New Study Finds no Connection between Salt And Heart Disease
A controversial new study is questioning the oft-repeated connection between the consumption of too much salt and the development of cardiovascular disease. The meta-analysis, published online today in the American Journal of Hypertension1, examined the results of seven clinical studies and found no solid proof that reducing salt consumption prevents heart conditions.
While the World Health Organization suggests that people should not consume more than 5g per day, many Westerners eat double this amount. Already, public-health officials are looking into ways to lower the sodium content of food. The Food Standards Agency in Britain is helping food producers to lower sodium levels. New York City's health department has launched a nationwide initiative to decrease salt intake by 20% for Americans over five years. Nature reviews the implications of this new study for policies.
Salt and Heart Disease: How can it be caused?
Consuming salt causes your body to retain water. This can increase blood pressure and lead to heart attacks, strokes, and other cardiovascular problems.
Rod Taylor, an statistician at University of Exeter (UK), led this new study. He said that a variety of studies and meta-analyses suggest that reducing one's salt intake may reduce blood pressure. However, his research team states that the ability to reduce salt intake enough is not certain to help prevent heart disease. An earlier meta-analysis showed that salt intake did not reduce blood pressure, but it had an average decrease of 0.2 points.
On the other hand, many studies comparing how much salt people consume with their incidence of cardiovascular disease have come up with clearer links. A 2009 meta-analysis3 of 13 such studies, incorporating 177,000 patients, found that a high-salt diet increased the risk of stroke by 23%.
Is it really so hard to show that cutting salt can prevent cardiovascular disease?
Observational studies, which look at the correlation between salt intake and the incidence of disease, can't directly pin reductions in cardiovascular disease on eating less salt, Taylor says. It is possible that people are choosing to cut down on salt but this may also be linked with other good behaviors. People may exercise more and eat less saturated fat, which are both factors that can protect them from cardiovascular disease.
"To inform policy and whether we should be advising people to reduce their salt, observational studies do fall short," Taylor adds. Taylor says controlled experimental studies, where patients are put on low- or high salt diets and monitored over time, will provide more clarity.
What was the New Study?
Taylor's team trawled through 2,600 published journal articles on the link between salt and cardiovascular disease, and came up with seven controlled studies that included a total of 6,250 patients who were tracked for 6 months or longer. Taylor's group divided the patients into three groups: those with normal blood pressure, high blood pressure, and those who had been diagnosed with heart disease. They then analyzed whether salt intake affected blood pressure, cardiovascular disease incidence, and death rates.
Low-salt diets resulted in a drop in blood pressure. Taylor's group found that there was no statistically significant change in heart disease risk in those who followed a low-salt diet versus people who did not. A low-salt diet did not result in lower deaths rates among people with high or normal blood pressure. Taylor states, "In one experiment in heart-failure sufferers, we found worryingly that salt reductions increased the death risk."
Why is this result different from those of other observational studies?
Taylor isn't sure why his team's review came to a different conclusion from previous observational studies. Taylor suggests that it could be because there's no connection between salt reduction and the prevention of cardiovascular disease. Taylor questioned this interpretation, as his team saw a reduction in blood pressure.
Maybe the results of this study were not statistically significant because they did not include enough patients. Francesco Cappuccio (who heads the World Health Organization Collaborating Centre for Nutrition in Warwick, UK) raises this possibility. Cappuccio states that the only issue is that the results are not statistically significant. The reason is because the meta-analysis was too small. It was noted that low-salt diets were showing a protective trend against heart disease.
Taylor thinks the best explanation is that patients cut their salt intake early on in the studies but eventually allowed their intake to creep up, masking any benefit. According to Taylor, "They are closely monitored for at least two years and their behavior is likely to have reverted back to the way it was."
Is it possible to get the truth about the relationship between salt intake and heart disease through a more extensive, well-supervised clinical trial?
Taylor thinks that studies that rely on dietary advice directed at individuals, like those his team analyzed, do not do enough to cut people's salt consumption. Rather, scientists should look to studies that investigate the effects of public-health efforts, such as clearer labeling of a food's salt content, to see if these can prevent cardiovascular disease and death. "We need to design studies that are population-level interventions," he says, "essentially where we take a community of individuals and we target them in various ways to change their behavior and help them sustain that behavior, rather than just give them a pamphlet and have them sit down with a counselor for an hour."
However, Cappuccio says that such studies are costly, impractical and unnecessary. Cappuccio says that such studies are costly, impractical and unnecessary. Salt, as with many nutritional factors, is one of the areas where it's necessary to act in light of overwhelming evidence. This applies even if there has not been a clinical trial.
Is The New Study Going to Have a Major Impact on Policy?
It depends on who you ask. Michael Alderman is an emeritus epidemiologist from the Albert Einstein College of Medicine, New York City. He believes that the new study confirms the growing body of evidence that salt reductions do not benefit people who only consume small amounts of it. According to Alderman, public policies that force salt reductions can be dangerously misguided. While reducing salt intake may lower blood pressure which can be good for your heart, it also increases insulin resistance and triglyceride levels, all of these risk factors could lead to cardiovascular disease.
Taylor is also concerned that salt reduction policies could lead to unintended economic and health consequences. According to Taylor, "While it is intuitively obvious that salt reduction across all areas of life would be beneficial," he said.
Cappuccio worries that this new study could be used to hide the truth. This will make it harder for public-health officials to persuade or even force food producers to lower salt content in prepared and processed foods. These are the main sources of salt in most Americans' diets. It's creating controversy, he says.