifically controlled studies. Performing such research on large numbers of people is very difficult.

A fraction of the population is genetically salt-sensitive; as salt intake rises, so does blood pressure. These people are well-advised to rigorously control their intake of salt. African-Americans are disproportionately salt-sensitive.

Likewise, some specific sets of patients need to restrict salt consumption. The big question is what is the appropriate level of salt consumption for the majority of healthy persons, or whether such determination can even be made.

Average sodium intake for Americans is 3,400 milligrams per day. (The great majority of this comes from salt, which is made up of sodium and chlorine).

The 2010 Dietary Guidelines for Americans recommends sodium intake be lowered to 2,300 milligrams per day. For those over 51, African-Americans, and those with diabetes, chronic kidney disease or hypertension, the limit is 1,500 milligrams per day. The rationale is that sodium intake influences blood pressure, which is a risk factor for heart disease.

However, a 2013 report by the Institute of Medicine specifically declined to endorse those limits, in part because the quality of information was insufficient.

"Overall, the committee found that both the quantity and quality of relevant studies to be less than optimal," the IOM report stated. But it concluded that the bulk of the evidence indicates a correlation between higher levels of sodium intake and cardiovascular disease. It also concluded there was insufficient evidence to conclude that lowering sodium intake below 2,300 milligrams per day either increases or decreases the risk of cardiovascular disease or death in the general population.

Moreover, the report cautioned that too low a sodium intake could cause health problems. But without any numbers to indicate what level of intake is insufficient or excessive, the report left room for differing interpretations.