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How can implementing a population-wide salt-reduction program be so hard?
For most of human evolution, the average daily diet contained a fraction of a gram of salt and our physiology developed accordingly. A few thousand years ago, with the discovery that salt could preserve food, average intake started to rise. Now, with salt poured into the food supply, average Australian consumption levels are many times our physiological need. Populations eating the level of salt upon which we evolved are now few, but they provide a window into normal physiological processes. One of the most notable findings is that their blood pressure levels do not rise with age.
Despite recent highly publicised reports, there is little debate about the adverse impact of salt on human health. The totality of the evidence is convincing and the unbiased findings from randomised trials of salt reduction particularly so. While a number of non-randomised studies have suggested health benefits of salt consumption, the publicity they receive greatly exceeds their real significance. Observational nutritional epidemiology is incredibly difficult to do well, and the diversity of findings almost certainly reflects methodological challenges, not discrepant science. Although direct evidence from a single adequately powered mortality and morbidity trial of salt reduction is lacking, the circumstantial evidence remains striking and the likelihood that reduction in salt intake will not reduce vascular risk is small.
The strength of the evidence base has persuaded multiple national and international organisations of the need to reduce salt consumption. All hypertension guidelines advocate consuming less salt, and more than 30 countries now have some form of population-based salt-reduction program in place. A series of influential reports has highlighted the large health gains that might be achieved from such national programs and the low costs required to deliver them. The issue is no longer whether salt reduction should be a goal, but how it can be achieved.
The reason salt reduction presents such a great public health opportunity is that almost everyone eats far more than they need. Average consumption in Australia is between 8 and 10 grams per day, with immediate and long-term implications for blood pressure. The early effects occur within weeks and the chronic effects over decades. As shown by Huggins and colleagues in this issue of the Journal, and previously noted by the Intersalt study, a daily intake 6 grams above physiological need will push up systolic blood pressure by a few millimetres of mercury in the short term and thereafter by about half a millimetre each year. This chronic effect translates into 25mmHg over 50 years, with enormous implications for individual and population risks of vascular disease.
Blood pressure is a leading cause of disease burden in Australia, and our strategy for preventing disease attributable to high blood pressure is hypertension control — individuals are diagnosed as hypertensive and treated within the medical system. Hypertension is currently the most frequent reason for a primary care consultation, with annual direct health care costs of more than a billion dollars. For those who need and receive it, antihypertensive therapy is a highly effective intervention.
Unfortunately, the clinical approach also has some limitations. First and foremost among these is that half of all disease caused by high blood pressure occurs among people without hypertension. Risks start to accrue well below the blood pressure level of 140/ 90mmHg that generally defines hypertension, and systolic blood pressure levels of 125–135 mmHg are associated with greater risks than a level of 120mmHg. While more moderate than the risks faced by those with hypertension, these blood pressure levels cause a very large number of adverse events because these are theblood pressure levels of most of the population.
The limited coverage achieved by the clinical hypertensioncontrol strategy further reduces its effectiveness. Only about half of hypertensive people are identified and treated;14 less than half ofthese get to target blood pressure levels,14 and almost none achieve a systolic pressure of 120 mmHg or below. Accordingly, clinicalmanagement of hypertension in Australia probably prevents only about a 10th of all blood pressure-related disease.
A plausible population-wide salt-reduction program thatremoved salt at the source could within a few years avert a similarproportion of disease burden at an annual cost of just $10–20 million. To achieve this, the Australian Government simply needsto set and enforce salt targets for foods, as has been done in the United Kingdom.7 Average salt consumption would fall, mean population blood pressure would immediately follow, and the long-term rise in blood pressure with age would be attenuated.
The real question is how this can be so hard. For almost no extra cost and at no risk, there is a high likelihood we could double the proportion of blood pressure-related disease averted within just a few years. With a proven overseas model to follow, our failure to take the action required is bordering on negligent. No one is going to lose their parliamentary seat and no one is going to go out of business if they make this happen. There are just going to be a lot of unnecessary strokes and heart attacks while the people pickling us figure this out.
Author Bruce C Neal, MB ChB, PhD, FRCP, Senior Director
The George Institute for Global Health, Sydney, NSW.
Published in the MJA 1 August 2011