Why do we need salt?
Why does the body need salt, and what happens if you have low salt levels?
Chris Smith answered this question...
Chris - Well, the reason the body needs salt is because every single one of our cells contains large amounts of salt; salt is the generic term for "ions" - charged particles - and most of the cells in our body, in fact all of the cells in our body, are electrical. In other words they pump these ions from one side of their cell membrane, which is a lipid or oily substance and therefore an insulator, so they pump ions from one side of that membrane to the other and this means there is an electrical potential difference across the membrane of a cell and this means that this gradient - this electrical difference - can be used by the cell to do other sorts of work. So cells, for instance, do have channels through which sodium can flow into the cell and it comes down its potential difference in concentration gradient and the result is that it can be used to pull in glucose at the same time. So sugars can get into cells.
So we need salts in our cells - that's how they regulate their size by bringing water in by osmosis; that's also how they regulate electrical activity. Nerve cells, for instance, couldn't carry information without actually having this electrical gradient across the membrane because all that's happening when a nerve cells fires off an impulse is that you get a sudden flood of sodium in to one patch of a nerve cell. This brings in lots of "plus" to that part of the cell and therefore an electrical signal goes whizzing down the nerve and gets built up and regenerated as it goes down the nerve and it travels at about 50 to a 100 metres a second; so very rapid transmission of information.
So we need salts in our body, we take in salt in our diet, we absorb salts and those salts are also include important things like calcium to make your bone strong but you're also losing salts all the time when you go to the toilet for example you lose calcium, you lose phosphates. This is both in urine and faeces so you have to continuously top up the number of salts that you have in your body because you have obligate or insensible losses.
Kat Arney - But we hear so much about salt being really bad for you in your diet giving you high blood pressure. So if you have a diet that's too low in salt is that also pretty bad for you?
Chris - Well the body is very good at scavenging salt from what you eat and what you drink, so it's very rare for people to get too low a level of salt in the body based on diet alone. Usually there's something pathological going on. Sometimes what happens is that people have a problem called syndrome of inappropriate ADH (SIADH). This is anti-diuretic hormone and the body saves too much water so it scavenges back water and as a result your blood can become too dilute and you have very low sodium levels and this can cause problems with your brain swelling. It can also cause the accumulation of water elsewhere around the body and it can cause heart failure so a bad thing to have. But that can cause low salt levels that can make people feel dizzy...