A new meta-analysis adds further fuel to the controversy over sodium and cardiovascular health, suggesting that reducing dietary sodium lowers blood pressure (BP) even among those whose starting systolic BP is as low as 120 mmHg.
Each 50-mmol reduction in 24-hour sodium excretion was associated with reductions in systolic BP of 0.66, 1.89, and 2.76 mmHg among normotensive patients, a mix of normotensive and hypertensive patients, and hypertensive patients, respectively.
“This is a quite important finding that it is beneficial not only in hypertensive individuals but those with normal blood pressure,” senior author Feng J. He, MD, Queen Mary University of London, United Kingdom, told theheart.org | Medscape Cardiology. “Also, the study showed quite clearly that the lower the salt intake achieved, the lower the blood pressure.”
The World Health Organization recommends a maximum dietary sodium intake of 2 g/day (5 g of salt) as a population-level intervention to reduce cardiovascular disease (CVD) and mortality. Previous studies, however, have reported that sodium reduction is of limited or no value in normotensive individuals.
Further questions were raised following evidence from the PURE study of a J-shaped relationship, in which both higher and lower sodium levels were associated with increased CVD risk.
Unlike the PURE study, which has been criticized for using spot urine tests to estimate sodium excretion, only 24-hour urine collection was used to estimate sodium excretion in the 133 randomized trials in the meta-analysis, He noted.
Among the 12,197 participants, each 50-mmol reduction in 24-hour sodium excretion was associated with a 1.10-mmHg reduction in systolic BP and a 0.33-mmHg reduction in diastolic BP (P = .03).
Falls in systolic BP were present across all subgroups, as classified by age, sex, and race, but were larger among those older than 55 to 65 years (– 3.88; 95% confidence interval [CI], –5.05 to –2.71), women (–1.32; 95% CI, –2.47 to –0.16), and blacks (–4.07; 95% CI, –6.14 to –2.00), the authors report in an article published online February 25 in the BMJ.
Reductions were also found for each group, as classified by mean baseline systolic BP:
- <120 mmHg; –0.39 (95% CI, –.061 to –0.18)
- ≥120 to <130 mmHg; –1.21 (95% CI, –1.87 to –0.55)
- ≥130 to <140 mmHg; –2.23 (95% CI, –2.89 to –1.57)
- ≥ 140 to <150 mmHg; –3.23 (95% CI, –3.88 to –2.59)
- ≥150 to <160 mmHg; –2.68 (95% CI, –3.55 to –1.81)
- ≥160 mmHg; –2.97 (95% CI, –4.34 to –1.60)
“It seems to me that the blood pressure barely changed in the normotensive individuals, so I think they exaggerate how much benefit there would be to the general population,” Suzanne Oparil, MD, a hypertension expert and director of the vascular biology and hypertension program at the University of Alabama at Birmingham, commented to theheart.org | Medscape Cardiology.
She noted that 24-hour urinary collections can vary, depending on how conscientious the individual is, and that studies involving Russian cosmonauts have shown large variations in 24-hour sodium excretion day to day. “So there’s no gold standard there,” Oparil said.
In addition, Oparil noted that the investigators used casual BP measurements rather than 24-hour ambulatory BP measurements and dismissed data showing that very low sodium intake sometimes stimulates counter-regulatory mechanisms, such as the renin-angiotensin system, which tends to drive blood pressure up.
In addition, the findings are based on study-level rather than patient-level data and do not include CV outcomes, she said.
“Clearly if you have high blood pressure and you eat a lot of salt, decreasing the salt will lower the blood pressure, but for people who have essentially normal or nearly normal blood pressure and are not consuming very much salt, I just don’t believe that reducing the salt intake has demonstrable benefit,” Oparil said.
Senior investigator He said that evidence showing a lack of benefit with sodium reduction in normotensive individuals is based on very short-term studies and that activation of the renin-angiotensin system and adverse metabolic effects linked to large decreases in dietary sodium do not appear to be present in longer-term interventions.
In the meta-analysis, interventions to reduce sodium varied in length from no more than 7 days to more than 6 months.
No overall association was identified between intervention duration and the magnitude of either systolic or diastolic BP reduction, likely because of a lack of statistical power, because only 19% of studies included interventions that lasted longer than 30 days, and only 4% were longer than 6 months, He said.
Nevertheless, the effect of each 50-mmol reduction in 24-hour sodium excretion on systolic BP was approximately twice as large in studies with interventions longer than 14 days vs 14 days or less (2.13 mmHg vs 1.05 mmHg; P = .002).
Long-term reductions in dietary sodium can be difficult for individuals, but positive health benefits have been reported from national programs that combine salt-awareness campaigns and collaboration with food industry, He said.
For example, the investigators previously reported that the United Kingdom’s salt-reduction program, which sets voluntary, incrementally lower salt-reduction targets for more than 85 food categories, led to a 15% reduction in salt intake (from 9.5 g/day in 2003 to 8.1 g/day in 2011), a 2.7-mmHg fall in population systolic BP, and a decrease in mortality from stroke by 42% and ischemic heart disease by 40%.
Going forward, more ambulatory blood pressure monitoring data reflecting different dietary conditions would be useful, Oparil suggested. “It would be worthwhile knowing more about the effects of salt reduction on diurnal variation in blood pressure; we know that out-of-office blood pressure, particularly nocturnal blood pressure, seems to be more associated with outcomes than in-office blood pressure,” she said.
Source : Medscape