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Response to controversial Lancet paper arguing against salt reduction programmes
A study published in science journal, The Lancet, this May argues against lowering salt intake to recommended levels in the general population, but there are a number of important flaws in the research.
This research combined the results of four studies looking at sodium levels in urine and whether they’re related to heart disease and stroke in people with and without high blood pressure.
The researchers found that in people with high blood pressure, both high and low sodium levels (reflecting a high or low salt intake) were linked to a higher risk of heart disease and stroke, but not sodium levels in between. In people with normal blood pressure, high sodium levels did not raise the risk of heart disease and stroke, but low sodium levels did.
Based on this, the authors argued that salt-reduction programmes should only be targeted at people who already have high blood pressure and who eat a lot of salt, and not the general population.
This study doesn’t stand up against the argument for lowering salt.
There are two major problems with this research, and it doesn’t stand up against the overwhelming evidence that lowering salt intake lowers the risk of heart disease and stroke. They explain that because of these flaws, this study does not overturn public health messages to reduce salt intake. The recommended maximum salt intake in the UK is 6g per day.
Two important flaws in the study
1. The studies measured sodium levels in urine at just one time point in each patient, but it has been demonstrated that a single urine test does not reflect a person’s salt intake. In fact, you would need to collect up to 11 24-hour urine samples (where the total amount of urine passed in 24 hours is collected and tested) to get a good idea of how much sodium (or salt) someone eats.
2. The study authors don’t discuss the problem of reverse causality. That is, when people are very unwell they eat little or no food, and as a result their salt intake is low. So it’s not the low salt intake that is causing their death, but rather, their illness that is causing their low salt intake.