What happens when you eat too much salt?

As a nation, the UK are above the intake guidelines for salt, which, for an adult, is 6g per day. To put that into perspective, there's about half a gram in a small packet of crisps, or one ham and cheese sandwich. But what does salt do to our insides? Viknesh Selvarajah from Addenbrooke's Hospital, in Cambridge, researches the impacts of salt and has a very unique perspective on the effects of high blood pressure, as he explained to Chris Smith...

Viknesh - I am a researcher in salt related disease and I had a stroke last year, interestingly enough. I had a bleed in my brain that was typical of a hypertensive stroke, which is a stroke you get with high blood pressure

Chris - Now we should be clear here. You're in your 30s. It's very young for someone, because most people who have strokes are older?

Viknesh - Yes. I had normal blood pressure, I had a healthy diet and I was a runner. So that was really unexpected and unlucky.

Chris - What happened that day?

Viknesh - Well, I was running a half marathon. I came back, I felt unwell and, about eight hours later, I lost my ability to speak. I lost all power of my right side, I lost my vision and I collapsed. I basically was close to dying but, fortunately, the bleeding stopped just in time for me to avoid surgery. The cause of the stroke was never clear.

Chris - Well you have made a very good recovery.  Admittedly, you do struggle a little bit with movement...

Viknesh - I do.

Chris - ... but for someone to be in that state and to now be being interviewed on a radio programme. That's a dramatic turn around.

Viknesh - Certainly. My wife has been very supportive and I have done a lot of physio, and we were actually drilling holes, hanging up curtains this morning. So, it's been a long, long journey but I've been improving slowly and steadily.

Chris - For a kidney doctor who spends a lot of his time worrying about his patients' blood pressure, this must give you enormous insight into what the consequences can be?

Viknesh - Certainly. I've always told my patients to watch their blood pressure because high blood pressure is associated with stroke but, from now onwards, I can tell them I know what it feels like to have a stroke and I know nobody wants to have one.

Chris - What is the relationship between salt and blood pressure? What's that guidance based on - what's the evidence?

Viknesh - Over the last 100 years or so there's been a number of studies looking at the relationship between salt intake and blood pressure and, without a doubt, it's right to say that populations which consume more salt have higher blood pressure. What is also interesting is we used to think that increasing our blood pressure with age was inevitable when, in actual fact, with populations with low salt intake such as indigenous populations, you don't see a rise in blood pressure with age.

Chris - How do you know, when you talk about indigenous populations, that this is not a genetic thing? These people are all genetically the same and whether or not they add salt to their diet is just a confounding variable. It's one of these bystander effects, it has nothing to do with the reality of the blood pressure.

Viknesh - In the very large studies carried out more than 50 years ago called the Intersalt study, looking at indigenous people in their own environment and those people who move to the cities, and they watched these people change when they entered different environments and had more salt, their blood pressures went up.

Chris - Do we know why?

Viknesh - The actual mechanism for salt increase in blood pressure is debatable. Most people think that increasing your salt intake increases the amount of blood volume and that pushes up your blood pressure.

Chris -   Why should eating more salt increase your blood volume?  What's the mechanism of that?

Viknesh - Sodium controls where water goes, so the extracellular volume, which is one third of all the water in your body, tends to be bound to sodium. So, if you eat more sodium you tend to drink more and you tend to keep more volume in the extracellular space.

Chris - So if there's more water in the blood vessels, they're stretching the blood vessels more so, therefore, the pressures going to be higher?

Viknesh - Exactly.

Chris - Why do we think that people's salt intake has risen in this way?

Viknesh - It's interesting. Mose of the salt that we eat isn't from the salt that we add to our food. It's actually from processed foods. Things like bread, cereals, cheese, sausages and bacon. We've become very used to eating processed food and in processed food we have incredible amounts of salt. A slice of bread used to contain the same amount of salt as a packet of crisps. When this was highlighted by an organisation called CASH, the food and beverage industry realised they had to cut back on salt. And that was quite alarming that people were eating salt and not realising how much they were actually eating.

Chris - What you're saying is that because we all this intake, we've almost adapted to expect that taste and so when things don't have that much salt, we don't like them?

Viknesh - We certainly rely on salt to make food taste better. It appears that salt helps us to enhance the taste of sweetness and increase the taste of bitterness and make the flavour of the food taste more full. It's interesting that the processes by which this occurs is not fully understood but it certainly implies that salt makes our food taste better to us.

Chris - So what would you prescribe then - a better chef?

Viknesh - I would certainly encourage people to read the labels on what they buy. If you buy processed food like bread or meat, read what's on the labels. Very often, what you see is not what you expect. We advise that you eat no more than 6 grams of salt a day. You'd be surprised how easily you can reach 6 grams.

Chris - Where did that 6 grams come from?

Viknesh - A number of studies done over the years show that 6 grams is associated with ideal blood pressures and, therefore, reduce risk of heart attacks, strokes, and heart failure.

Chris - And what proportion of people do you think actually hit that 6 grams a day target?

Viknesh - The minority. The most recent survey called the National Diet and Nutrition Survey, the average salt intake for a male in England was 9 grams. The average salt intake for a woman was 7 grams so, overall, we are still eating too much salt.