Dr Viknesh Selvarajah, Addenbrooke's Hospital, Cambridge
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, thereís 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 Addenbrookeís 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. Youíre in your 30s. Itís 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 movementÖ
Viknesh - I do.
Chris - ... but for someone to be in that state and to now be being interviewed on a radio programme. Thatís 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, itís been a long, long journey but Iíve been improving slowly and steadily.
Chris - For a kidney doctor who spends a lot of his time worrying about his patientsí blood pressure, this must give you enormous insight into what the consequences can be?
Viknesh - Certainly. Iíve 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? Whatís that guidance based on - whatís the evidence?
Viknesh - Over the last 100 years or so thereís been a number of studies looking at the relationship between salt intake and blood pressure and, without a doubt, itís 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 donít 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. Itís 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? Whatís 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 thereís more water in the blood vessels, theyíre stretching the blood vessels more so, therefore, the pressures going to be higher?
Viknesh - Exactly.
Chris - Why do we think that peopleís salt intake has risen in this way?
Viknesh - Itís interesting. Mose of the salt that we eat isnít from the salt that we add to our food. Itís actually from processed foods. Things like bread, cereals, cheese, sausages and bacon. Weíve 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 youíre saying is that because we all this intake, weíve almost adapted to expect that taste and so when things donít have that much salt, we donít 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. Itís 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 whatís 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. Youíd 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.
Doesn't answer the obvious question, that if my blood pressure if excellent, should I still shy away from salt?
Your body needs both sodium and chloride ions to function properly. However, it is easy to satisfy those needs, and excess salt can lead to problems. Everybody's needs and limits are slightly different, but in general there is a very wide window of healthy salt intake.
The podcast mentioned a "target" salt intake of 6g per day; it also mentioned that in the UK, men average about 9g/day, and women 7g/day (from memory).
Medical textbooks suggest that if your kidneys are working, and you ingest a reasonable amount of potassium along with the sodium, you are unlikely to eat a harmful quantity of salt because your tongue will tell you to stop and your kidneys will excrete the excess. Blood is isotonic with sea water, so everything will taste pretty disgusting before it begins to leach water from your body. But who cares about medical textbooks when the government tells you that salt is bad for you?
I've often wondered whether such correlations as salt/blood pressure and smoking/cancer have a more subtle element to them.
Only excess salt in your body is bad, your body needs salt to sustain its life. Alan McDougall, Fri, 27th May 2016