Role of Sodium Intake in Preventing Hypertension
Experts differ on the ideal sodium intake to reduce CVD and mortality risks.
Blood Pressure and Sodium Intake: The Established Link
The link between dietary sodium intake and blood pressure has been established by numerous clinical studies over the last 30 years. The connection is strongest between excess sodium intake and hypertension, but decreasing sodium intake can also lower blood pressure in “high normal” or prehypertensive individuals.1
Several studies, including DASH-Sodium (Dietary Approaches to Stop Hypertension; ClinicalTrials.gov identifier: NCT00000544), have demonstrated that reducing sodium intake to as low as <1.5 g/d lowered blood pressure to a modest degree.2,3 The findings from DASH-Sodium in particular have strongly influenced guidelines for blood pressure management. The American Heart Association (AHA) recommends maintaining sodium intake levels below 1.5 g/d to help prevent cardiovascular disease (CVD).2,3
However, it is unclear whether lowering blood pressure by reducing sodium intake improves CV outcomes and overall mortality. “There is much controversy regarding the effects of sodium on mortality, especially with lower sodium intake,” Nancy R. Cook, ScD, of Brigham and Women's Hospital in Boston, said in an interview. “While decreasing sodium intake clearly improves blood pressure, not all studies have found a benefit for CVD or mortality.”
To that point, excess sodium intake can raise the risk for CVD, but surprisingly, very low levels of sodium intake have also been associated with increased CVD and mortality. In contrast, a study that evaluated 10- to 15-year follow-up data from TOHP (Trials of Hypertension Prevention; ClinicalTrials.gov identifier: NCT00000528) demonstrated that the lowest levels of sodium intake correlated with the lowest CV risk.1
To investigate the relationship between sodium intake and total mortality, Dr Cook and colleagues examined long-term mortality data from phase 1 and phase 2 of TOHP (TOHP I and TOHP II). Their findings were recently published in the Journal of the American College of Cardiology.1
Long-Term Mortality in TOHP
In TOHP I, 744 participants were randomly assigned to active sodium reduction or usual care for 18 months.5 In TOHP II, 2382 participants were treated with sodium reduction, weight loss intervention, combined sodium plus weight loss intervention, or usual care for 36 to 48 months.6 Both studies found that urinary sodium excretion and blood pressure were significantly lower in the reduced-sodium groups compared with usual care.5,6
In the current trial, mortality data spanning 23 to 26 years after randomization were collected on TOHP participants. Mortality rates among patients treated with sodium reduction and usual care in TOHP I were 10.1% and 10.6%, respectively; in TOHP II, 6.9% and 7.7%, respectively. This translated into a 15% lower mortality risk, albeit nonsignificant, in patients receiving the reduced sodium intervention (hazard ratio [HR]: 0.85; 95% confidence interval [CI], 0.66-1.09; P =.19).1
Sodium intake, as measured by 24-hour urinary sodium excretion, directly correlated with mortality in a nonsignificant but linear fashion (P for trend =.30). Every 1000-mg/24-h increase in sodium excretion was associated with a greater risk of death (HR: 1.12; 95% CI, 1.00-1.26; P =.052).1
“We found a direct linear relationship between sodium intake and mortality, meaning that mortality was highest with high sodium intake and lowest with low sodium intake,” Dr Cook said. “This was not surprising since we had previously found benefits with low sodium for both blood pressure and CVD.”
Low Sodium Intake: Helpful or Harmful?
Although Dr Cook and colleagues found a linear relationship between sodium intake and mortality, recent data from observational studies and a meta-analysis suggest that very low sodium intake may actually increase the risk for CVD and mortality. Thus, the relationship between sodium intake and mortality may be U-shaped.1
“Numerous studies in the past 5 years have shown that low sodium intake, less than 3 grams per day, compared with average sodium intake in the population, 3 to 5 grams per day, is associated with an increased risk of total mortality and CVD events,” Andrew Mente, PhD, of McMaster University in Hamilton, Canada and lead author of a related editorial, said in an interview. “This finding has been replicated in multiple studies using different methods to estimate sodium intake.”