Salt restricted diets have been a mainstay of treatment for high blood pressure therapy for over 100 years. A study published this month in the Journal of the American Medical Association challenges this view, purporting to link higher salt intakes with reduced risk of cardiovascular disease.
In this new study, the authors collected the urine of about 3600 European adults over a single day period, and analyzed the content of sodium. This group was then followed over a period of about six years, and statistics were kept on cardiovascular events (like heart attack and stroke) and blood pressure.
There were a number of controversial findings in this study, but I am particularly interested in the conclusion that people who eat more salt have less risk of dying from cardiovascular disease. This conclusion has generated some controversy as it flies in the face of current scientific consensus.
While it may be true that in this small group of people, diets lower in salt were correlated with better heart outcomes, I believe the limitations of the data make it unlikely that these findings are important. Among the many limitations, here are what I believe are the most important:
- This is a pretty small group for this type of prospective cohort study. We’re only talking about 84 cardiovascular deaths here, a number small enough as to be potentially quite influenced by chance.
- A single day of urine sodium output is a poor marker for salt intake, potentially influenced by hydration, kidney health, other mineral intakes, etc. Dietary recall over a number of days would probably have been a better measure, and would have given more robust results.
- The amount of sodium intake in the untreated population is very high, on average about twice that seen in a salt restricted diet, so even the “low sodium” people in this study were likely to have been eating more salt than recommended for blood pressure reduction.
- Since there was no intervention in this study (the authors just observed patient habits, they did not attempt to change them), it is potentially true that people who knew they had higher cardiovascular risk were more careful about their salt intake. It does not appear that the authors corrected for this potential problem.
It does appear possible that there are some specific situations where salt restriction may lead to increased problems, most specifically higher risk of kidney problems in diabetics and people with heart failure.
For the most part, though, studies with better design have shown that low salt diets are modestly helpful in preventing or treating high blood pressure. For about a quarter of people with high blood pressure, the effect is even greater, making salt restriction a key therapy in blood pressure control. Since high blood pressure is one of the most important risks for heart attack and stroke, even small reductions of individual blood pressures can have dramatic effects across a population.
I think we’ll continue to see debate over how much salt restriction is appropriate, and whether this intervention should only apply to certain individuals or to everybody. But I think we should avoid allowing a single study with a relatively poor design stand a century of nutrition research on its head.
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