Does excess salt in food cause thousands of deaths per annum in the UK?

Question

The body responsible for promoting public health in England (prosaically called Public Health England) has faced recent criticism that it it has failed to secure sufficient reductions in the salt added to food.

The Guardian reports that average consumption is 8 grams per adult per day, instead of the 6 gram target, and that this excess is responsible for many thousands of deaths:

Voluntary action by the food industry and retailers has failed to bring the nation’s salt intake down to safe levels and thousands of people have died unnecessarily as a result, say campaigners.

Public Health England (PHE) faced harsh criticism as it published the first comprehensive report on the salt reductions that have been achieved by food manufacturers and retailers in 14 years. It showed that only just over half of the targets were met.

“Such poor progress in PHE’s attempt to reduce salt intake is a national tragedy,” said Graham MacGregor, professor of cardiovascular medicine and chair of the pressure group Action on Salt.

“This report confirms what we know already – that voluntary targets need comprehensive monitoring and guidance but this has been completely lacking from PHE. As a result, thousands of unnecessary strokes and heart attacks have occurred and billions of pounds wasted by the NHS; and tragically more than 4,000 premature deaths per year have occurred.”

But this complaint occurred in the same week as the publication of this meta-analysis that seriously questioned the scientific basis for such scare stories. as MedPAge Today reports:

low-sodium diet has a shaky foundation in heart failure, a systematic review showed.

Out of more than 2,600 studies on sodium restriction in heart failure, only nine small trials with a total sample of 479 -- none of which were free from bias -- made it into an analysis by a group led by Kamal Mahtani, PhD, of the University of Oxford, England.

In the end, the investigators found "no clinically relevant data on whether reduced dietary salt intake affected outcomes such as cardiovascular-associated or all-cause mortality, cardiovascular-associated events, hospitalization, or length of hospital stay,"

We appear to have two views of the science in serious conflict. Who is right? Does the extra 2g of salt per adult per day produce four thousands more premature deaths per year in the UK?

Note: versions of this question have been asked before (Are low-salt diets effective? and Is salt "white death"? ) but both the answers and the questions are too vague and unsatisfactory. I'm open to merging them (if that is possible) but the most important thing is to get better answers. I've tried to ask the question in a more specific form that should encourage quantitative answers.

Answer 1

The meta-analysis and the Guardian article are not talking about the same thing.

The specific Guardian claim is [my emphasis]:

thousands of unnecessary strokes and heart attacks have occurred ... more than 4,000 premature deaths per year

The scientific article you link says:

In this systematic review of 9 unique studies of 479 unique patients with heart failure, an overall paucity of robust high-quality evidence was available to support or refute the use of reduced dietary salt intake.

To paraphrase, we have no evidence to tell us if salt intake contributed to these particular heart attacks or not. the linked article really has nothing to say about the Guardian claim - it doesn't address the claim regarding stroke and it really has nothing to say about heart attack.

Compare this to Rexhaj, Emrush; Messerli, Franz H; Cerny, David; Bohlender, Juergen (2017) who say:

Studies involving >150,000 individuals showed a U-shaped association between salt consumption and cardiovascular events.

That is, both too much and too little increases stroke and heart attack.

Going back to 2014, Feng J He, Sonia Pombo-Rodrigues, Graham A MacGregor specifically studied Salt reduction in England from 2003 to 2011: its relationship to blood pressure, stroke and ischaemic heart disease mortality which is bang on to the Guardian's topic. They concluded:

The reduction in salt intake is likely to be an important contributor to the falls in BP in England from 2003 to 2011. As a result, the decrease in salt intake would have played an important role in the reduction in stroke and IHD mortality during this period. Despite considerable progress being made on salt reduction, the mean salt intake in England (8.1 g/day in 2011) was still 35% higher than the recommended level of 6 g/day, and 70% of the adult population (80% men and 58% women) had a daily salt intake above the recommended level. Therefore, continuing and much greater efforts are needed to achieve further reductions in salt intake to prevent the maximum number of stroke and IHD deaths.

They also mention "the reduction in salt intake is likely to have occurred across the whole population as it was predominantly achieved by a gradual reduction in the amount of salt added to all processed foods, which accounts for approximately 80% of total salt intake."

So the facts are:

• The English are consuming less salt in 2011 than they were in 2003,
• This is probably due to a reduction in salt in processed foods,
• Nevertheless, most are still consuming more than the recommended daily intake (i.e. they are on the high side of the U-shape for salt),
• Controlling for other factors, this reduction in salt is correlated with lower blood pressure,
• Controlling for other factors, higher blood pressure is correlated with a high risk of stroke/heart attack,
• At the same time there has been a reduction in mortality from stroke/heart attack.

So, salt reduction has probably saved lives and further salt reduction would probably save more lives. Whether that amounts to 4,000 per year I can't tell.

Answer 2

A lot of the medical research into the relation between lifestyle factors such as the diet and occurrence of illnesses attempt to find relations between a single factor such as salt intake and some health outcome by eliminating a large number of confounding factors. But such methods can only work if the human body were to be very fragile w.r.t. the factor that is studied. Usually this is not the case and then you get misleading results that then tends to contradict the results of older studies of entire populations where the same factors are different.

A good example is the fact that populations that traditionally stick to the prehistoric salt intake of no more than few tenths of a gram of salt a day have blood pressures that remain low well into old age. We can read here:

For their study, the researchers took blood pressure measurements from 72 Yanomami aged one to 60, and found no trend towards higher or lower readings as the participants aged. The researchers also measured blood pressure in 83 members of the nearby Yekwana tribe, which is more exposed to Western influences including dietary -- and here they found a clear trend towards higher pressure with advancing age.

As the article points out, the Yanomami are free of atherosclerosis strokes and heart attacks are very rare, but they also eat lots of fruits and vegetables, get a lot of exercises, so it's not clear whether one can attribute their heart health to their low slat intake. However, it's wrong to downplay salt intake as an important factor just because statistical studies on the unhealthy British population that eats a massive amount of salt have contradictory result. This method of looking at a single factor at a time is a flawed method. We do the same thing w.r.t. fat intake where on the one hand we have population based studies with results like this:

In the African population of Uganda coronary heart disease is almost non-existent. This statement is confirmed by adequate necropsy evidence. In the Asian community, on the other hand, coronary heart disease is a major problem.

and this more recent one:

...the Tsimane, a forager-horticulturalist population of the Bolivian Amazon with few coronary artery disease risk factors, have the lowest reported levels of coronary artery disease of any population recorded to date.

Here too there are statistical studies where many confounding factors (such as e.g. salt intake and exercise) are eliminated and then you don't find spectacular results anymore, with some studies even suggesting that increasing fat intake improves health.

So, the entire approach where you look at single factors is totally flawed, the human body is very flexible and is capable of adapting itself quite well to very poor lifestyle factors, which will then make single factors such as lack of exercise, high fat intake, high salt intake etc. look to be not as bad as suggested by population based studies. What matters is that when all these factors are combined at the population level, then that does lead to very large health effects.

Therefore, while we can indeed criticize the conclusion of 4000 lives saved per year, it is also clear that if we were to drastically reduce salt intake, stop eating processed foods (at least in cases where the processing has significantly changed the nutrient profile), exercise a lot more then there is plenty of evidence from population based studies to suggest that heart attack and stroke risks would plummet by at least a factor of ten.

Given that there are 160,000 deaths in Britain caused by cardiovascular disease, we should assume that the number of lives saved can be of the order of 140,000 per year and simply stick to the advice of Public Health England on all relevant matters such as salt intake, exercise, fruit and vegetables intake, fiber intake, fat intake etc. etc., instead of trying to only look at each single factor in isolation and then dismissing or downplaying all of them one by one.