Salt raises CVD risk
Reducing salt (sodium chloride) intake is one of the key pieces of dietary advice handed out by practice nurses and gps when faced with a hypertensive patient. We all accept that raised blood pressure is probably the single most important risk factor for cerebrovascular disease and a very important risk factor for most other forms of CVD. Salt intake is an important determinant of blood pressure, with perhaps the best known trials, the Intersalt study1,2 and DASH-sodium trial,3 demonstrating this point. The theory that salt reduction should reduce the risk of subsequent CVD would therefore appear to be well founded.
The long-term benefits of reducing salt on CVD have not been shown in randomised controlled trials until now. The results of the recently published observational follow-up of the Trials Of Hypertension Prevention (TOHP)4 are very encouraging.
This follow-up study looked at more than 3,000 patients from the original TOHP I and TOHP II trials. These trials recruited from 10 clinic sites in 1987-88 (TOHP I: 744 patients) and nine sites in 1990-92 (TOHP II: 2,382 patients). Patients included were aged 30-54 years with prehypertension (systolic BP <140mmHg and mean diastolic BP 80-89mmHg in TOHP I; systolic BP <140mmHg and mean diastolic BP 83-89mmHg in TOHP II). They were randomised to a salt reduction intervention or control. The interventions took the form of education and counselling on reducing salt intake for 18 months (TOHP I) and 36-48 months (TOHP II). Salt intake was assessed through 24-hour urinary sodium collection. Final data for TOHP I and TOHP II were collected in 1990 and 1995 respectively.
The follow-up study was conducted between 2000 and 2005, with information collected via telephone or postal questionnaires on all cardiovascular events that had occurred since the end of the trials. Positive responses were subsequently validated via the patients' medical records. It should be noted that there were no direct measurements of blood pressure, weight and sodium intake during the follow-up study. The primary outcome measure was CVD – a composite of myocardial infarction, stroke, coronary revascularisation or cardiovascular death.
The intervention groups achieved an impressive sodium reduction of 44mmol/24 hours (approximately 2.6g of salt) and 33mmol/24 hours (approximately 2.0g of salt) over the TOHP I and TOHP II study periods respectively. The follow-up study obtained responses from 2,415 (77%) of patients, which was perhaps not as high as had been hoped but was still respectable, with 200 respondents reporting a cardiovascular event.
The relative risk of a cardiovascular event was 25% lower in the salt reduction group (relative risk 0.75, 95% confidence interval [CI] 0.57-0.99, P=0.044); when further adjustment was made for baseline sodium excretion and weight the risk of an event was 30% lower in the salt reduction group (relative risk 0.70, 95% CI 0.53-0.94, P=0.018). In secondary analyses, 77 patients died, 35 in the intervention arm and 42 controls, but this small difference did not achieve statistical significance (relative risk 0.80, CI 0.51-1.26, P=0.34).
It would therefore appear that lifestyle interventions to reduce salt intake can have a benefit on long-term cardiovascular outcomes, and this study is the first to show this using data from randomised controlled trials. This study would also tend to support the notion that good habits can be developed and maintained by intensive lifestyle advice such as that used in the intervention groups of the original TOHP trials. This should add further support to the notion that salt intake needs to be addressed and that dietary advice to patients with CVD should not be overly lipid-centred, with the deleterious effects of excess salt consumption overlooked as a result.
The challenge now is to persuade patients to become more salt aware, and in particular to minimise their intake of processed foods and not add salt to food during preparation or consumption. If this could be achieved then there is much to be gained in terms of both blood pressure control and overall cardiovascular morbidity and mortality.Author
Dr Peter Savill
BSc MB BS PGDipCard
GPSI Cardiology, Southampton