A Fresh Foray in the Salt Wars: Life Expectancy Higher With Greater Sodium Intake
One expert points to other variables that could increase mortality, saying, “I don’t think this study is informative.”
Greater sodium intake on a national level is associated with a longer average life expectancy and reduced all-cause mortality, according to an ecological study based on data from 181 countries. But the meaning of these findings, which run counter to advice from international societies and organizations, remains unclear.
After adjusting for countries’ gross domestic product and average body mass index, there was an increase of 2.6 years of healthy life expectancy at birth—and an additional 0.3 years at age 60—for each additional gram of daily sodium intake, researchers led by Franz Messerli, MD (Bern University Hospital, Switzerland; Jagiellonian University, Krakow, Poland; and Icahn School of Medicine at Mount Sinai, New York, NY), report.
Moreover, all-cause mortality was inversely associated with sodium intake, with 131 fewer deaths for each additional gram of daily sodium intake, the analysis, published online December 22, 2020, ahead of print in the European Heart Journal, shows.
This is actually the most rejected paper in my career. Franz Messerli
“If indeed salt is a cardiac nemesis, then there should be [an inverse] correlation between life expectancy and salt intake,” Messerli told TCTMD, but “to my great surprise,” the opposite was seen. He pointed out that various groups recommend daily limits on sodium consumption—1.5 g for the American Heart Association, 2 g for the World Health Organization (WHO), and 2.3 g for the European Society of Cardiology—that were associated with short life expectancy and increased mortality compared with higher levels of intake in this study.
Messerli acknowledged the myriad limitations in an analysis like this and stressed that dietary recommendations cannot be made on the basis of the results. “Don’t increase your salt intake, but if you’re perfectly normotensive, there’s no reason to follow the recommendation of the American Heart Association to reduce the sodium intake below 1.5 g per day,” he said, adding that “these recommendations have had, I’m not saying no effect at all, but very little effect.”
For hypertensive patients, he continued, “I think it’s reasonable to attempt to reduce your salt intake.”
Nancy Cook, ScD (Brigham and Women’s Hospital, Boston, MA), who was not involved in the study, cautioned against reading too much into the findings. “This is an ecologic analysis, which is a very crude association analysis,” she commented via email. “While descriptive, it can’t be used to infer causality at all. These types of studies are subject to confounding by many factors, as well as reverse causation and mismeasurement. As a simple example, many African countries have both low sodium consumption and low life expectancy, while the US has higher sodium consumption and higher life expectancy. That doesn’t mean that sodium is the cause of the difference!”
She said she was surprised the paper was published, as the results may be misleading. “I don’t think this study is informative,” Cook said. “It can lead to a great deal of misunderstanding if readers don’t understand the design and methods used.”
Halt the Salt or Not?
The study is the latest volley in what has come to be known as the salt or sodium wars, a back-and-forth over the merits of restricting sodium in the diet to stave off hypertensive CVD. If increased levels of sodium intake are detrimental, then that should translate into shorter life expectancies, Messerli and colleagues propose.
The investigators tested that idea in the current study, which incorporated data from 181 countries. They included global health estimates from the WHO, gross domestic product estimates from the United Nations, and country-level income data from the World Bank. Estimates of country-level sodium intake were taken from a prior study.
These types of studies are subject to confounding by many factors, as well as reverse causation and mismeasurement. Nancy Cook
Average daily sodium intake ranged from 1.48 g in Kenya to 5.98 g in Kazakhstan. Average life expectancy at birth ran from 42.7 years in the Central Africa Republic to 74.8 years in Singapore and at age 60 from 9.8 years in Sierra Leone to 20.3 years in Japan.
Life expectancy tended to increase along with sodium consumption up to intake of 4 to 5 g per day, which equals a salt intake of 10 to 12.5 g per day. Above that threshold, life expectancy started to level off and eventually declined at higher levels of consumption.
To further account for socioeconomic differences across countries that could influence the relationship between sodium intake and life expectancy, the researchers performed a sensitivity analysis confined to the 46 countries with the highest incomes. Sodium intake remained positively associated with healthy life expectancy at birth (3.4 years per gram increase) and inversely associated with all-cause mortality (168 fewer events per gram increase). The relationship with life expectancy at age 60 was no longer significant.
The study “argues against dietary sodium intake being a culprit of curtailing life span or a risk factor for premature death,” Messerli et al write. They say the results “are startling in view of the numerous studies showing a direct association between dietary sodium intake and blood pressure. However, in most studies, the slope of the correlation between sodium intake and blood pressure is rather shallow.”
Pointing to a recent review in EHJ, they note that research suggests most people in the world consume a moderate amount of sodium, 2.3 to 4.6 g per day (equal to 1 to 2 teaspoons of salt), and that CVD risk increases only when consumption tops 5 g per day.
But Messerli et al, who acknowledge that “the number of potential confounders is infinite” in their study, “refrain from making any projections or dietary recommendations as time and again were inappropriately made with regard to reducing sodium intake despite the lack of evidence of hard endpoints. Whether changing sodium intake ultimately will affect life expectancy or all-cause mortality, individually or at country level, remains unknown and cannot be inferred from the present data.”
Still, the paper is still likely to get a salty reception from those who support restrictions of dietary sodium. Messerli said, “this is actually the most rejected paper in my career,” kicked back to him eight times before finding a home in EHJ. “The salt-restriction evangelists are a powerful group, and obviously they didn’t like the paper and they didn’t want to have it published.”
Messerli FH, Hofstetter L, Syrogiannouli L, et al. Sodium intake, life expectancy, and all-cause mortality. Eur Heart J. 2020;Epub ahead of print.
- Messerli reports having served as a consultant or advisor for Pfizer, Medtronic, WebMD, Menarini, the American College of Cardiology, and Sandoz.