August 18th, 2014
An Expert’s Perspective: Why Salt Is Not Like Tobacco and Why Guidelines Are Tricky
At the center of this week’s renewed debate on salt was Salim Yusuf, the longtime influential and occasionally controversial cardiology researcher and clinical trialist based at McMaster University in Hamilton, Ontario. I spoke with Yusuf before the publication of the New England Journal of Medicine papers, which include his own two papers from the PURE study.
Yusuf was troubled by the tone of some of the salt debate. He’s no stranger to scientific controversies and intense disagreement, but “scientific criticism is one thing, personal attack on those who hold a different viewpoint is another,” he said. Because he and others have presented data that suggest that moderately high levels of sodium may not be as bad as some had thought, and that very low levels of sodium may actually be harmful, “we’ve come under direct attack.”
Yusuf wanted to emphasize that the PURE findings should by no means be interpreted as suggesting that there is no relationship between salt and blood pressure, or that elevated blood pressure due to salt is never harmful. “Our data don’t contradict that there is an association between salt and blood pressure,” but, he points out, the relationship is not a simple linear relationship.
He therefore disagrees with some of the details of the accompanying NEJM paper from NUTRICODE, which calculates that each year there are 1.65 million cardiovascular deaths that can be attributed to high sodium consumption. The true number is more likely to be less than half a million, and this fits in with the sensitivity analyses presented in the paper by Dariush Mozaffarian, though these are not generally noticed or publicly discussed.
Surrogates and Sodium, Diet and Tobacco
“We know from a ton of data in cardiovascular disease that you can’t totally trust surrogate outcomes to make clinical decisions,” said Yusuf. He cited the recent ACCORD trials conducted by the NHLBI as only the most recent demonstration of this phenomenon. In that study lowering glucose or lowering blood pressure (by a rather large degree) did not lead to a significant reduction in cardiovascular disease outcomes.
But the low-salt advocates “believe that blood pressure is an incontrovertible surrogate, and we know that is not so.” Based on this belief, “they’re going after salt as the next tobacco.” But salt, said Yusuf, is not tobacco. “It’s not like smoking, where the optimal number is zero.”
“People forget that sodium is an essential nutrient,” required in every single cell in the body. “There will have to be some point where very low sodium is harmful.” He compared sodium to vitamins: “we know that supplementing vitamins well beyond an appropriate amount does no good and may even do harm. Equally, low levels of vitamins cause disease and so there is an optimal range for most nutrients that are key to normal physiology.”
The Responsibilities of Guideline Authors
Guidelines, especially national policy guidelines that make huge impacts on large populations, need to be based on firm evidence, said Yusuf: “If we get it wrong then we really harm a lot of people.” He compared the salt recommendations with the push some decades ago to reduce all kinds of fats and replace it with carbohydrates. The approach of recommending a reduction in all fats (not just saturated or trans) and replacing it with more carbohydrates is now having to be reconsidered and this may have contributed to the obesity and diabetes epidemics. “This is an example of how not getting policy right can have negative consequences.”
But also, said Yusuf, “think of the wasted energy. You can’t go to the lay public and say, ‘Do a lot of things, especially extreme changes to improve your life.’ You can only do two or three things. Your messaging has to be parsimonious.” Similarly, “is it wise to go to the government and ask them to legislate many things? You have to choose the two or three that are the most important and for which we have the most evidence.”
“In the end there are only a few public health policies on diet and lifestyle that we can recommend as a society in order to have credibility and so we have to choose them very carefully and focus on the ones for which we have the best evidence and those which are most feasible.”
In addition, said Yusuf, it is important that when new evidence emerges, guidelines should be re-evaluated objectively “rather than people or organizations digging in, and refusing to consider new evidence even when it may challenge one’s own thinking. Policy must be based on reliable evidence and not on personal positions. The last IOM report was an exercise that was laudable in that, under difficult circumstances, the committee did a fair job in pointing out what was known and areas that needed research. In the last few years, the guidelines on sodium have changed from targets as low as 1500 mg per day to about 2300 (IOM) or 2400 mg/day (NHLBI), and this means that despite the pressure, the new evidence is having an impact. A lot of new information has now become available since the IOM report and so it may be timely to reconvene a group of wise and unbiased individuals, who have not taken firm positions, by the IOM or by a global organization such as the WHO or the WHF, and re-examine current recommendations. People who have a dog in the fight, including me, can provide evidence but should have no role in formulating the policy recommendations. It should be done by those who have no professional or financial conflicts.”