WEDNESDAY, Oct. 20 (HealthDay News) -- Americans still consume more salt than they should, despite decades of warnings linking high-salt diets with an increase in blood pressure and a higher risk of heart disease and stroke.

A new Harvard study finds salt intake is about the same today as it was nearly 50 years ago, an amount well above recommended guidelines, noted Dr. Adam M. Bernstein, the study's lead author.

Bernstein, a research fellow in the Harvard School of Public Health's department of nutrition, and colleague Dr. Walter C. Willett analyzed 38 studies, published between 1957 and 2003, that reported the amount of salt that the more than 26,000 participants passed in their urine. This test is the most reliable estimate of salt intake.

The researchers thought they would find that salt intake had increased over time because Americans eat more processed foods today than in 1957. But decade after decade, people consistently consumed about 3,700 milligrams of sodium a day, the data showed. Current sodium guidelines advise up to 2,300 milligrams (about one teaspoon) a day for adults, and 1,500 milligrams for those who have or are at risk for high blood pressure.

The study appears in the November issue of the American Journal of Clinical Nutrition.

Since the 1980s, the U.S. government has recommended cutting back on salt in order to reduce blood pressure and cardiovascular disease risk. But the ongoing U.S. National Health and Nutrition Examination Survey (NHANES), which provides a snapshot of Americans' health and nutrition status every two years, regularly suggests that Americans consume more salt now than they did 20 or 30 years ago, Bernstein said. However, those data are based in part on survey participants recalling what they ate, rather than on the more accurate urine samples.

The study also noted that although Americans' salt intake has remained relatively constant for almost 50 years, their rates of high blood pressure and heart disease have increased in the past 20 years. But America's ever-rising obesity rates may play a more critical role in this rise than salt intake, the researchers said.

The study's main message "is that for all the intense effort to get Americans to limit their salt intake, the evidence suggests that this hasn't happened," said Dr. David McCarron, lead author of an accompanying journal editorial.

The average salt intake of the Americans in the Harvard study is similar to that found in international studies, the editorial said. This similarity suggests that humans may need a set amount of salt and are hard-wired to seek it, said McCarron, an adjunct professor with the department of nutrition at University of California-Davis.

McCarron led a 2009 study that analyzed urine samples in 19,151 people in 33 countries over a 24-year period. The average daily sodium intake was 3,726 milligrams a day, even across diverse populations and diets, and with no evidence of change over time. In a 12-year study of more than 13,000 people from Switzerland, also published in 2009, people averaged around 3,680 milligrams a day. "It's spooky how consistent this number is," said McCarron.

In light of these studies, the editorial said, the government should limit its sodium guidelines to those at risk for high blood pressure and heart disease rather than issue broad, one-size-fits-all guidelines for everyone. McCarron has consulted with the Salt Institute and the food industry in the past.

Fully 69 percent of American adults meet these risk guidelines, a 2009 study by the U.S. Centers for Disease Control and Prevention found. These high-risk folks include people over 40, blacks, and those with slightly elevated blood pressure (pre-hypertension).

And there's a call to do more to get Americans to ease up on salt. In April, a report by the Institute of Medicine called for the U.S. Food and Drug Administration to set national standards for salt added to processed foods. According to that report, 32 percent of U.S. adults have high blood pressure. And the American Medical Association estimates that cutting the amount of salt in processed foods by half could save 150,000 lives in the United States each year.

Bernstein said he supports the government's sodium guidelines and disagrees with the "set point" theory. "A set point would not explain why other populations have substantially different sodium intakes," he said.

More information

There's more on lowering daily salt intake at the U.S. Food and Drug Administration.