Salt - Is It One More Bogeyman Thrown At Us Via Poor Science?

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.

Francesco Cappuccio, Cephalon Professor of Cardiovascular Medicine & Epidemiology, University of Warwick and Ceu Mateus, Senior lecturer in Health Economics , Lancaster University.

The Independent.