As dietary sodium levels go up over time, so does the risk for high blood pressure, suggests a new study that followed more than 4,000 adults in Japan for four years. 

The study did not measure dietary sodium directly, instead using urine samples to estimate levels of sodium consumed, but the results align with other large studies of sodium intake and blood pressure risk, researchers say.

“A small amount of salt (less than 3,000 milligrams a day) is necessary for us, but excessive salt is dangerous,” said coauthor Dr. Tomonori Sugiura of Nagoya City University Graduate School of Medical Sciences in Japan.

To excrete excess amounts of sodium, the body has to increase blood pressure, Sugiura told Reuters Health by email.

Dietary sodium levels are much higher in Japan than in the U.S., but most people eat too much in both countries, Sugiura said.

U.S. government dietary guidelines recommend consuming no more than 2,300 mg of sodium a day, equivalent to about two-thirds of a teaspoon of salt, and that people with existing high blood pressure or risk factors for it limit total sodium to 1500 mg daily. Past research indicates the average American consumes about 3,600 mg of sodium a day.

Scientists are still debating whether high levels of sodium intake among people with normal blood pressure could cause chronic high blood pressure or other cardiovascular problems to develop.

But if it does contribute, sodium intake would be a risk factor that would be easy to modify to benefit public health, the authors write in the Journal of the American Heart Association (AHA).

The researchers used urine tests from checkup appointments to estimate the dietary salt intake of 4,523 Japanese adults without high blood pressure. The participants, who ranged in age from 22 to 85 years old, had annual physicals including sodium tests and blood pressure measurements.

The study team used these records to follow people for three or more years to see if they developed high blood pressure. During the study, 1,027 individuals developed high blood pressure, including 26 percent of the men and almost 17 percent of the women.

Having higher sodium levels at the beginning of the study and showing greater increases in sodium levels each year were both tied to higher risk of developing high blood pressure, the researchers found.

At the start of the study, the participants were consuming an average of 4,200 milligrams of dietary sodium per day. Those who would go on to have high blood pressure were eating an average of 4,500 milligrams per day.

Men tended to have higher urinary sodium levels than women. And people who developed high blood pressure also tended to be heavier and older.

“Salt is one of the most important factors in the prediction of hypertension,” Sugiura said. “Other important lifestyle factors include obesity and too much alcohol intake.”

People with diabetes, obesity, a history of cardiovascular disease, and those with systolic blood pressure greater than130 millimeters of Mercury (mmHg) or diastolic blood pressure greater than 85mmHg should pay the most attention to their sodium intake, Sugiura said.

Trials that actually test changes in diet, as opposed to observing a population as this study did, have shown that modest reductions in sodium intake will decrease the instance of high blood pressure, according to Dr. Paul K. Whelton of Tulane University School of Public Health and Tropical Medicine in New Orleans.

“For people with established (high blood pressure), reducing sodium intake reduces the need for hypertension therapy,” Whelton, who was not part of the new study, told Reuters Health.

Exact sodium guidelines vary by country and advisory source, but in a way they are irrelevant because “no one is even close to it,” he said. More than 90 percent of U.S. adults exceed recommendations, he said.

While individuals can try to limit their sodium intake on a daily basis, the vast majority of sodium in our diets comes from processed food, so gradually reducing the amounts in those foods would have important health benefits without asking people to actually change their behavior, Whelton said.

“Adding sodium at the table not very important, about 80 percent of what we get is added during processing,” he said.

So far, manufacturers have not been pressured to reduce sodium levels, due to powerful commercial lobbies, he said. Salty processed foods make consumers thirsty, and many of the same manufacturers also make sodas, he noted.

Whelton has found in his own studies that “the more you can reduce your sodium intake the better off you were, even those with a small reduction.”