Three months after the federal government urged most Americans to sharply cut their salt intake, a new study questions whether the recommendation will benefit those without high blood pressure. The finding from Belgian researchers, published today in the Journal of the American Medical Association, indicates that healthy people who eat the least amount of sodium don’t have any health advantage over those who eat the most. In fact, they had slightly higher death rates from heart disease.
The findings are likely to add fuel to an already heated debate over public health guidelines regarding salt consumption. Eating less salt has been shown to modestly lower blood pressure in people with hypertension, but more than a dozen studies since the mid-1990s have reached conflicting conclusions about whether lowering salt intake helps healthy people avoid high blood pressure and its serious consequences: heart disease, stroke, and kidney failure.
In the latest research, the scientists estimated sodium intake of nearly 3,700 participants, with an average age of about 40, by measuring sodium in their urine at the beginning and end of the study. They found that those with the lowest levels of sodium -– equivalent to consuming an average of nearly 2,500 mg, or just over one teaspoon per day -- had no greater protection against high blood pressure after eight years than those who consumed the highest levels, nearly 6,000 mg per day on average. None of the participants had hypertension when the study began.
Heart disease deaths decreased with higher sodium intakes: 50 deaths occurred in the third of participants with the lowest sodium intakes, 24 deaths in the third who had medium intakes, and 10 deaths in those with the highest intakes. One component of blood pressure -– the top number, or systolic pressure -- was slightly higher in those who ate the most sodium but not enough to drive them into the high blood pressure range.
“Our findings do not support a generalized reduction of salt intake in the population,” study co-author Dr. Jan Staessen, a professor of medicine at the University of Leuven in Belgium, wrote in an e-mail. He did emphasize, though, that reducing salt intake is still crucial for those with high blood pressure or heart failure.
Some nutritionists criticized the study's methodology, however, and said it was not rigorous enough to justify a retreat from the government's latest salt advice.
Reducing salt intake has been a top priority for public health officials for some time. US dietary guidelines now recommend that about half of Americans should aim to reduce their sodium to 1,500 milligrams a day; this group includes anyone over 50 and those with high blood pressure, kidney disease or diabetes. It also includes all African Americans, since they’re more likely to develop high blood pressure from their salt intake. Everyone else is supposed to limit intake to 2,300 mg a day.
A 2009 RAND Corporation study estimated that reducing Americans' average intake of sodium to the amount recommended by health officials could save the nation as much as $18 billion annually in avoided health care costs and improve the quality of life for millions of people.
Even kids are targets of sodium reduction efforts: Massachusetts schools will soon be serving reduced-sodium lunch items and snack offerings. "Ninety percent of the sodium our kids eat comes from their prepared foods, not the salt shaker," said Dr. Lauren Smith, medical director at the state Department of Public Health.
Some, though, question whether these public health efforts are overkill, especially for those who don’t have high blood pressure.
Hillel Cohen, an epidemiologist at the Albert Einstein College of Medicine in New York, has extensively studied the link between salt consumption and health outcomes and has found that lower salt levels do not correspond to lower risks for heart problems.
"When the data results are mixed, perhaps we should be less strident with our public health guidelines," Cohen said.
Harvard School of Public Health professor Dr. Walter Willett, a nutrition researcher, suggested the results of the latest study be taken "with a huge grain of salt," since it estimated sodium intake -– which can vary widely from one day to the next -- over an eight-year period with just two measurements. What’s more, the study didn’t include any African Americans, so it’s impossible to say whether the results apply to them.
African Americans tend to have more severe high blood pressure compared with whites and are considerably more likely to have bad outcomes from that hypertension, including strokes and heart and kidney failure.
Dr. Laura Svetkey, director of the Duke Hypertension Center, said observational studies, like the Belgian one, can't show cause and effect, so it’s impossible to conclude that lower salt consumption led to more deaths from heart disease. She said, for instance, there was no way for the researchers to know whether some of the people they studied were advised by their physicians to lower their salt intake because they were developing hypertension. If some of these people died of heart disease during the study, yet had also lowered their salt intake, that might have contributed to the surprising finding that cardiovascular deaths were highest in those who ate the least salt, she said.
What’s clear is just how unclear the science is concerning the effects of excess sodium in those who are at low risk for heart disease.
There is stronger evidence for other lifestyle factors when it comes to preventing or reversing hypertension. “In most people who are overweight or obese, even small amounts of weight loss can lower blood pressure more than reducing salt intake,” Svetkey said. Research suggests that overweight folks who lose just 5 percent of their body weight –- 11 pounds for those who weigh 220 pounds -- can cut their risk of developing high blood pressure almost in half.
The second most powerful way to lower blood pressure without medication, Svetkey said, is through diet -- eating mainly fruits, vegetables, whole grains, and low-fat dairy foods, and severely cutting back on animal fats, sweets, and sugary beverages. Exercise and lowering alcohol intake also may slightly lower blood pressure.
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