Headlines declared the news this week that low-sodium diets don't benefit health. The stories were based on a new study in the Lancet that concluded cutting back on sodium doesn't improve health for most and that it may, in fact, increase the risk of cardiovascular problems.
The findings run counter to messages repeated for years by groups like the World Health Organization and the Heart and Stroke Foundation: that the vast majority of us consume too much sodium and it's contributing to high blood pressure, heart disease, stroke and other health problems. They also highlight an increasingly contentious battle between the health experts who advocate for sodium reduction and the small but vocal group of researchers who say big decreases aren't a good idea. Each side accuses the other of bias and a refusal to acknowledge basic facts.
Dr. Norm Campbell, chair in hypertension prevention and control at the University of Calgary, said in an e-mail that "low-quality research and financial conflicts of interest seem to be the main issues driving the salt controversy."
Dr. Andrew Mente, lead author of the new study and associate professor at McMaster University, said in an interview that the lower-sodium crusaders are biased because they have built their careers on the idea that too much salt is bad for health. Some of his studies have been funded by companies that make blood-pressure drugs and several co-authors have financial ties to those companies, but the suggestion this poses a conflict of interest is "extremely far-fetched," Mente said.
For the average person, these wildly differing positions are confusing, even maddening. The flip-flops are just the type of thing to convince a person to tune out public-health messages altogether.
But looking at the evidence, nothing has changed. The vast majority of studies done have shown too much sodium can have negative health consequences. Until an equally convincing mountain of evidence emerges to suggest that low sodium levels are harmful to health, public-health messages will remain the same.
The study, led by researchers at McMaster University in Hamilton, looked at data from more than 133,000 people from nearly 50 countries. The data was collected from a number of different studies, such as a large trial studying the effectiveness of a blood-pressure medication. The individuals, who ranged in age from 45 to 63, were divided into those who had high blood pressure and those who didn't. The researchers used a urine sample and a blood-pressure reading to guide their analysis. After following the group for a median of 4.2 years, they concluded that individuals with high blood pressure and high sodium levels in their urine were more likely to suffer a serious heart problem or die. But for people without high blood pressure, elevated sodium levels were not linked to poor health outcomes. In fact, the researchers say, people with low sodium levels were tied to a "significantly increased risk" of death or a serious cardiovascular event.
"Our findings suggest that sodium reduction should be confined to only those individuals with hypertension and high sodium intake," the researchers concluded.
But there are several major problems with this analysis, severe enough to have prompted the American Heart Association to issue a strongly worded statement condemning the study.
The biggest issue singled out by experts is how sodium levels were measured. Instead of doing a 24-hour urine test, which allows researchers to get a fairly accurate picture of a person's sodium intake in a day, researchers relied on a spot urine test – a single sample – and used a formula to then estimate sodium levels. Researchers also didn't collect data on what people had eaten the day the sample was taken. Dietary histories are often collected to help experts get an accurate picture of an individual's sodium consumption. Mente said that this method is accurate.
But Dr. Elliott Antman, professor of medicine at Harvard Medical School, said using a spot urine test is an extremely unreliable way to estimate an individual's overall sodium intake. For instance, a woman who eats a healthy diet may have indulged in a salty meal the previous night, which could skew the results. While the researchers used a formula to get a more accurate 24-hour measurement, Antman analyzed their data and found their estimates weren't accurate in many instances.
It's also a leap to base conclusions about a person's health on one snapshot, said Antman, who is also a senior cardiologist at Boston's Brigham and Women's Hospital and immediate past-president of the American Heart Association.
"It's unrealistic that even a single measurement could reliably inform you about the relationship between sodium intake and cardiovascular events," he said.
Mary L'Abbé, professor and chair in the department of nutritional sciences at the University of Toronto, said spot samples "cannot be used to estimate the sodium intakes of individuals."
L'Abbé highlighted the fact this group of researchers has previously published similar findings. A main difference in the new study is that they included more participants.
"However, more poor quality data doesn't make a poor quality study any better," L'Abbé said in an e-mail.
Existing health problems
What about the conclusion that people consuming low amounts of sodium – less than 3,000 milligrams a day – are more likely to die and suffer serious heart problems? According to Mente, going below this level can trigger a series of events in the body which can explain the risks.
But the American Heart Association said there's a more likely explanation. The study included people with serious illnesses, including heart failure and heart disease. Those individuals may be following a strict diet or be eating very little food because of their illness. Because those people have a number of health risks, it makes sense the study found they experienced a greater number of heart events and deaths. But it doesn't mean their low-sodium diet caused the problem.
Better health news
It's easy to point the finger at media organizations that pick up on conflicting health news, and they do deserve some criticism. But in many cases, universities, hospitals and other reputable organizations are only too happy to spin questionable or potentially misleading research to attract interest. The press release written for the Lancet sodium study declared "Low salt diets not beneficial" and stated they may "increase the risk" of heart problems and death.
In the end, the consumers of these stories are the ones who lose when they hear conflicting messages and aren't in a position to tell what's real. It's in everyone's best interests for research to continue on all sides of a debate. But trying to sway the public on an incomplete story is not responsible.