It turns out that the accepted wisdom on dietary salt, that too much causes dangerous increases in blood pressure, might be a little oversimplified.
The body strives to maintain a constant level of salt in the blood; when there is too much, the body draws water in from surrounding tissues to keep the correct concentration. This process is called homeostasis and is important for physiological function.
However, increases in blood volume cause pressure in blood vessels to rise, which can then lead to a stroke, heart attacks, and other problems.
Over the past few decades our view on dietary salt has become a little more nuanced: research suggests there are two different type of people – about half of all adults are ‘salt sensitive’, meaning consuming salt will increase their blood pressure, while the rest are ‘salt tolerant’, meaning they can consume salt without an increase in blood pressure.
Although these categories have been around for years, people were unsure as to why this is the case. Interestingly, there is a greater death rate for adults who are ‘salt sensitive’ irrespective of their blood pressure, so we know it’s important.
The leading idea was that when a salt tolerant person consumed salt, their kidneys started removing the salt, i.e. the salt was progressively removed so its effects were compensated for.
The corollary was that people who are salt sensitive are unable to do this effectively and so drugs have been developed that work on the kidneys to remove this extra salt.
However, this model has shifted, by studying cosmonauts in the Mars-flight simulations. Subjects were given a constant diet, including around 9g of salt a day, long-term, but there was wild variation in the amount of salt that came out – sometimes less than expected, but sometimes more.
So, how could a person put out more salt than they took in? Only, reasoned German scientist Jens Titze, if there was a store of salt somewhere in the body. He found there was a weekly rhythm of sodium excretion, independent of weight gain, suggesting that sodium can be stored in the body without associated water.
Sure enough, to prove his idea he climbed into an MRI – a machine that uses strong magnets to produce detailed pictures of organ and fluid distribution inside the body – and pinpointed a sodium salt in his skin that didn’t affect water location.
He then convinced others to join him and found the same. This data was initially not enough to convince reviewers.
“I’ve had the impression that not many scientists or clinicians really believed it — or that it was neglected because this area of research was so different from conventional wisdom,” Titze said.
Three further methods have since been used to prove his discovery. It seems now that high blood pressure is caused by a problem with salt storage, as opposed to salt removal. Preliminary data suggests that the cells involved in this balance are a type of white blood cell called macrophages, known to be involved in salt balance in the kidneys.
This work may lead to an entirely new generation of medications for blood pressure; Titze’s lab in Germany and the Mullin’s lab of QMRI here in Edinburgh are exploring these findings. So when you hear someone saying ‘that’ll go straight to my thighs’ about their food, it turns out they might be right in more ways than one.
Image: André Robillard