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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.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. centres 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 per cent 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 per cent 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.
What about the implication that kids would be better off if they simply cut back on their salt intake? Governmental organisations including the CDC and IOM advocate for population-wide salt reduction, but some researchers question the science behind these policies. “Cutting back on salt does reduce blood pressure, but it may not reduce the risk of dying early,” explains Katarzyna Stolarz-Skrzypek, a cardiologist at Jagiellonian University Medical College in Poland.
In a 2011 study published in JAMA The Journal of the American Medical Association, Stolarz-Skrzypek and her colleagues compared the urinary sodium levels of 3,681 people with their risk of dying over the course of eight years. They found, surprisingly, that the more sodium their subjects ate, the less likely they were to die. In particular, the death rate among those eating the least sodium was 4.1 per cent, but it was only 0.8 per cent among avid salt consumers.
One factor behind this strange trend is that low-salt diets do more than just lower blood pressure. “Cutting sodium can cause other physiological changes such as increased resistance to insulin, which can set the stage for diabetes and increase the risk of death from heart disease,” Stolarz-Skrzypek says. “Too little sodium can also increase sympathetic nerve activity, which raises the risk of heart attacks, and boost the secretion of aldosterone, a hormone produced by the adrenal gland that is bad for the cardiovascular system.” A 2011 review published by the Cochrane Collaboration, an international, independent, not-for-profit research organisation funded in part by the World Health organisation, concluded that low-salt diets are associated with “increases in some hormones and lipids, which could be harmful if persistent over time.”
Looked at another way, the CDC study suggests that parents of overweight and obese kids should focus on weight loss, not salt reduction. Multiple studies in both adults and children suggest that weight has a bigger effect on blood pressure than salt does, and once kids reach a healthy weight, eating too much salt may not cause problems. Plus, going from obese to an appropriate weight reduces not only blood pressure but also the risk for conditions such as cancer, depression and type 2 diabetes. Shedding pounds isn’t easy, but considering that an estimated 75 per cent of our sodium intake comes from store-bought processed foods rather than what is added during cooking or at the dinner table, cutting back on salt isn’t either—and ultimately, doing so may not be as beneficial for us as we think it is.
This story was originally published by Scientific American. Reprinted with permission.
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