I usually like to keep my blogs short and to the point, but following a critical article about the Agency's salt policy in The Times earlier this week, I felt this needed a more detailed response. As Chief Scientist, my job is to ensure that the Agency draws its conclusions and bases its advice on the totality of evidence. So it concerned me when I read this selective view of science in relation to such an important public health issue.
The article in question highlighted some studies that suggest there are no benefits to reducing people’s consumption of salt. It describes findings from a study that argues public policy to reduce salt intake is unrealistic because our appetite for salt is controlled by our central nervous system. The study analysed 24-hour urinary sodium excretion (the most accurate way to measure salt intake) from previously published surveys, which included data for 20,000 people in 33 countries and reported that salt intake ranged from 7 to 12 g a day. From this the authors concluded that salt intake is controlled within the range defined by these surveys. It’s a very odd argument that because usual intakes of salt are within a certain range this is how much we need. It’s not surprising so many populations have intakes within this range, in some ways diets across the world are similar, for example bread, breakfast cereals, savoury biscuits, tinned vegetables, soups, and ready-prepared meals form a large part of the diet, and can all be high in salt – in fact, most of our salt intake (about 75%) comes from foods we buy.
The Times article mentions a study in the British Medical Journal in 2002 that concluded that while salt avoidance was helpful to those on medication for hypertension, there were no clear benefits for anyone else. This may be a reference to a systematic review by Hooper et al (2002) which assessed the long-term effects of advice to reduce dietary salt in adults with and without hypertension. This review found that the significant reductions in blood pressure observed at six and 12 months were not sustained over time. The findings reflect the difficulty in making substantial
The Times article also states that a Cochrane review concluded ‘there is little evidence for long-term benefit from reducing salt intake’– but the study could only assess the short-term effects! And although they couldn’t assess the long-term effects, the review authors acknowledge that reduced sodium intake in those with elevated blood pressure has a ‘useful effect to reduce blood pressure in the short term’.
The article also includes the views of a dietitian, who states that salt reduction is only important for people with high blood pressure. Well, the 2006 Health Survey for England showed 31% of men and 28% of women had high blood pressure – and it’s likely that lots more might also have high blood pressure that hasn’t been diagnosed. Because the risk of high blood pressure is so widespread, the population as a whole may be at a relatively high risk of cardiovascular disease. This just adds weight to our advice high blood pressure is a serious public health problem in the UK.
But in fact the evidence shows, the risk from cardiovascular disease is not restricted to people with high blood pressure. A large study (Prospective Studies Collaboration, 2002) that combined the results of 61 studies of blood pressure and mortality with data for one million adults with no history of cardiovascular disease, found a relationship between increasing blood pressure and risk of death from cardiovascular disease. This is another reason why the Agency agreed a public health approach to reducing salt intake was needed.
Additionally, the DASH sodium trial, a rigorous and large randomised controlled trial in which salt intake was tightly controlled, demonstrated that blood pressure was reduced in response to decreasing salt intakes for people both with high blood pressure and normal blood pressure.
Contrary to these robust published trials, another scientist is quoted in The Times claiming that results from a randomised clinical trial showed that people with a lower salt diet suffered significantly more cardiovascular deaths and hospitalisations than people with a higher salt diet. We’ve looked and have been unable to find such a trial.
The Agency recognises that we do need some salt in our diet as it is one way of obtaining sodium, which is an essential nutrient. Our recommendation is to reduce consumption of
The sodium content of the blood is tightly regulated over a narrow physiological range. This means if we consume more salt than we need, the excess must be excreted by the kidneys to maintain the sodium content of the body. However, there is an upper limit to the rate at which sodium can be lost from the body. Intakes above this point can cause an increase of sodium in the body and this causes water to be retained in the body. This might not matter in the short-term, but if the amount of salt that exceeds the capacity for excretion is large, or if it’s maintained over a long period of time, it can lead to tissue damage and the development of higher blood pressure.
The Agency’s advice to consume no more than 6g of salt a day is based on independent expert advice from the Scientific Advisory Committee on Nutrition (SACN) and this is based on a wide range of published scientific evidence (approximately 200 studies). We’re not alone on this – the 6g intake target is consistent with advice from the world’s leading scientific bodies, including the World Health Organization and the Institute of Medicine in the US.
Agency advice is based on the totality of the available evidence, guided by independent experts, rather than the views of one or two individual scientists. The benefits of salt reduction are clear. The scientific consensus is that excessive salt consumption increases the risk of high blood pressure which, in turn, increases the chance of cardiovascular disease. That’s why the Agency will continue to campaign for a reduction in the salt intakes of the UK population, which are still considerably greater than the 6g a day target.
I appreciate presentation of alternative views; considering all the available evidence ensures we can be confident our advice is based on sound science. But this article highlights how damaging it can be when only one side of an argument is presented and the full body of scientific evidence is not considered – is this bad science or sensationalist journalism?