Study questions benefits of low salt diet, experts quick to dismiss

The new study, published in the Journal of the American Medical Association (JAMA), ​examined health outcomes related to salt intake, including the incidence of death, illness and hypertension in relation to measures of urinary sodium excretion.

The research reported that lower sodium excretion was associated with an increased risk of cardiovascular death, while higher sodium excretion did not correspond with increased risk of hypertension or cardiovascular disease complications.

Speaking to FoodNavigator on behalf of Consensus Action on Salt and Health (CASH), Professor Graham MacGregor said that the observational study “is extremely paradoxical, suggesting that salt puts up blood pressure yet relates inversely to cardiovascular events – even though it is known that raised blood pressure is the biggest risk factor for cardiovascular disease, accounting for approximately 62 per cent of strokes and 49 per cent of all heart disease.”​

“The associations between systolic pressure and sodium excretion did not translate into less morbidity or improved survival … On the contrary, low sodium excretion predicted higher cardiovascular mortality,”​ said the researchers, led by Dr Katarzyna Stolarz-Skrzypek of the University of Leuven, Belgium.

“Taken together, our current findings refute the estimates of computer models of lives saved and health care costs reduced with lower salt intake … They do also not support the current recommendations of a generalized and indiscriminate reduction of salt intake at the population level,”​ they added.

Controversy​

The new JAMA​ study has already come in for some criticism. Prof MacGregor, of the Wolfson Institute of Preventive Medicine, and chairman of CASH explained that the paper has “what appear to be severe methodological problems”​, adding that “it is difficult to critically assess this paper.”​

In addition, officials at the Centers for Disease Control and Prevention (CDC) felt so strongly that the study was flawed that they criticized it in an interview with the New York Times​ – something they normally do not do.

Dr. Peter Briss, medical director at the CDC, told the NY Times that it is hard to draw any conclusions from the study given its relatively small size, and use of relatively young participants.

MacGregor and Briss both highlighted that the study goes against a raft of evidence showing that salt reduction is beneficial to the general population.

“This paper does not provide any substantiated evidence to the contrary,”​ said MacGregor, whilst Briss added that the study “might need to be taken with a grain of salt.”​

“There is no evidence that a modest reduction in population salt intake has any harmful effects on health, as such salt reduction is the simplest and most cost-effective method of reducing cardiovascular disease next to tobacco reduction,”​ added MacGregor.

Study details​

The researchers’ studied data from a prospective population study, involving 3681 participants without cardiovascular disease (CVD).

Among the 3681 participants followed, CVD deaths decreased with increasing measures of 24-hour sodium excretion, from 50 deaths in the lowest group, to 24 for medium excretion, and 10 in the highest excretion group.

“Lower sodium excretion was associated with higher CVD mortality,”​ said Dr Stolarz-Skrzypek and colleagues; noting that this inverse association between sodium excretion levels and death from CVD was statistically significant.

The researchers added that systolic blood pressure, but not diastolic pressure, changes over time aligned with changes in sodium excretion, “but this association did not translate into a higher risk of hypertension or CVD complications.”​

“It is unlikely that these findings were due to reverse causality because we excluded patients with a history of CVD … Moreover, these observations were consistent when we censored cardiovascular deaths over a time span ranging from 6 to 21 years, or excluded cardiovascular deaths occurring within 3 years of enrolment,”​ they explained.

Source: Journal of the American Medical Association (JAMA)​
Volume 305, Issue 17, Pages 1777-1785, doi:10.1001/jama.2011.574
“Fatal and Nonfatal Outcomes, Incidence of Hypertension, and Blood Pressure Changes in Relation to Urinary Sodium Excretion”​
Authors: K. Stolarz-Skrzypek, T. Kuznetsova, L. Thijs, V. Tikhonoff, et al​