Salt and cardiovascular disease

Blood pressure is the most powerful predictor of stroke and other cardiovascular events. The importance of salt (sodium chloride) intake in determining blood pressure and the incidence of hypertension is well established. Furthermore, randomised controlled clinical trials of moderate reductions in salt intake show a dose dependent cause-effect relation and lack of a threshold effect within usual levels of salt intake in populations worldwide.1 The effect is independent of age, sex, ethnic origin, baseline blood pressure, and body mass.

Prospective studies,2 3 4 5 with one exception,6 also indicate that higher salt intake predicts the incidence of cardiovascular events. While widespread support exists for reducing salt intake to prevent cardiovascular disease, the lack of large and long randomised trials on the effects of salt reduction on clinical outcomes has encouraged some people to argue against a policy of salt reduction in populations.6

In this week's BMJ, Cook and colleagues7 report the long term effects of reduced dietary sodium on cardiovascular disease in people participating in the controlled randomised trials of hypertension prevention follow-up studies (TOHP I and II). More than 3000 participants without hypertension were randomised to a reduced sodium intake for 18 months (TOHP I) or 36-48 months (TOHP II), or to a control arm. The reductions in sodium intake were 44 mmol/day and 33 mmol/day (equivalent to ∼2.6 g and ∼2.0 g of salt), respectively. The results show that people originally allocated to either sodium reduction group had a 30% lower incidence of cardiovascular events in the next 10-15 years, irrespective of sex, ethnic origin, age, body mass, and blood pressure. The benefits exceed those estimated by a recent meta-analysis.8 Cook and colleagues' study is the first to report a beneficial effect of dietary salt reduction on cardiovascular outcomes based on randomised trial data.

The study strengthens the support for dietary recommendations for lower salt intake to prevent cardiovascular disease in the general population. In 1985, the World Health Organization recommended that the average salt intake should be reduced to 5 g per day or less. However, few countries have policies for targeted reduction in salt intake.

Differences exist between developed and developing countries. In Westernised countries, people derive salt mostly from bread and processed food and only a small proportion comes from discretionary use (up to 20%). A population wide policy of salt reduction in developed countries can only be implemented with the collaboration of the food industry. Over the years, however, the need to sustain a profitable market has led to opposition from the food industry or slow progress.9

In England and Wales some progress has been made, but levels of salt intake are still far from the recommended 6 g of salt per day. Future options are to do nothing, to establish voluntary target levels of salt for a wide range of foods, or to legislate so that the food industry has to comply. Given the inertia of the past 20 years, the first option would not contribute to progress. The “voluntary” option would support existing work, but it is unlikely to achieve the set targets. The recent position of the industry in rejecting the “traffic light” proposal for labelling, whereby highly salted foods would carry a red alert warning, is one measure of the gap still remaining. The legislation option would require the food industry to reduce the salt content of processed food to within set levels. The experience in Finland suggests that legislation has added value to the previous option and at this stage is necessary and justified.

Conversely, in many developing countries, like those of sub-Saharan Africa, where the main source of salt is still discretionary, community based and context specific initiatives can be effective and should be pursued,10 given the increasing burden of cardiovascular disease related to hypertension.

Without considerably modifying the environment by allowing greater availability of low salt foods, people will find it difficult to exercise their “choice” when trying to reduce dietary salt. Doctors and health professionals have long used dietary counselling to deliver non-pharmacological management of hypertension. Advising patients to reduce salt intake with a lifestyle package quickly delivered in a busy primary care setting is ineffective, however.11 A baseline assessment of salt intake (through a 24 h urinary collection for the measurement of sodium) is not part of the National Service Framework requirements and is not included in the Quality and Outcome Framework. The current system is therefore unlikely to make health professionals and consumers more aware of how much salt people eat or to increase motivation and knowledge on how to reduce it. In a climate of scarce healthcare resources, one of the most cost effective ways to reduce the burden of cardiovascular disease is being overlooked.12 And yet the evidence gets stronger.