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The great salt debate: does consuming less really save lives?

Sodium chloride – a controversial substance. www.shutterstock.com

The great salt debate: does consuming less really save lives?

Sodium chloride – a controversial substance. www.shutterstock.com

For years, public health officials have been telling us that too much salt is bad for our health. Others have questioned the evidence for this claim. It’s Salt Awareness Week, so what better time to ask two experts for their views on the topic.

The case for reducing salt intake

Francesco Cappuccio: Lowering salt intake reduces blood pressure, stroke and other blood-vessel-related disease, so avoiding 1.25m premature deaths a year worldwide. This is effective in men and women of all ages, ethnic groups and incomes. The United Nations and the World Health Organisation have thoroughly reviewed all the evidence with panels of experts from five continents. They concluded that population programmes of moderate salt reduction (to 5g per day) are feasible, effective, cheap, powerful, quick and equitable.

This important shift in public health action has not happened, however, without stubborn opposition from organisations and people with vested interests in maintaining high population salt intake. Key components of this denial strategy include misinformation (with pseudo controversies) and the use of poor science to create uncertainty and support inaction. The polarised positions are centred on whether reducing a population’s average salt intake by a moderate degree (to 5g per day) is justified and whether it may cause harm.

The wisdom of the global health organisations is challenged on the basis of some studies which seem to indicate that a low salt intake may be associated with early death. These studies are flawed because they include errors in the assessment of salt intake. They use methods to determine a person’s salt intake that cause bias, such as reverse causality (high mortality in low sodium groups is due to the inclusion of sick people undergoing treatment with numerous drugs), residual confounding (other factors explaining the results), and often insufficient statistical power (the study is too small). Studies that avoided making these errors did not find harm.

Despite these scientific inadequacies, some continue to ignore them and refuse to correct their methods of investigation. Rather, they make claims of alleged conspiracies by global health organisations or of the systematic drive of a small group of advocates in favour of public health action. But they provide no credible motives to explain these speculations. On the contrary, parts of the food industry are known for conspiring to bias research, as well as using unscrupulous opinion leaders to divert attention from salt (as well as fat and sugar) with well fabricated theories. This has been well documented.

The opponents also confuse the concept of “no evidence of effect” with that of “evidence of no effect”. The sceptics call for randomised controlled trials showing that a moderate reduction in salt intake reduces cardiovascular outcomes, claiming it is the usual standard for most strategies recommended for cardiovascular prevention. But they are mistaken. There are no randomised controlled trials on cardiovascular disease to back up policies such as reducing obesity, increasing physical activity, preventing diabetes, reducing air pollution or banning exposure to asbestos (I could go on and on). Science will always be imperfect. Public health policies are almost invariably implemented based on the appraisal of the best available evidence.

Finally, the suggestion that “guideline committees should only include independent methodologists” (experts in general methods but not in the subject matter) is intriguing. Following this principle, poor science was produced using flawed studies which were subsequently retracted. Despite official retractions, this flawed evidence is still quoted to support the argument of harm.

Would you be reassured to receive expert advice from a cardiologist for a toothache or from a urologist for a recurrent cough? As Michael Greger of the World Hypertension League recently suggested, take the sceptics with a pinch of salt!

In this article I’m using the word “salt” to refer to sodium chloride measured in grams per day. However, many scientific articles also refer to sodium expressed as either grams per day or milligrams per day. To make a comparison you should use the following equivalence: 1g (or 1,000mg) of sodium = 2.5 g of salt or sodium chloride.

High blood pressure is associated with premature death. www.shutterstock.com

The case against

Céu Mateus: A major cause of premature death and disability around the world and in the UK is cardiovascular disease which includes heart attacks and strokes. There is evidence that high blood pressure is a risk factor for cardiovascular disease, so the positive correlation between salt intake and blood pressure is used to advocate a reduction of sodium intake at the population level in order to reduce cardiovascular disease and premature death.

The recent article by Trinquart et al., which reveals a polarization of the scientific literature on salt reduction, shows that sometimes science is not based on hard facts but is an echo of opinions reverberating around. It helps to understand the weaknesses and flaws of the research done so far to support the argument that less salt is going to save you.

Between 1978 and 2014 only 68 studies were undertaken but very few randomised control trials – only 27% of the total. Of those 68 studies, 50% say that reducing salt intake won’t save you and 10% are inconclusive. Of the 14 systematic reviews done in the same period, 43% are inconclusive and 21% contradict the fact that less salt saves you.

Still, research does show that reduced salt intake has positive results in the short term, leading to lower blood pressure – although this is more significant for people with hypertension than for people with normal blood pressure. Prospective cohort studies (where study participants are followed for a period of time), however, show a higher risk of cardiovascular disease and death for low versus moderate sodium intake.

We are born with unchangeable genetic characteristics that might trigger conditions such as haemophilia and cystic fibrosis, and, for the time being, there is nothing we can do to change that. However, there are many diseases related to unhealthy habits and lifestyles. So it’s said that if we change our behaviour we can be healthier and live longer. Which habits are those? Smoking, drinking, diet (eating too much, too much sugar, too much salt, too much fat) and physical inactivity.

The links between smoking, drinking alcohol and being physically inactive on life expectancy are well established – if you do one of the above you have a good chance of dying before people that don’t – while the link between obesity and life expectancy is not so well established, particularly at older ages.

But dietary habits depend heavily on food availability and affordability. It’s easier to eat healthily when you’re well off. The burden for changing one’s diet lies more heavily in people living in low-income households.

Regarding salt consumption, the key seems to lie in moderation rather than very low levels of sodium intake. Indeed, low sodium intake may adversely affect certain risk factors, including blood lipids and insulin resistance, and so potentially increase the risk of heart disease and stroke. For people with normal blood pressure there is no convincing evidence that a reduction in salt intake reduces cardiovascular disease – although there is some evidence of benefit for people with abnormally high blood pressure.

In observational studies, following people “eating as usual”, the results on cardiovascular disease and death seem to be better for people eating less salt. But it should be noted that people who are more concerned about salt in their diet are also more likely to eat more fresh food, less fat and less refined sugars, exercise more and smoke less. So the overall results are influenced by the healthy lifestyle and not solely by salt intake.

Cardiovascular disease and death are at the end line of complex interactions between social determinants of health, such as childhood development, education and income. The population, in general, would benefit from a healthier diet but, first of all, people have to have money to put healthy food on the table. Governments and agencies put the weight of the decision on the shoulders of the population and tell people “you should know better and eat healthier”, but this is a very simplistic vision of the problem.