In Britain it started with Sid, the “giant slug with a message”, who slicked his way onto television screens back in 2004 as part of a government health campaign to warn people about the dangers of consuming too much salt. “Stay away from fast cars, loose women and SALT!” he screamed.

Sid’s message - that liberal sprinklings of sodium, the main component of salt, don’t only kill slugs but humans too - has now become conventional wisdom worldwide. High salt intake is linked to high blood pressure, or hypertension, a key risk factor for strokes, heart attacks and other cardiovascular diseases. Together those rank as the world’s number one killers. The World Health Organization (WHO) puts cutting salt intake alongside quitting smoking as one of the top 10 “best buys” in public health.

“Blood pressure is the biggest cause of death in the world ... and salt is the most important thing that puts it up,” says Graham MacGregor, a professor of cardiovascular medicine at the London-based Wolfson Institute of Preventive Medicine and chairman of the influential World Action on Salt and Health lobby group (WASH). “Cutting back on salt gives a direct beneficial effect on the biggest cause of death in the world. That’s why it’s so important.”

Or is it? Recent scientific papers suggest the basis for a global crackdown on salt is not what you’d call rock solid. Two 2011 studies indicate that the evidence is inconclusive, or that reducing salt may even be harmful.

“There’s a view that salt is the root cause of all high blood pressure worldwide and some people religiously hold on to that belief,” said Tony Heagerty, head of the cardiovascular research group at Britain’s Manchester University and a former president of the International Society of Hypertension. “But the evidence for that is actually pretty flimsy.”

It’s a debate that has flared over the past few months, with each side harnessing a legion of experts in hypertension, heart disease, nutrition and scientific analysis. The salt industry has, naturally, jumped on studies that question the conventional wisdom, and at least one food manufacturer has started to add salt back to some of its processed foods. At times the row has become personal. Trapped in the middle are consumers, who may worry they have become unsuspecting guinea pigs in a grand global experiment.

“The two sides are totally polarized and there’s no agreement or consensus on what the answer is,” says Peter Sherratt of the UK Salt Association. Any new scientific paper which supports the anti-salt position is lauded as proof salt consumption is dangerous, but any piece of evidence or science showing salt is beneficial, or reducing it dangerous, is criticized as unrepresentative, he said.

The debate has big implications for business. Salt for food use accounts for only a fraction of the 250 million tonnes of annual global production. Looking at the United States alone, 1.5 million tonnes of so-called human nutrition salt was sold in 2009 with a value of more than $321 million.

But the U.S. snack foods industry - a key consumer of salt which includes major companies like Pepsico’s Frito-Lay and Kraft’s Nabisco - has a combined annual revenue of $27 billion, according to analysis by company profile builder Hoover’s. Then there’s the business of selling drugs to treat high blood pressure. Worldwide sales of anti-hypertensives were around $35 billion in 2009, according to research by Deutsche Bank.

Heagarty disclosed that 15 years ago, his department accepted 2,000 pounds ($3,200 at today’s rates) from the U.S.- based industry lobby, the Salt Institute, but said he has no current financial conflict of interest.

WORTH THE SALT?

Salt has been taxed, monopolized, treasured and fought over for thousands of years. Today’s scientists are waging a modern-day salt war.

In the 1970s, American researchers experimenting on rats found very high doses of salt raised blood pressure. Some of the most-cited evidence on salt and health came in a 1988 international study called InterSalt, which surveyed more than 10,000 men and women in scores of populations across the world. The study included four remote tribes in Brazil, Kenya and New Guinea whose people had the lowest salt intake and were also found to have the lowest blood pressure and very few, if any, cases of hypertension. Although these findings were disputed by parties including the Salt Institute, it wasn’t long before a scientific consensus emerged that too much salt is bad for you.

A 2005 study in the PubMed journal found almost 1 billion people around the world have high blood pressure, which makes the heart work too hard, hardens the walls of the arteries and can cause other problems such as heart failure, kidney disease, and blindness. Cardiovascular diseases are the leading cause of death globally, claiming 17.1 million lives a year. A substantial number of these deaths are put down to smoking, which raises the risk of hypertension, strokes and heart attacks.

In the past few years, governments have begun to act. Under its health-promoting mayor Michael Bloomberg, New York City pledged in 2010 to coordinate a U.S.-wide effort to cut salt in restaurant and packaged foods by 25 percent. National sodium reduction strategies have been adopted across Europe and in Australia, China and India.