PLEASE PASS THE SALT – OR NOT? DEBATE ON SODIUM INTAKE (April 12, 2018)
This promised to be fun and interesting – and it was. The session started off with a great lecture from a dietician, Patty Willms, on strategies to reduce salt intake. She highlighted that the food with the greatest contribution to salt intake is not from the salt shaker, chips, fries, or even pizza – but plain old bread. Cold cuts and cured meats, sandwiches, pizza, soup, and chicken make up the rest of the list.
Her simple heuristic for identifying food that was low sodium is whether a farmer could grow it. This underlines the fact that most sodium is indeed added during processing and is not naturally present or even needed. Confusing marketing strategies also make it difficult to understand how much sodium is in anything – see the terms below:
- Sodium free: < 5 mg per serving
- Very low sodium: < 35 mg sodium/serving
- Low sodium: < 140 mg sodium/serving
- Reduced or less sodium: at least 25% less sodium than the usual sodium
- Light in sodium: if sodium is reduced by 50% or more than usual
Needless to say, it is more accurate to actually read the sodium content from nutritional labels. The American Heart Association has produced a useful sodium tracker for those who would be meticulous enough and want to tally things up. Otherwise, there are many apps available – she used a bunch of them and found them usually pretty accurate. The DASH eating plan, however, is a simpler eating plan to follow: just fill half the plate with vegetables. She also provided some good links and resources for grocery shopping, and the ‘Milligrammys,’ awards which go to restaurants with ridiculously high sodium food.
The Debate featured long time opponents, Lawrence Appel and Michael Alderman. They call each other Larry and Mickey, but the moderators had to (almost) separate them before the start when they started arguing on the topic on the floor!
Prof Appel has been a long time clinical, and importantly for this topic, was part of the team that did the aforementioned DASH trial. He showed data from the DASH-Sodium trial: low sodium diet, in addition to the DASH eating plan, resulted in a further decrease in blood pressure of ~ 6/3 mm Hg. What about hard outcomes? Admittedly, there are few outcomes of reducing salt and benefit, but even doing DASH, which was a feeding study of ~400 participants, cost many million dollars, and doing a proper, long-term trial would be prohibitively expensive. There are many cohort studies, but he cautioned against using any of those, given many methodological issues, including confounding, that plague them. Sicker people eat less of everything, including sodium, and are more likely to die, which cannot always be adjusted away.
Lastly, Prof Appel emphasized the intra-individual variation in 24-hour sodium excretion, which makes meticulous measurement based on multiple 24-hour urine samples more reliable, rather than one, or even a spot sample. He also was skeptical of the utility of the entire phenomenon of salt sensitivity, which according to him, is hard to identify and not meaningful at a public health level. The J-curve (greater mortality at both ends of sodium intake, high as well as low) could be a fallacy mostly of undercollection of 24-hour urine at the low end.
Prof Alderman was unfazed by the data shown by his opponent. He emphasized that we prefer to have trials with hard outcomes, especially for an intervention that has been championed beyond guidelines into policy at governmental levels – but we have no such data available for salt. Even the long-term (post-trial completion) data from Trials to Prevent Hypertension (TOHP) showed only a non-significant effect on all-cause mortality. Overall, he cautioned that salt intake, whether in the US, or globally, has remained static over the last few decades, despite a lot of public health measures – which could be taken to mean that it is a default state of usual intake.
Blood pressure is a surrogate outcome, and some intriguing data also suggests harmful effects of low sodium intake on other surrogates, such as renin. His own interest was piqued because he saw mere counseling for low sodium intake didn’t work: indeed, all sodium-lowering and BP-lowering trials are feeding trials. The methods of estimating sodium intake using a spot sample (as was done in the large Prospective Urban Rural Epidemiology [PURE] studies) is methodologically sound. The J-curve exists for all other biological variable – it has to exist for sodium too, as has been shown in many other cohort studies. The question then boils down to what level of sodium intake represents that nadir? According to Prof Alderman, it is not 2.3 grams/day, but more along ~ 2.5 to 5 grams/day.
The author of the PURE study (discussed here on #NephJC) chimed in to confirm the veracity of the J-curve, which is seen whether one believes 24-hour collections, spot samples, or food frequency questionnaires. Indeed, even the CRIC study, in which Dr Appel was involved, showed, that there was a non-linear relationship between sodium and mortality in CKD patients.
Overall, it was a spirited and entertaining debate, and one suspects that enough doubt was sown about the sodium and mortality link that the support for the proposition was lukewarm at the end. Everyone did agree on one thing though: reducing sodium decreases blood pressure, but the question about what’s the safe and necessary target remained unresolved.