Science Interviews


Mon, 23rd May 2016

What happens when you eat too much salt?

Dr Viknesh Selvarajah, Addenbrooke's Hospital, Cambridge

Listen Now    Download as mp3 from the show The War on 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, 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.


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Doesn't answer the obvious question, that if my blood pressure if excellent, should I still shy away from salt?

I would say crisps, but most of the crisps have replaced salt with flavourings, so the salt content is way down on what it use to be. Simon Waters, Wed, 25th May 2016

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.

I will point out that "low sodium salt" contains significant amounts of potassium chloride (which your body also needs) but has a much smaller window for healthy intake (oral LD50 of KCl is about 3g/kg vs about 12g/kg for NaCl). Too much potassium can cause heart problems... chiralSPO, Wed, 25th May 2016

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).

That target sounded a little high to me.

Australian recommendations are that 1.1-2.3g of salt per day is medically adequate (but Australians average about 10g/day!). See:

So it seems that the "target" is a target to get down below, not a target to aim at. evan_au, Wed, 25th May 2016

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?

Apropos "Lo Salt": I use it to demonstrate natural radioactivity. Not only is KCl a favourite covert murder weapon, it's also radioactive!  alancalverd, Wed, 25th May 2016

Yes, I considered mentioning the radioactivity in my first post on this thread, but opted against it in case someone didn't get my sarcastic claim that it was dangerous. Detecting the decay of 40K in Low Sodium salt is an excellent high school/undergraduate demonstration--I wish more people had seen/done it... chiralSPO, Wed, 25th May 2016

It seems that there is a chronic rise in blood pressure with increasing salt burden over time. No one knows why, but salt does appear to affect endothelial function (lining of blood vessels), which might be part of the reason. The evidence is substantial that genetically-matched individuals who are / are not exposed to salt intake have higher and lower blood pressures respectively, as they age. chris, Wed, 25th May 2016

I've often wondered whether such correlations as salt/blood pressure and smoking/cancer have a more subtle element to them.

Given a genetically matched pair, i.e. identical twins, why would one habitually consume significantly more salt, sugar, tobacco, alcohol, whatever.... than the other? If we exclude genetics, it must be environment. So who eats lots of snack foods and smokes a lot? My first guess would be the person with the most stressful life. Perhaps this can account for the anomalies of those heavy users who live to a ripe old age "against the odds".

An old colleague used to open his lectures on heart disease by pointing out that the French smoke more, drink more, and eat more fat than the British, but have a lower incidence of cardiovascular death. Why? Because they enjoy smoking, drinking and eating, and don't worry about rules, regulations, tax deadlines, government health warnings, working hours, or any other sources of stress.    alancalverd, Thu, 26th May 2016

Only excess salt in your body is bad, your body needs salt to sustain its life. Alan McDougall, Fri, 27th May 2016

You're right, Alan, that there are lots of potential confounding factors, but the Intersalt study addressed this by including groups who are ancestrally related and inhabit similar environments but have different dietary practices ie one group adds seawater to cooking, the other does not.

This found the relationship alluded to above: dietary salt burden is linked to blood pressure elevation with age.

I absolutely agree with you though that there are lots of other very strong environmental drivers. chris, Sat, 28th May 2016

Prolonged exposure to high plasma sodium concentrations i.e. >140mM induces a fluid shift from the interstitial to intravascular space by facilitating deterioration of the negatively-charged heparan sulphate moiety of the endothelial glycocalyx - thus upregulating Na entry into endothelial sodium channels.
exothermic, Sat, 28th May 2016

Pflugers Arch. 2011 Oct;462(4):519-28. doi: 10.1007/s00424-011-0999-1. Epub 2011 Jul 28.
Salt overload damages the glycocalyx sodium barrier of vascular endothelium. exothermic, Sun, 29th May 2016

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