Salt has long been a staple of life. Once upon a time, it was a form of currency; roads were built to transport it; cities arose to produce and trade it. And, of course, people ate it. Today, we continue to consume it the whole world over.
It’s hard to believe something so integral to the human experience, spanning centuries and civilizations, might be bad for the body. But as the Mrs. Dashes of the world would have us know, salt consumption has been linked to high blood pressure, which in turn has been linked to heart disease — a reason, perhaps, to keep one’s salt intake in check.
Just how much salt are we consuming? And how much damage can we attribute to the delicious granules that top our pretzels, coat our fries, and cure our meats? A group of researchers from the Global Burden of Diseases Nutrition and Chronic Diseases Expert Group (NUTRICODE) used survey data and statistical models to estimate the sodium consumption of adults from 187 nations. They also performed a meta-analysis of over a hundred randomized interventional studies to calculate the effects of sodium on blood pressure and analyzed pooled data from cohort studies to calculate the effects of high blood pressure, in turn, on cardiovascular mortality.
Their findings, published in this week’s NEJM, might make you think twice before reaching for the saltshaker. In 2010, the average daily sodium consumption was 3.95 grams, well above the World Health Organization recommendation of 2 grams per day. In fact, 181 out of the 187 nations — accounting for a whopping 99.2% of adults — exceeded the recommended intake level. The consequence? Nearly one in every ten deaths from cardiovascular causes in 2010, the authors estimated, could be attributed to excess sodium consumption. That translated to 1.65 million deaths globally. What’s worse, over 40% of these deaths occurred prematurely (i.e., in people younger than 70 years of age).
While no region of the world was spared, there was considerable variability across geographies in sodium-associated cardiovascular mortality rates. The highest rates were in Central Asia and Eastern and Central Europe. Among individual countries, Georgia had the highest rate (1967 deaths per 1 million adults per year), while Kenya had the lowest (4 deaths per 1 million adults per year). Perhaps most striking, more than 80% of sodium-related cardiovascular deaths around the world — four out of every five — occurred in low- or middle-income countries.
There may be a silver lining. The authors also found that reductions in sodium intake were linked proportionally to reductions in blood pressure (P<0.001 for a linear dose-response relationship). For a white, normotensive 50-year-old, for instance, a reduction of 2.3 grams per day lowered systolic blood pressure by 3.74 mmHg (95% CI: 2.29 to 5.18). The exact effect varied by age and race, with greater reductions in older people (vs. younger people), blacks (vs. whites), and people with hypertension (vs. without).
Does that mean low sodium intake translates to better health? Not necessarily, according to two other articles published in this week’s NEJM. The Prospective Urban Rural Epidemiology (PURE) study collected fasting morning urine specimens from over 100,000 adults, representing 18 different countries and a range of income levels, and calculated 24-hour urinary sodium excretion (a proxy of sorts for sodium consumption). This was correlated with blood pressure.
The investigators found a non-linear relationship. For people who excreted a lot of sodium (in this study, defined as over five grams per day), there was a strong association, meaning each additional gram of sodium was linked to a steep rise in blood pressure (2.58 mmHg per gram). In contrast, for people with low sodium excretion (less than three grams per day), the association with blood pressure was not statistically significant (0.74 mmHg per gram; P=0.19). Remarkably, only ten percent of study participants fell into this low-sodium category, and only four percent had sodium excretion levels consistent with the current U.S. guidelines for sodium intake.
Based on these results, Dr. Suzanne Oparil of the University of Alabama at Birmingham suggests in an accompanying editorial, a low-sodium diet might not be the most useful public health recommendation. She writes, “The authors concluded from the findings that a very small proportion of the worldwide population consumes a low-sodium diet and that sodium intake is not related to blood pressure in these persons, calling into question the feasibility and usefulness of reducing dietary sodium as a population-based strategy for reducing blood pressure.”
The PURE study investigators also tested the correlation between sodium excretion and a composite outcome of death and major cardiovascular events (heart attacks, strokes, heart failure). They observed that, compared to a reference sodium excretion range of four to six grams per day, a higher level of sodium excretion (seven or more grams per day) was linked to a greater risk of the composite outcome (odds ratio 1.15; 95% CI, 1.02 to 1.30). But, when they looked at people with very low sodium excretion (below three grams per day), they found an increased risk as well (odds ratio, 1.27; 95% CI, 1.12 to 1.44).
Complicating matters further, the authors also looked at the urinary excretion of potassium. Compared to a reference level of 1.5 grams per day, a higher potassium excretion level was linked to a reduced risk of the composite outcome. “The alternative approach of recommending high-quality diets rich in potassium might achieve greater health benefits, including blood-pressure reduction, than aggressive sodium reduction alone,” Oparil writes.
To the scholars in us, these findings are an inspiration for further research. But how should we apply this knowledge to our daily lives, when we’re sitting in diners wondering whether to shake or not to shake? It may be presumptuous to read too much into the findings, to infer causality where it has yet to be established. Still, as a species, we’re better equipped today than we’ve ever been to make smart choices about our diet. And until more data become available, moderation may be the way to go.
The results of this new research are also summarized in a short animation.