March 5th, 2020

Podcast 255: Salt talks — transcript included

(4 votes, average: 3.00 out of 5)

Here we have an interview with Prof. Feng He, whose English is much better than my Mandarin. Thus, I’ve attached a transcript to make her ideas on salt intake (no level is too low) and blood pressure (there’s a dose-response relation with salt) more immediately available than it might be to your ears alone.

She’s coauthor of an article in The BMJ — a meta-analysis — that finds the effect of salt is greater with age, and in non-white populations and those with hypertension.

Links to articles mentioned and apps:

Running time: 28 minutes

Here is a transcript of the interview:

TRANSCRIPT OF INTERVIEW WITH PROF. FENG HE WHO HAS REVIEWED AND CORRECTED IT

Joe Elia:
Salt and sodium intake have challenged medical researchers for generations. What seems indisputable is that too much salt is not good, especially in hypertension. After that things get fuzzier. National advice on restricting salt intake has been challenged by findings that restricting salt too much can increase cardiovascular risk.

Professor Feng He and her colleagues looked at the question of whether existing studies showed a relation between reductions in dietary sodium and changes in blood pressure. They performed a meta analysis of 133 studies comprising some 12 thousand 200 participants. Those studies all randomly allocated participants to either reduced sodium intake or usual (and thus higher) intake. All studies collected 24-hour urine sodium data to estimate those intakes. In short, they found a dose – response relationship between intake and blood pressure change.

Professor He works at the Wolfson Institute of Preventive Medicine at Barts and the London School of Medicine and Dentistry, Queen Mary University of London. Welcome to Clinical Conversations, Dr. He.

Dr Feng He:
Thank you. Thank you for having me.

Joe Elia:
You have been researching salt for about 25 years. What have you observed about the research over that time? Have you changed your mind about the effect of dietary salt and health and blood pressure?

Dr Feng He:
No, I haven’t changed my mind, because when you look at salt reduction as a public health strategy, you need to look at the totality of evidence. You need to look at all different types of studies, including epidemiological studies, population-based intervention studies, randomized trials in humans, animal studies, and also physiological studies.

So all of these different types of studies have consistently shown that a high salt intake is a major cause of raised blood pressure. Salt intake is also an important determinant of the increase in blood pressure with age. I remember when I was in medical school, we were taught that systolic blood pressure increases with age — that this is a normal physiological phenomenon. But later scientific evidence has clearly shown that this is not normal, and that high salt intake is an important contributor for the increase in blood pressure with age.

If you look at the societies now, you know, there are still some societies isolated to tribes. They don’t have added salt in their diet, and that’s like our human ancestors, And their blood pressure, their average blood pressure for the adult population is only 90/60 mm Hg, and their blood pressures do not increase with age and these populations do not have cardiovascular disease at all. You may say, “No, people in isolated tribes die at a much younger age.” However, if you compare those isolated tribes with Western populations of the same age group, in Western societies so many people suffer or die from cardiovascular disease.

Joe Elia:
Yeah. The fact that the research is ongoing tells us that not everyone believes that the results are final yet. Would you agree with that?

Dr Feng He:
No. Actually you know, you never get a final result because you know what? For any dietary factors, it is extremely difficult to do randomized outcome trials. Remember, for salt reduction you have to randomize many thousand individuals to the high-salt or the low-salt diet and keep these two groups for many, many years to see whether there’s a difference in cardiovascular outcome.

The problem with this is, firstly, there’s overwhelming evidence that shows that a high salt intake is harmful to human health. It’s unethical to put a group of people on a high-salt diet for so many years.

Secondly, you know, on the lower-salt group in the current food environment, it is extremely difficult for individuals to keep to the lower-salt diet for many, many years. That’s almost impossible in the current food environment.

And thirdly, many countries have started salt reduction initiatives — governments and NGOs have programs as do health professionals. Even in the media — in the control group if you want them to be on the high-salt diet, they’ll receive all these messages from the media — from the radio, from television, from newspapers. They [high-salt controls] will lower their salt intake, too. So in the end you wouldn’t see a difference in salt intake between the two groups and then there’s a severe contamination between the two groups.

So it’s difficult to do such outcome trials you know, to keep two groups on the low and the high salt there for many, many years.

Joe Elia:
I see. Is it possible to lower salt intake too much?

Dr Feng He:
No, if you look at those isolated tribes, they don’t have added salt to their diet. You know, there’s lots of food, and some of the food naturally already has sufficient salt, like meat. Our ancestors, they did not have added salt, but they lived healthily. Now in the isolated tribes, they are still living in the lifestyle of hunter-gatherers. They don’t have added salt. Their salt intake as measured by 24-hour urinary sodium excretion is less than 1 g a day. It’s much lower. You know, they live perfectly well. They don’t have cardiovascular disease.

So, you know, in our current food environment in Western societies there’s no way you could reduce salt intake to such a level. Currently in most countries the average salt intake is about 10 g per day and the WHO recommended level is 5 g per day. And for the US it’s 6 g per day and for the UK it’s also 6 g per day for the general population.

However, for the US, in almost half the population, the target actually is much lower. It’s 4 g per day for individuals with high blood pressure, people of African origin, and people with kidney disease, because they are at increased risk. So their target should be even lower — to 4 g per day.

Joe Elia:
Well, what prompted you and your colleagues to undertake this meta-analysis?

Dr Feng He:
This actually is an updated meta-analysis. I don’t know whether you know, we have published a meta analysis in 2013 in the same journal, The BMJ. At that time what we looked at was a modest reduction in salt intake and over a longer duration as the current public health accommodation. For example, from 10 g per day to 5 g per day to see whether that had significant effects on blood pressure. What we showed was that a modest reduction in salt intake for a longer term, like for a duration over a month or longer, there’s a significant effect on blood pressure in both hypertensives and normotensives.

So for the new meta-analysis, not only did we update it to include many more trials but also there’s a focus on the dose-response relationship with salt reduction and blood pressure. And we also looked at duration, whether the duration [of reduction] has any effect on the effect of the salt on blood pressure.

So this time the inclusion criteria were different. We basically included almost all of the salt reduction trials with 24-hour urinary sodium measurement. So with these many studies we have shown a clear dose-response relationship; so the greater the reduction in salt intake, the greater the fall in blood pressure.

The current public health recommendation is from the current level of approximately 10 g per day to the WHO-recommended level of 5 g per day. That will have a significant effect on lowering blood pressure. However, if you lower salt intake further, down to 3 to 4 g per day, the effects on blood pressure would be bigger. So there’s a clear dose-response relationship: the greater the reduction in salt intake, the greater the fall in blood pressure.

Joe Elia:
So you found that lowering salt intake is good for blood pressure, even among people without hypertension. But you were careful to limit your findings to blood pressure and not cardiovascular disease or other things, and…

Dr Feng He:
This is very good question. Because as I mentioned earlier, it’s extremely difficult to do a randomized trial for cardiovascular outcomes. Hardly any trials have looked at the longer-term salt reduction on cardiovascular outcome. And having said that, I don’t know if you have seen our previous meta-analysis published in the Lancet. Basically because there’s an insufficient number of studies looking at long-term salt reduction on cardiovascular outcome, what we looked at in that meta-analysis was the trials whose aim was to look at the blood pressure.

However after the trials completed, the researchers followed this population up for many years after the trial, so even that type of study you know, wasn’t exactly a long-term outcome trial;  but still, that type of study has indicated that a reduction in salt intake has a significant effect on reducing cardiovascular events.

Joe Elia:
But the question that you asked was a simpler one and in addition to lowering blood pressure across the board, you found that studies that were of a short duration, for instance two weeks or less, didn’t show the effect as much as those studies that were longer-term. Is that right?

Dr Feng He:
That’s right. That’s right. Basically these studies show that the dose-response relationship is much stronger in the longer-term trials compared to short-term trials. So it’s likely that the shorter-term studies have underestimated the impact of salt reduction. The problem with looking at the duration and looking at the effect of duration on the effect of salt reduction is quite difficult, because at the moment not many longer-term salt-reduction trials, only a few trials have had a duration lasting six months or longer.

The problem with this type of trial is, initially, people achieve their reduction of salt intake, but with time you know, with the current food environment it’s impossible for individuals to keep the lower-salt diet for long term. So by the end of, say, a few years their salt level has already gone back to the higher level, so that’s why longer-term studies cannot see a greater effect. Because over the longer term they have not achieved a greater reduction of salt intake.

So if you really want to look at the long-term effect, the duration, the effect of duration on blood pressure, we should have longer-term trials, with the individuals kept on the low-salt diets throughout with multiple measurements of blood pressure throughout. The only study that can show this is the DASH Sodium study. I do not know whether you have heard of the DASH Sodium study. In this DASH-Na study the compliance is perfect because it’s a feeding study. All of the food and the drinks are provided to the participants. So the individual can keep the lower salt diet over the whole study duration. That study has shown that, with a longer duration the effect of salt reduction on blood pressure is bigger, compared to short-term study.

A footnote: Dr. He wanted to add this observation after her comments on DASH: “Countries that have achieved a reduction in salt intake for several years, for example, Finland and the UK, have demonstrated a much greater impact of salt reduction on population blood pressure”.

Joe Elia:
The effects seemed especially stronger in older people, non-white populations, and those with higher baseline systolic pressures.

Dr Feng He:
That’s right.

Joe Elia:
Okay. And so I guess that’s the population it would seem that would have been exposed, especially older people, to this kind of food environment as you describe it, that is going to have loaded their bodies up with salt over many years, isn’t it.

Dr Feng He:
That’s one of the reasons but there are other reasons. In our human body we have these hormonal systems like the renin-angiotensin system, and this system is actually maintaining our blood pressure. And because for individuals with older age and people of African origin and also people with high blood pressure, their renin-angiotensin system is suppressed. And so usually, like in young people, if you reduce your salt intake, the renin-angiotensin system would react and then there’s an increase in plasma renin activity and increase in angiotensin II.

This is like a compensatory mechanism to maintain our blood pressure. So for elderly and for people of African origin and also for those with high blood pressure, this system is not as active as in young people or compared to their counterpart in the white population or people with normal blood pressure. That’s one of the mechanisms for those subgroups to have a greater fall in blood pressure for a given reduction in salt intake.

Joe Elia:
What do you think these findings mean for people who are skeptical about over-restricting salt intake? There have been some researchers in Europe — I know of one group — that have data saying that over-restriction of sodium leads to greater cardiovascular risk.

Dr Feng He:
Yes, I’m fully aware of these publications. We have published several papers and there’s lots of debate about this. The problem with their studies is that there are severe methodological problems. For example, their study measured salt intake using spot urine. Spot urine measured sodium concentration. For example, if I just have two glasses of water now, and two hours later I collect spot urine. If you measure my sodium concentration in that spot urine, it’s much lower because it’s diluted.

And also this spot urine, they converted spot urine sodium concentration to 24-hour urine sodium to estimate individuals’ daily salt intake.

Joe Elia:
I see.

Dr Feng He:
They used a formula to convert this spot urine sodium to 24-hour urine sodium. This formula included age, gender, sex, height, weight. We all know age is an important determinant of any health outcome and death, and that age is also associated with salt intake. And also the other factors — gender and body weight and a 24-hour urine creatinine — all of these factors are important confounding factors because they both related to salt intake, and also related to health outcome.

So in this study you know, they can’t control such confounding factors.

Joe Elia:
I see. Okay.

Dr Feng He:
So there’s a lot of methodological [errors]. We call it measurement errors. Using spot urine is one of the contributing factors for the J-shaped findings. We published a paper in the International Journal of Epidemiology and another one in Hypertension, and clearly showed that this formula — the variables like age and gender, height and weight, the creatinine concentration — they all are important contributors to the J-shaped findings.

That’s the only one of the factors. Another factor in lots of cohort studies is that they included people who are not well, who are sick. And this is called “reverse causation,” because we know that if you are not well you can’t eat, and then you have lower salt intake. And then because you’re not well, you have a chronic disease, you’re more likely to die, so the lower salt intake in these individuals is the consequence of their underlying disease and it’s [lower salt intake] not the cause. So there are lots of problems with the J-shaped findings. And so you see our recent paper [which] clearly shows that these different factors have contributed to the J-shaped findings.

If you use accurate measurements of salt consumption like we did, we analyzed the Trials of Hypertension and Prevention [TOHP] follow-up data. Actually that study was done in the US, and we collaborated with Professor Nancy Cook at Harvard University [Brigham and Women’s Hospital]. In the TOHP study, all the participants had multiple nonconsecutive 24-hour urine measurements and measured their salt intake, and so if you look at it, this salt intake, you can see there’s a clear linear association. So the lower the salt intake, the lower the risk of death, down to a salt level of actually 3 g per day. There’s no J-shaped or U-shaped relationship.

Joe Elia:
There is none. Okay. What do you hope will happen as a result of your continuing work on this and your current published analysis?

Dr Feng He:
I hope definitely there’s a clear message that salt reduction is extremely beneficial to the whole population, not only in those with high blood pressure but also in individuals with blood pressure in the normal range. So firstly, the general public need to be more salt-aware and also reduce their salt intake. And for the clinicians, they need to give their patients appropriate advice on how to reduce their salt intake, because in our clinic, sometimes the patient will say “Oh, my salt intake must be low because I never use salt in my cooking or at the table.”

But when we measure it, you know that salt intake is extremely high. The patients, they did not know that the food they usually eat — bread, breakfast cereal — are really high in salt. So in most of the Western countries like the US, UK, and in many other developed countries, about 80 percent of salt in our diet is added to our food by the food industry.

So for the food industry, they needed to make a gradual and sustained reduction in the amount of salt they add to all of their products. And the UK has been very successful in reducing the population salt intake. In 2000, 2003 we [the UK] started a salt reduction program in collaboration with the Food Standards Agency and also our group Action on Salt. What we did is to set incrementally lower salt targets for over 85 categories of food.

And the principle is a small reduction — a 10 to 20 percent reduction — and then you repeat it at two- to three-year intervals. And if you do it gradually, you know, small reductions, the general public wouldn’t notice any difference in their taste. And they can continue to buy the food that they usually buy and their salt intake will come down. The UK salt-reduction program has been really successful. From 2003 to 2011 salt intake in the population was reduced by 15%, from 9.5 g per day in 2003 to 8.1 g per day in 2011.

So this you know, 15% reduction in population salt intake has led to a significant reduction in population [systolic] blood pressure by 2.7 mm Hg, and this was associated with a significant reduction in population mortality from stroke and ischemic heart disease.

Joe Elia:
Oh. I think we should all be reading labels more carefully when we buy food.

Dr Feng He:
Definitely. For individuals in developed countries it’s important that when we do shopping, we choose the lower salt option, and actually now there’s an app available. You can use this app and scan the bar code and then it will give you the lower salt option and you know, there is a similar product that tells which ranks high in salt, which ranks low in salt.

Joe Elia:
What is the name of that app?

Dr Feng He:
It’s [Salt Switch]. Actually there’s a more comprehensive one, it’s called Food Switch.

Joe Elia:
Just a footnote here, Dr He contacted me after our interview and wanted to be clear that she had misspoken about the name of one of the apps. They are Salt Switch and Food Switch and I’ve included links to both on the website, podcasts.jwatch.org.

Dr Feng He:
And then in the UK, Australia, and in China, and in India, there’s an app, freely available for download and when you go shopping you just scan the bar code and it will give you the “traffic light” labelling. It tells you which one is healthier, and it gives you alternatives to buy.

Joe Elia:
Well, I want to thank you very much for talking with me today, Dr He.

Dr Feng He:
Thank you. Thank you very much. It’s so good to talk to you.

Joe Elia:
That was our 255th episode. All of them are available free at podcasts.jwatch.org. We come to you from NEJM Journal Watch and the NEJM group. The executive producer is Kristin Kelley, and I’m Joe Elia. Thank you for listening.
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