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Hypotheses about Salt and Blood Pressure

Summary:
There is a lot we don’t know about salt and blood pressure. There seems to be a positive correlation between salt consumption and high blood pressure, but a lot of that evidence could be explained by the importance of processed food as a source of salt. Processed food tends to have a lot of sugar as well as a lot of salt; if sugar raises blood pressure, that could get falsely blamed on salt. Because some of the relevant environments of evolutionary adaptation for human beings were on the seashore and so probably had a lot of salt, one would suspect that within some reasonable range, our bodies are able to get rid of excess sodium. So unless salt

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Hypotheses about Salt and Blood Pressure

There is a lot we don’t know about salt and blood pressure. There seems to be a positive correlation between salt consumption and high blood pressure, but a lot of that evidence could be explained by the importance of processed food as a source of salt. Processed food tends to have a lot of sugar as well as a lot of salt; if sugar raises blood pressure, that could get falsely blamed on salt.

Because some of the relevant environments of evolutionary adaptation for human beings were on the seashore and so probably had a lot of salt, one would suspect that within some reasonable range, our bodies are able to get rid of excess sodium. So unless salt consumption is very high, I don’t see why we would have a vulnerability to excess salt.

Now think of interventions to lower salt consumption, these might lower blood pressure without salt having been the cause of the high blood pressure in the first place.

Let me describe an analogy. Warfarin acts to as a blood thinner (that is, to reduce the danger of blood clots) by blocking the action of Vitamin K. For the Warfarin to remain effective, the one being treated needs to avoid eating too much Vitamin K or needs to raise the Warfarin dose to compensate for Vitamin K consumed. So Vitamin K can frustrate Warfarin without adjusting the dose. But although less Vitamin K can reduce blood clotting, extra Vitamin K doesn’t increase blood clotting beyond a certain point. The excess would just be ignored or reprocessed by the body.

In the case of sodium, if one has a sodium deficiency then the body might need to reduce the quantity of blood so the sodium concentration doesn’t get too low. That would reduce blood pressure. If blood pressure is too high for some reason other than salt consumption, this sodium deficiency lowering blood pressure might have some beneficial effect.

Note that while it is strange if the body can’t get rid of excess sodium, if there is a sodium deficiency there are fewer ways to get the sodium levels up, mainly conservation and making someone crave salt. If, for medical reasons, the individual doesn’t given in to a salt craving, the body can’t remedy the sodium deficiency that way and has to reduce blood pressure to keep sodium concentrations up, once available avenues of conservation of sodium are exhausted.

I may be totally wrong, but what does this predict? First, that most of the correlation between salt and blood pressure is about the confound of the sugar that tends to come along with salt. This accounts for most of the relationship between salt and is probably a linear relationship accounting for the linear relationship between salt and blood pressure that meets the eye in a scatter plot. Second, intervention evidence that gets the causal effect of salt on blood pressure is mostly about reducing the amount of salt and could work through sodium deficiency reducing blood pressure even if it is easy for the body to get rid of excess sodium. Third, there is a prediction that an experiment that probably wouldn’t pass human subjects review—feeding people extra salt—would not cause a rise in blood pressure.

I am not wedded to these hypotheses; I’d be glad to hear a refutation. I am interested. And I also realize I might be wrong about Vitamin K.

Miles Kimball
Miles Kimball is Professor of Economics and Survey Research at the University of Michigan. Politically, Miles is an independent who grew up in an apolitical family. He holds many strong opinions—open to revision in response to cogent arguments—that do not line up neatly with either the Republican or Democratic Party.

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