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Monday, April 23, 2007

Dietary sodium reduction seems to reduce cardiovascular events long term

Dietary sodium reduction seems to reduce cardiovascular events long term

According to the findings of this long term follow up study, sodium reduction, previously shown to lower blood pressure, may also reduce long term risk of cardiovascular events. The study followed up participants in two randomised lifestyle intervention trials - the trials of hypertension prevention I (TOHP I) and II (TOHP II) - for subsequent cardiovascular outcomes. The original trials found small but significant direct effects of sodium reduction on reducing blood pressure in adults with high normal blood pressure. TOHP I involved 10 clinic sites between 1987-90 and TOHP II involved 9 sites between 1990-5.

In this study, the authors determined the long term effects, over a period of 10-15 years, of sodium reduction on cardiovascular disease and mortality. Data was collected on all events occurring since the end of the original trials. A co-ordinating centre conducted the follow-up centrally by mail and phone. Questionnaires (seeking detailed information on cardiovascular and other health outcomes) were posted in January 2000, followed by phone calls as needed. Additional questionnaires were sent to responders at two year intervals through early 2005. The primary outcome measure used in the study was cardiovascular disease, a composite of myocardial infarction, stroke, coronary artery bypass graft (CABG), percutaneous transluminal coronary angioplasty (PTCA), or death with a cardiovascular cause. In the original studies, 744 participants in TOHP I and 2382 in TOHP II were randomised to a sodium reduction intervention or control.

The results found:

  • Follow-up information on cardiovascular outcomes or death was obtained for 2415 participants (77%).
  • 200 participants reported a cardiovascular event. Risk of a cardiovascular event was 25% lower among those in the intervention group (relative risk 0.75, 95% confidence interval 0.57 to 0.99, P=0.04), adjusted for trial, clinic, age, race, and sex
  • The risk was 30% lower after further adjustment for baseline sodium excretion and weight (0.70, 0.53 to 0.94), with similar results in each trial.
  • In secondary analyses, it was found 67 of the total 3126 participants died; 35 in the intervention groups and 42 in the comparison groups. After adjustment for baseline characteristics, including weight and sodium excretion, there was a 20% lower mortality among those in the sodium reduction intervention (0.80, 0.51 to 1.26, P=0.34 – not significant).
The authors note limitations of their study, including the lack of direct measurement of blood pressure, weight, and sodium intake during follow-up. They also note the less than complete rate of follow up, although add that the rates were high (77% for morbidity and 100% for mortality).

[Editor's note: to add perspective, the relative reduction in cardiovascular events is similar to that associated with statins - but with no significant adverse effects. Applied across the whole population, the absolute risk reduction would be small, nevertheless because of the numbers involved the reduction in cardiovascular events would be large. A modelling paper published last October looked at this issue and gives an indication of the potential differences between interventions in populations and interventions in at-risk individuals.]

Br Med J, published early online 20 April 2007; doi:10.1136/bmj.39147.604896.55 (link to abstract)

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