Findings Are Latest Challenge to Benefits of Aggressively Low Sodium Targets

A long-running debate over the merits of eating less salt escalated Wednesday when one of the most comprehensive studies yet suggested cutting back on sodium too much actually poses health hazards.

Current guidelines from U.S. government agencies, the World Health Organization, the American Heart Association and other groups set daily dietary sodium targets between 1,500 and 2,300 milligrams or lower, well below the average U.S. daily consumption of about 3,400 milligrams. The new study, which tracked more than 100,000 people from 17 countries over an average of more than three years, found that those who consumed fewer than 3,000 milligrams of sodium a day had a 27% higher risk of death or a serious event such as a heart attack or stroke in that period than those whose intake was estimated at 3,000 to 6,000 milligrams. Risk of death or other major events increased with intake above 6,000 milligrams.

The findings, published in the New England Journal of Medicine, are the latest to challenge the benefit of aggressively low sodium targets—especially for generally healthy people. Last year, a report from the Institute of Medicine, which advises Congress on health issues, didn't find evidence that cutting sodium intake below 2,300 milligrams reduced risk of cardiovascular disease.
The new report has shortcomings, and as an observational study it found only an association, not a causative effect, between very low sodium and cardiovascular risk. Still, it spurred calls to reconsider the targets. This "adds a pretty big weight on the side that low salt intake is associated with harm," said Suzanne Oparil, professor of medicine at the University of Alabama at Birmingham and an expert on high blood pressure. Without evidence from randomized trials to back them up, the low-sodium targets are "questionable health policy," she said. Dr. Oparil was author of an editorial that accompanied the findings.

"It's about time that major groups who are making recommendations on sodium take a more measured approach," said Salim Yusuf of the Population Health Research Institute, or PHRI, at McMaster University in Ontario and senior author of two papers on the new study. The American Heart Association, a strong proponent of the low-sodium targets, isn't persuaded. Certain methods in the study, including how dietary sodium was estimated from urine samples, call "into question our ability to have confidence" in the findings, said Elliott Antman, AHA president.

"We hold fast to the recommendations that there is a need to reduce sodium intake in the diet," said Dr. Antman, a cardiologist at Brigham and Women's Hospital in Boston.
The Food and Drug Administration said it intends to review the studies. The agency said it "continues to recognize the need to reduce the sodium content of the food supply" to help reduce sodium intake.
Participants in the study, known as the Prospective Urban Rural Epidemiology study, or Pure, consumed an average of 4,930 milligrams of sodium a day, based on estimates derived from a single urine sample obtained when they enrolled in the study. The research was funded through a variety of public, private and corporate sources, according to PHRI.
Researchers followed participants for an average of 3.7 years. They found that 4.3% of those who consumed less than 3,000 milligrams of sodium either died or suffered a heart attack or stroke or developed heart failure in that time, versus 3.1% with intake between 3,000 and 6,000 milligrams. The percentage rose to 3.2% at levels above 6,000 milligrams and to 3.3% above 7,000 milligrams. Sodium, one of the components of salt, is a nutrient that is key to many cellular functions, many of which would likely "function on a lower level" with low sodium levels, said Niels Graudal, an internal-medicine specialist at Copenhagen University Hospital in Denmark who wasn't involved in this study. Too little sodium could trigger a hormonal response from the renin-angiotensin system that regulates blood pressure and actually increase blood pressure, researchers said. Very low sodium is also associated with higher blood fats called lipids, another risk for cardiovascular disease, Dr. Graudal said.

Little is known about what the right sodium levels are, but high levels are associated with high blood pressure, also known as hypertension—a key risk factor for heart attacks, strokes and heart failure. Estimates are that more than one billion adults world-wide have hypertension. Current guidelines are largely derived from short-term studies that found that low-salt diets helped people already diagnosed with hypertension or with borderline high blood pressure to get their readings significantly lower.

But studies that show the resulting blood-pressure reduction in such patients reduces risk of death or serious cardiovascular problems are lacking.
"There is not a single study, not one, showing [such a] benefit for having a sodium intake of less than 2,300 milligrams," said Brian Strom, chancellor of Rutgers Biomedical and Health Sciences in New Jersey who wasn't involved in the study but chaired the Institute of Medicine panel that reported on sodium last year.
Dr. Antman of the AHA said conducting a long-term randomized trial to prove such a benefit would be "very challenging."
Absent that, the heart association believes other research, including a recent British study associating a reduction of salt intake in the population during the past decade with a lower rate of death from stroke and heart disease, support aggressive dietary sodium targets. Underscoring the divide among heart experts over sodium intake levels, a separate study Wednesday in NEJM, from researchers at the Harvard School of Public Health, estimated that globally, there were 1.65 million deaths from cardiovascular causes in 2010 attributed to sodium consumption above 2,000 milligrams a day. The findings were derived from a complex calculation of results from 107 randomized studies and other data.

Dr. Antman said the result offered "a sense of the staggering numbers of people who are dying" from excess dietary sodium. Dr. Oparil, who is a past president of the heart association, termed researchers' efforts "herculean," but said because of the "lack of high-quality data" and "numerous assumptions" that went into the analysis, "caution should be taken in interpreting the findings."
Dr. Antman also said that the effect of a high-sodium diet accumulates over many years, leading to elevated blood pressure and a stiffening of blood vessels that results in increased risk for heart disease and strokes.
Reducing dietary sodium has been a public-health goal for several decades, but meeting the recommended targets is a daunting challenge for most people. Fewer than 1% of Americans are now in compliance, Dr. Strom said. More than three-quarters of dietary sodium comes not from the salt shaker but from processed food and restaurant fare, according to the FDA. Grocery-store aisles and restaurants, according to information on a Texas A&M University website, are rich in sodium: 264 milligrams in two slices of whole-wheat bread; 1,107 in a cup of chicken noodle soup; 639 in a hot dog; 1,093 in a frozen pot pie; 709 in a fast-food cheeseburger.

Such levels help explain calls for efforts to work with food makers to reduce sodium content. "Very high levels of sodium intake appear to be associated with bad outcomes," said Michael Lauer, director of cardiovascular sciences at the National Heart, Lung and Blood Institute. "That's an important message to keep in mind."

최근 적은 양의 소금을 섭취할 경우 건강 위험에 노출될 가능성이 높다는 연구들이 확대되고 있다.
최근 WHO, AHA (the American Heart Association) 등은 일일 나트륨 섭취량을 현재 미국인들의 평균 섭취량인 3,400 mg을 훨씬 밑도는 1,500-2,300mg 이하로 설정하였다.
그러나 17개의 나라에서 평균 3년 이상 100,000 명 이상의 사람들을 추적한 최근 연구에서는 하루에 3,000 mg 미만의 나트륨을 섭취한 사람이 3,000-6,000 mg을 섭취한 사람보다 사망 위험 혹은 심장마비나 뇌졸중과 같은 심각한 질병에 대한 노출 정도가 27% 이상 높았고, 나트륨 섭취량 6,000mg 이상으로 높을 경우에도 증가하였고 보고하였다.
지난해 개최된 의학학회 보고에서 2,300 mg 이하의 나트륨 섭취가 심혈관계 질환의 위험을 감소시키는 것에 대한 증거가 불충분하다고 보고하였고, 또 다른 연구보고에서는 저 나트륨섭취와 심혈관계질환 사이의 연관성이 발견되었다고 보고하였다. 여러 연구들에서 3.7년 동안 환자들을 추적한 결과, 나트륨을 3,000 mg 이하로 섭취하는 사람은 3,000-6,000 mg의 나트륨을 섭취하는 사람보다 사망 혹은 심장마비, 뇌졸중과 같은 질병에 노출되는 확률이 높았고 7,000 mg 이상 나트륨을 섭취한 사람은 6,000 mg 이상 나트륨을 섭취한 사람보다 질병에 노출되는 확률이 증가하는 것을 발견했다. 소금의 구성성분 중 하나인 나트륨은 많은 세포의 기능을 위한 필수적인 영양소중의 하나로 낮은 수준의 나트륨은 세포의 기능을 저해하고 지질과 다른 심혈관계 질환과도 연관이 있으며, 또한 혈압의 증가와 혈압을 조절하는 레닌 안지오텐신 의 호르몬 시스템에 영향을 줄 수 있다라고 하였다.
현재까지 적절한 나트륨 섭취수준은 알려지지 않았으나 아직까지는 높은 수준의 나트륨이 고혈압, 심장병, 심장마비, 뇌졸중과 같은 질병에 연관이 있다고 본다. 식이 나트륨을 줄이는 것은 몇십년동안 공공의 건강목표였지만, 권장 목표를 달성하는 것은 대부분의 사람들에게 어려운 과제로 보여진다. FDA에 따르면 “식이나트륨의 4분의3 이상은 가공식품, 레스토랑의 식사로 온다” 라고 하였다. 그러므로 이러한 나트륨 함량을 줄이기 위해 식품업체에서도 함께 나트륨함량을 줄이기 위한 노력을 해야 할 것이다. 저 나트륨의 섭취와 여러 건강성의 위험에 대한 연구가 진행 중이지만 아직까지는 높은 나트륨의 섭취는 건강에 안 좋은 결과로 나타남을 명심해야 할 것이다.