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Open Season on Salt: What the Science on Hypertension Really Shows

By Melinda Wenner Moyer
9/26/2012 12:43:04 PM

The latest news reports about salt are enough to make a parent ponder a household ban on pizza and cold cuts. A study published last week in Pediatrics found that children eat, on average, 3.4 grams of sodium daily-more than twice the amount recommended for adults by the Institute of Medicine (IOM). News outlets, including the Associated Press and USA Today, explained that, according to the study, the quarter of American kids who eat the most sodium are twice to three times as likely to develop high blood pressure as the quarter who eat the least. The take-home message from these stories is clear: kids need to cut down on salt or they will suffer serious health consequences.

Image: Flickr/reallyboring

It's a compelling argument. Problem is, it may be wrong.

The study that these articles reference, which was published by researchers at the U.S. Centers for Disease Control and Prevention (CDC), did not actually find a statistically significant association between salt intake and blood pressure in kids. And the doubling or tripling of risk described by some outlets isn't an accurate portrayal of the findings either. As lead author Quanhe Yang explained to Scientific American in an interview, high salt intake doubles the odds that kids have hypertension or pre-hypertension (and again, this doubling is not statistically significant), but odds and risk are two very different things. "I am not sure the best way to convert this odds ratio into a risk ratio," Yang says, but if he had to guess, the risk would probably be lower than the odds.

Yang's study does provide compelling insights. It shows that among obese or overweight children, increased salt intake is linked to higher blood pressure, an association that is statistically significant. Scientists have long known that obesity increases hypertension risk in adults and kids, but the CDC's study suggests that being overweight might also make kids more sensitive to salt's blood-pressure-boosting effect.

Still, the kids' blood pressure changes were not huge: The overweight children who ate the most sodium-an average of about 4.6 grams per day-had an average systolic blood pressure (the top number in the blood pressure ratio) of 112.8 millimeters of mercury (mmHg), whereas those who consumed the least-an average of 2.3 grams of sodium-had an average systolic pressure of 109 mmHg. (The two groups' average diastolic pressures, the bottom number in the ratio, were the same.) In other words, among overweight and obese kids, a doubling of sodium intake was associated with a 3 percent increase in systolic blood pressure. This difference may not be clinically significant for individuals, because "systolic blood pressure changes from minute to minute" by as much as 5 mmHg, says Michael Alderman, a professor emeritus at Albert Einstein College of Medicine and editor in chief of the American Journal of Hypertension.

An average systolic blood-pressure difference of 3 percent could, however, have consequences for overall public health. But Yang says that it's impossible to tell from his study whether eating more salt actually causes blood pressure to rise. "This is a cross-sectional study; we cannot say anything about causality," he explains. Although he and his colleagues tried to control for potentially confounding variables, it's possible that kids who eat more salt also have other habits that predispose them to high blood pressure. (For instance, research suggests that children who eat lots of salt also drink lots of soft drinks, which are associated with blood pressure increases, too.) Indeed, research doesn't always support the notion that salt causes high blood pressure: A large, multicenter study known as INTERSALT compared urinary sodium levels-an accurate indicator of prior sodium consumption-with hypertension in more than 10,000 people in 1988 and found no statistically significant association between them. In fact, the population that ate the most sodium had a lower median blood pressure than the population that ate the least.

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