Nov. 9, 2011 -- Everyone knows that too much salt is bad for you, right? Well, according to new research, not everyone is convinced.
Researchers say that means cutting back on sodium may not have a substantial health benefit.
But critics say the review draws faulty conclusions because it relies on too many small, short-term studies. They say the weight of research evidence shows clear health benefits when people cut back on sodium.
The review is an analysis of data from more than 167 studies of people with normal or high blood pressure who were randomly assigned to eat either high- or low-sodium diets.
It found that eating less than 2,800 milligrams of sodium a day helped lower blood pressure. But the reductions were small -- an average of 1% for people who had normal blood pressure to begin with and 3.5% for people with high blood pressure.
But cutting back on salt appeared to have other effects, too.
People on lower-sodium diets had an average 2.5% increase in cholesterol and a 7% increase in bad blood fats called triglycerides compared to people who were eating more than 3,450 milligrams of sodium -- an amount that's close to what the CDC says the average American eats every day.
Researchers say it's not clear why cutting back on sodium may affect blood fats.
Lower-sodium diets also boosted levels of the hormones renin and aldosterone, which can raise blood pressure. Researchers say that may be one reason that slashing salt from the diet has only modest effects on blood pressure.
"The theory that you can reduce the risk of cardiovascular disease by reducing salt intake and thereby blood pressure is tempting. But our study shows that the effect of reduced salt intake on blood pressure in healthy persons is only 1%," says study researcher Niels A. Graudal, MD, DrMedSci, in an email to WebMD.
"Furthermore, reduced salt intake leads to an increase in lipids [blood fats], which is bigger than the reducing effect on blood pressure. Therefore it is likely that reduced salt intake does not have a beneficial effect. On the contrary the net effect may be harmful," says Graudal, who is a senior consultant in the departments of rheumatology and internal medicine at Copenhagen University Hospital in Denmark.
The study is published in the American Journal of Hypertension.
Should the FDA Regulate Sodium in Food?
The study is being released just a day before the FDA is scheduled to hear public testimony on reducing sodium in the food supply.
The CDC says more than 90% of Americans eat more sodium than government guidelines recommend.
Public health experts say high sodium diets are one reason that about one in three American adults has blood pressure levels that are higher than normal.
The FDA doesn't regulate the amount of sodium in food. But in recent months a chorus of public health groups, including the Institute of Medicine and the American Public Health Association, has called on the agency to step in to force food manufacturers to lower the amount of sodium they add to prepared and processed foods.
Last week, in a news release, Michael F. Jacobson, executive director of the Center for Science in the Public Interest, called the high amounts of salt in processed foods "the single deadliest ingredient in the food supply, contributing to the premature deaths of tens of thousands of Americans each year."
But food industry representatives and some independent scientists say salt has become a red herring in heart disease prevention. They say calls to regulate it aren't being based on sound science.
"All the figures that you see that are associated with the benefits of salt reduction are not really benefits associated with salt reduction, they are benefits associated with blood pressure reduction," says Morton Satin, vice president of research at the Salt Institute, which is based in Alexandria, Va.
Satin agrees that reducing high blood pressure is important for health. But he says there are better ways to do it besides cutting out salt.
"There is an impact of reducing salt that is beyond simple blood pressure reduction," Satin says. "Using salt as the main lifestyle means of reducing blood pressure does have negative consequences, and that's really the issue."
Graudal says that's exactly the point. Since doctors don't really understand all the health effects of lowering dietary sodium, they say it's premature to be considering regulation of sodium in the food supply.
Should People Worry About Salt, or Not?
Experts who reviewed the study for WebMD say its conclusions are flawed because it relies too heavily on small, short-term studies.
In some cases people in the studies that were included in the review were only on low-sodium diets for a few days.
"This is a key issue. When there is a large, abrupt reduction in sodium, it takes time to acclimate," says Lawrence Appel, MD, PhD, professor of medicine, epidemiology, and international health at Johns Hopkins Medical Institutions in Baltimore.
"It's very convenient for somebody to summarize a complex area in one paper. But the truth is in the details, and much of that is obscured by the fact that many of these studies are poorly done," says Appel, who is also a spokesman for the American Heart Association.
Other experts agree that the study has some limitations. But they say it raises interesting questions that deserve further study.
"This doesn't change our practice, but it does raise questions about low-sodium diets," particularly for people with normal blood pressure, says Jonathan Whiteson, MD, director of the cardiac and pulmonary rehabilitation at New York University's Langone Medical Center in New York City.
"This is not one-stop shopping. We know that no one medication is the same for one group as it is for another in terms of its effects; therefore, we can't assume that one diet is the same for one population as it is for the next," Whiteson says.
Other doctors say the study is probably correct that changes to sodium are going to affect more than just blood pressure. But they question whether that matters.
"The question is: What does this translate into?" says Tara Narula, MD, a cardiologist at Lenox Hill Hospital in New York City. "Over time, does that mean that somebody has a problem, or is that OK?"
"Certainly I would not tell my patients not to lower their sodium because it might then raise your cholesterol," Narula says. "I haven't seen enough information at this point to make me change my practice, and I'd be hard pressed to find other cardiologists who would say that based on this study that they would not recommend low-sodium diets to people, especially those that have hypertension and heart failure."
"Over 50 public health organizations can't be wrong on this one," says Appel.