Three large international studies addressing sodium intake’s effect on blood pressure and on cardiovascular and mortality outcomes are not likely to quell the controversy surrounding this issue. Rather, since the findings of one study directly oppose those of the other two, the results promise to fan the flames a bit higher.
All three studies were reported online August 14 in the New England Journal of Medicine.
Sodium and blood pressure: PURE
The first report concerned a substudy of data from the Prospective Urban Rural Epidemiology (PURE) study involving 102,216 adults aged 35-70 years residing in 667 communities in 18 low-, middle-, and high-income countries worldwide. Urinary sodium and potassium levels were used as surrogates for dietary intake of these elements, and these excretion levels were correlated with the participants’ blood pressure levels, said Andrew Mente, Ph.D., of the Population Health Research Institute, Hamilton (Ont.) Health Services, McMaster University, and his associates.
Current guidelines recommend a maximum sodium intake of 1.5-2.4 g/day, depending on the country. Only 0.6% of the study population achieved the lowest level of 1.5 g/day, the level recommended in the United States, and only 10% achieved less than 3 g/day. The largest segment of the study population, 46%, had a sodium excretion of 3-5 g/day, and the next largest segment, 44%, had a sodium excretion of more than 5 g/day.
"This suggests that, at present, human consumption of extremely low amounts of sodium for prolonged periods is rare," the investigators noted.
The investigators found, after multivariate adjustment, that for each 1-g increment in sodium excretion, there was an increment of 2.11 mm Hg in systolic blood pressure and 0.78 mm Hg in diastolic blood pressure (P less than .001 for both) for all areas of the globe.
However, this correlation was nonlinear. The association between sodium and blood pressure was weak in the largest subset of participants who had an excretion of 3-5 g/day, and was nonsignificant in those who had an excretion of less than 3 g/day.
The association between sodium intake and blood pressure was stronger in people who had an excretion of more than 5 g/day and in those who already had hypertension at baseline. It also increased with increasing patient age.
Taken together, these findings indicate that sodium’s effect on blood pressure is nonuniform and depends on the background diet of the population as well as the individual’s age and hypertension status, Dr. Mente and his associates said (N. Engl. J. Med. 2014 Aug. 14;371:601-11 [doi:10.1056/NEJMoa1311989]).
Sodium and cardiovascular mortality: PURE
The second report also was a substudy of the PURE study, this time headed by Dr. Martin O’Donnell of the Population Health Institute and McMaster University. The researchers performed a prospective cohort study involving 101,945 PURE participants to assess the association between baseline urinary sodium and potassium excretion, again as a surrogate for intake, with mortality and incident cardiovascular (CV) events during 3.7 years of follow-up.
The primary composite outcome of death or a major CV event occurred in 3,317 participants (3.3%). The mean 24-hour sodium excretion was 4.9 g.
Surprisingly, the lowest risk of death and CV events was seen not in people with the recommended levels of sodium excretion but in those whose sodium excretion was much higher, at 3-6 g/day. Risks actually increased at levels of sodium excretion that were lower than 3 g/day, as is recommended, as well as at levels that were higher than 6 g/day. Moreover, the association between high sodium excretion and high CV and mortality risk was significant only among adults who already had hypertension at baseline.
"The projected benefits of low sodium intake ... are derived from models ... that assume a linear relationship between sodium intake and blood pressure and between blood pressure and cardiovascular events. Implicit in these guidelines is the assumption that there is no unsafe lower limit of sodium intake," Dr. O’Donnell and his associates wrote (N. Engl. J. Med. 2014 Aug. 14;371:612-23 [doi:10.1056/NEJMoa131889]).
The findings from both of these PURE studies call those assumptions into question.
Sodium and cardiovascular mortality: NUTRICODE
The third report was a review of the literature regarding sodium intake’s effect on CV mortality worldwide; the gathered data then served as the basis for a complex statistical model that estimated how many deaths could be attributed to sodium consumption in excess of a reference level of 2.0 g/day. This study was performed by the Global Burden of Diseases, Nutrition, and Chronic Diseases Expert Group (NUTRICODE) and was headed by Dr. Dariush Mozaffarian, a cardiologist and epidemiologist with Tufts University and the Harvard School of Public Health, both in Boston.