— But more potassium always helps, researchers find
Not everyone needs to cut back on salt in order to reduce their cardiovascular disease risk, according to an analysis from the controversial Prospective Urban Rural Epidemiology (PURE) study.
For every 1-g increase in sodium intake, people across 18 countries had an overall additional 0.73 cardiovascular events per 1,000 years (P<0.0001), Andrew Mente, of McMaster University and Hamilton Health Science in Hamilton, Ontario, reported in the Aug. 11 issue of The Lancet.
The relationship was non-linear, however, such that each extra gram of sodium's link to cardiovascular event rates on the community level depended on the population's average daily consumption.
- The lowest tertile (mean 4.04 g sodium/day) had significantly lower risk with higher consumption (-1.00 events per 1,000 years, P=0.0497)
- The intermediate tertile (mean 4.70 g/day) showed no relationship of risk with sodium intake (+0.24 events per 1,000 years, P=0.8391)
- The highest tertile (mean 5.75 g/day) had a non-significant trend for greater risk (+0.37 events per 1,000 years, P=0.0712)
Blood pressure inched up by 2.86 mm Hg per 1-g increase in a community's mean sodium intake, but the increase was significant only among communities in the highest tertile of sodium intake (P<0.0001 for heterogeneity).
The investigators showed that most communities in their study had sodium intake averaging 3-5 g/day -- the exception being in China, where most of the 103 communities sampled exceeded 5 g/day. The World Health Organization currently recommends a target of 2 g/day sodium (equivalent to approximately 5 g/day salt).
"The rationale, however, is based on the association between sodium intake and blood pressure and the assumption that any approach to reducing blood pressure will translate into fewer clinical cardiovascular outcomes. Nevertheless, the claim that the effects of salt on cardiovascular disease are exclusively mediated through its effects on blood pressure has never been proven," Mente and colleagues said.
Indeed, the associations between sodium intake and cardiovascular events did not change when adjusted for age, sex, and blood pressure, suggesting that the effects of sodium intake on cardiovascular events are largely unrelated to the effects of sodium intake on blood pressure, the researchers argued.
What was clear from their data was that all major cardiovascular outcomes decreased with increasing potassium intake in all communities.
"Our findings suggest that a population-specific strategy for sodium reductiontargeted at countries or communities with sodium intake greater than 5 g/day would be preferable to a population-wide strategy of sodium reduction to reduce cardiovascular disease and premature deaths. In contrast, there is a stronger case for increasing the consumption of foods that are rich in potassium (e.g., fruits and vegetables) population wide," the authors concluded.
"The data for sodium continue to be messy but this study has one clear and totally unambiguous implication: Eat your veggies! The study absolutely confirms the heart-healthy benefits of eating fruits and vegetables," agreed Marion Nestle, PhD, MPH, of New York University.
For now, "it's hard to say" if the sodium intake target should be moved, Nestle toldMedPage Today. "I wouldn't want to be at the high end of sodium intake, not least because of stroke risk. Anything that raises blood pressure seems a good idea to avoid," she said.
PURE is a large ongoing epidemiology study. The authors previously reported several controversial findings, that diets rich in fats don't increase mortality risk, for one, and that eating more than a few servings of fruits, vegetables, and legumes doesn't amount to more cardiovascular benefit.
In the current analysis, 369 communities were assessed for blood pressure and 255 for cardiovascular outcomes over a median of 8.1 years. Participants were adults ages 35-70 years in the general population who had no cardiovascular disease at enrollment.
The link between sodium intake and cardiovascular events was driven mostly by stroke, with a positive association again found only among communities in the highest tertile of intake. In contrast, higher consumption of sodium did not lead to more deaths, Mente's group reported.
A key limitation to the PURE data was that morning fasting urine was used to estimate 24-hour sodium and potassium excretion levels, which in turn were used as surrogates for intake. The observational data were also derived from a predominately Asian population, the editorialists noted.
"Nevertheless, the findings are exceedingly provocative and should be tested in a randomized controlled trial," according to Messerli's group. "Indeed, such a trial has been proposed in a closely controlled environment, the federal prison population in the USA," they said, noting that resorting to such a population highlights the fact that "curtailing salt intake is notoriously difficult."
"We think that incentivizing people to enrich their diets with potassium through eating more fruit and vegetables is likely to need less perseverance," they concluded. "Perhaps salt-reduction evangelists and salt-addition libertarians could temporarily put aside their vitriol and support the hypothesis that diets rich in potassium confer substantially greater health benefits than aggressive sodium reduction."
The study was funded by the Population Health Research Institute, Canadian Institutes of Health Research, Heart and Stroke Foundation of Ontario, and European Research Council.
Mente and colleagues declared no competing interests.
Messerli disclosed getting grants, advisory board and speaking honoraria from Menarini, Servier, Pfizer, Novartis, Medscape, and Medtronic.
Reviewed by Robert Jasmer, MD
Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner
Read the original article on Medpage Today: PURE Study: Only Cutting the Saltiest Diets Helps the Heart