(Health.com) -- It's no secret that fast-food fare like burgers, french fries, and fried chicken tends to be high in sodium. According to a new study, however, American fast-food customers may be getting a larger dose of sodium than their counterparts in other countries -- even if they order the exact same items off the menu.
In the study, published this week in the Canadian Medical Association Journal, researchers analyzed the posted nutritional information for more than 2,000 items sold in multiple countries by the world's six largest fast-food chains: Burger King, Domino's Pizza, KFC, McDonald's, Pizza Hut, and Subway.
Overall, the researchers found, fast food tended to be saltier in the United States than in the other countries included in the study: Australia, Canada, France, New Zealand, and the UK. What's more, the sodium content of the same menu items at the same chains varied by country, sometimes widely.
For most menu items, the sodium content differed by no more than 20% to 30% from country to country. Burger King's Double Whopper, which contained an average of 1,050 milligrams of sodium, varied by 240 milligrams at the most, for instance.
But in some cases the difference was dramatic. The Chicken McNuggets sold in the United States contained 2.5 times more sodium than the McNuggets sold in the UK. Likewise, the sodium content of a Subway club sandwich was more than twice as high in the United States as it was in France.
It's not clear from the study what accounts for these variations. Several factors could make it difficult for restaurant chains to reduce or standardize their sodium use across countries, says Joy Dubost, Ph.D., director of nutrition for the National Restaurant Association, a trade organization that represents all of the chains covered in the study.
"There are challenges not identified by this study," DuBost said in a statement, "including availability of acceptable reduced-sodium items in the supply chain, consumer variability in taste preference across the U.S. and among the various countries, regulatory constraints, as well as availability of new and existing alternatives to sodium."
Local suppliers and regulations are probably more influential than local tastes, says Norm Campbell, M.D., one of the study authors and a blood-pressure specialist at the University of Calgary, in Alberta.
If restaurants were largely responding to consumer demand for saltier items, Campbell says, one would expect to see a close relationship between the sodium content of fast food and a country's per capita sodium intake (a rough index of a culture's taste for salt). "We did not see that," he says.
Although they can't pinpoint the reasons for the sodium disparities, Campbell and his colleagues say the study findings show that limitations in food-processing technology are not a barrier to providing lower-sodium products, as the food industry has claimed.
"We found multiple examples of low-salt choices, and for the same product across different countries there's variation in the amount of salt that's added," Campbell says. "From that perspective, it would appear that it's not very challenging to lower the amount of salt in food products."
Consuming too much sodium can raise blood pressure and contribute to hypertension, one of the leading risk factors for cardiovascular disease. In recent years, public health officials across the globe -- including those in the UK, Brazil, and New York City -- have set voluntary salt-reduction targets for food companies.
In the UK, the government's push to reduce sodium began in earnest in 2006, and the relatively low sodium levels seen at UK restaurants in the study suggests the initiative may be working, says Gary Beauchamp, Ph.D., director of the Monell Chemical Senses Center, a nonprofit institute in Philadelphia that specializes in taste research.
Salt has a number of properties besides taste enhancement that make it attractive to food manufacturers. Sodium is a preservative, and it also can also make certain foods easier to process. "Salt is the magic ingredient -- it does all sorts of things to food," Beauchamp says.
However, consumer taste is the bigger obstacle to sodium-reduction efforts. People in industrialized nations have become accustomed to saltier foods, and companies worry that reducing salt will make their products less competitive.
For that reason, the Institute of Medicine (IOM), an independent nonprofit organization, has recommended a gradual, industry-wide sodium reduction in both packaged and restaurant foods. "Then people would acclimate to the change slowly and wouldn't even notice it," says Beauchamp, who served on the IOM committee that issued the recommendation in a 2010 report.
"Most of the science says if you reduce salt by 10% it's completely unnoticeable," Campbell says. "What we really want is very gradual reductions which don't affect the consumer base. Consumers enjoy the food and the health of the population improves."
Some fast-food chains have already begun cutting sodium. Danya Proud, a spokesperson for McDonald's USA, pointed out in a statement that Campbell and his colleagues used nutritional info from 2010. Since then, Proud said, the company has cut the sodium content of its chicken items by 10%. (According to current nutritional info, Big Macs sold in the United States still contain 1,040 milligrams of sodium, as they did when the study was conducted.)
McDonald's expects to reduce the sodium in all of its national menu items by an average of 15% by 2015, Proud said. "We are also listening to our customers, to ensure we continue to evolve to meet their taste and nutrition expectations."