In another blow against decades of accepted medical wisdom, one of the most prestigious, long-running studies reports that lowering sodium intake doesn’t reduce blood pressure.
The study also implies that most Americans are consuming a perfectly healthy amount of salt, the main source of sodium. But those who are salt-sensitive — about 20 percent to 25 percent of the population — still need to restrict salt intake.
Consuming less than 2,500 milligrams of sodium daily is actually associated with higher blood pressure, according to the Framingham Offspring Study report. The American Heart Association recommends consuming no more than 2,300 milligrams of sodium daily, equal to a teaspoon of ordinary iodized table salt.
High blood pressure is a known risk factor for heart disease and stroke. Hence, lowering salt intake is supposed to lower blood pressure and thus reduce the risk of cardiovascular disease and stroke. But the study found that supposition to be unfounded.
Moreover, the lowest blood pressure was recorded by those who consumed 4,000 milligrams or more a day — amounts considered dangerously high by medical authorities such as the American Heart Association.
Those taking from 2,500 milligrams to 4,000 milligrams a day had very slightly higher blood pressure, but significantly below the low-sodium group. The average American consumes 3,400 milligrams of sodium a day.
Higher levels of calcium, potassium and magnesium were also associated with lower blood pressure. The lowest readings came from people who consumed an average of 3,717 milligrams of sodium and 3,211 milligrams of potassium a day.
The study is an offshoot of the groundbreaking Framingham Heart Study. Both are projects of the National Heart, Lung and Blood Institute and Boston University.
The new report was delivered in Chicago during the Experimental Biology meeting by Lynn L. Moore, an associate professor of medicine at Boston University School of Medicine.
The report directly contradicts advice from the American Heart Association, which recommends consuming less than 1,500 milligrams of sodium a day to reduce blood pressure and risk of heart disease.
The American Heart Association justifies its recommendation on a 2001 study in the New England Journal of Medicine. The study is cited in a “scientific statement” by the association.
The NEJM study examined 412 participants — both those with and without hypertension — for 30 days. They were randomly assigned to eat either a control diet or the Dietary Approaches to Stop Hypertension, or DASH, diet, which is rich in vegetables, fruits and low-fat dairy products.
The Framingham Offspring Study based its findings on a population of more than 2,600 men and women, whom it followed for 16 years. That means it can capture the long-term results of salt consumption, which the New England Journal of Medicine study couldn’t do because of its short duration.
Cheryl Anderson, a member of the American Heart Association’s Nutrition Committee, said the study appeared to have some weaknesses in data collection, according to its abstract. Anderson didn’t attend the presentation, because she was delivering her own at the same time.
The best standard for such studies of sodium intake is a 24-hour multiple urine collections protocol, which the study didn’t have, said Anderson, an associate professor in the Department of Family and Preventive Medicine at the University of California, San Diego.
The study used dietary records, which aren’t as accurate as direct measurement, Anderson said.
And it’s also possible that some of those in the study may have changed their sodium intake because they developed high blood pressure, which would confound the correlation, she said.
“When I put it in the broader context of the general literature around dietary sodium assessment and blood pressure, particularly from what we know about clinical trials, [the study] didn’t bother me in any way,” Anderson said.
“Until I see the whole paper, this is not going in any way to disrupt my current thinking around that fact that high sodium intakes are likely to increase your blood pressure.”