by Craig Weatherby
Salt intake has been rising among Americans, and this growing dietary excess harms their health… right?
Actually, the response from many sodium-savvy researchers would be "not really"—making the obvious exception for folks with sodium-related hypertension or at very clear risk for it.
A growing body of evidence undermines both the notion of fast-rising salt intake causing an epidemic of hypertension in the general public (not just high-risk groups)... and the consensus that lowering salt intake well below the current American average is healthful for everyone.
(To learn about the “salt hypothesis” of hypertension and its persistence in the face of stubborn facts, see our sidebar titled “Quick history of the salt controversy,” below.)
Salt intake has been rising among Americans, and this growing dietary excess harms their health… right?
Actually, the response from many sodium-savvy researchers would be "not really"— while making the obvious exception for folks with sodium-related hypertension or at very clear risk for it.
A growing body of evidence undermines the notion that salt intake is rising and that this increase is causing an epidemic of hypertension in the general public.
(To learn about the “salt hypothesis” of hypertension and its persistence in the face of stubborn facts, see our sidebar titled “Quick history of the salt controversy”, below.)True, the available evidence shows that for many but not all adults, cutting sodium intake drops blood pressure.
And sodium experts are baffled by the truly extreme proposal—made this summer by a USDA advisory panel—that the 2010 Dietary for Guidelines for Americans should call for all adult Americans to cut their salt intake in half.
But as one delightfully wry physician wrote, “so does applying leeches.”
More to the point, a net health benefit from universal salt restriction has never been proven, nor could one ever be tested ethically.
Indeed, low sodium intake is linked to serious health conditions, and researchers expert in sodium's myriad bodily roles say that extreme sodium-cutting in the general public is a risky experiment.
If your doctor says you need to reduce sodium intake drastically to treat hypertension, by all means follow that advice … if it seems well justified by his or her reasons.
New guidelines lack scientific support
This statement made last summer by a committee advising the USDA on that agency's proposed 2010 Dietary Guidelines for Americans is true as far as it goes:
“The projected health benefits of a reduced sodium intake [among the general adult population] are substantial and include fewer strokes, cardiovascular disease events, and deaths...” (DGAC 2010).
But it utterly ignores ample evidence that the extreme universal sodium restriction they propose as the official U.S. nutrition advice through 2015 could, if followed, wreak unholy havoc on other aspects of Americans' health.
Given the uncertainties about sodium and hypertension and clear risk of harm from strict, universal sodium restriction, it seems passing strange that USDA's expert advisors advocate such an extreme yet evidence-light position for the fast-approaching 2010 Dietary Guidelines... which will last through 2015.
Quick history of the salt controversy
Starting in the late 1940's, physicians began to routinely prescribe low-sodium diets for patients with high blood pressure (hypertension).
In 1954, Dr. Lewis K. Dahl proposed a direct link between the sodium intake of a population and the subsequent development of hypertension, which became known as the “salt hypothesis” (Dahl LK 1954).
But in the very same year, the U.S. Food and Nutrition Board warned against applying this advice more broadly, noting that “harmful results may follow the restriction of sodium intake ... these changes are potentially dangerous, perhaps leading to reduced kidney function.”
Doubts notwithstanding, as the result of well-placed advocacy efforts, physicians suddenly began prescribing low-salt diets for all their patients, not just those with documented high blood pressure.
But in the 1970s and 1980s, research fueled a growing realization that doctors' advice about sodium and hypertension was not supported by good evidence.
Studies began to link low-sodium diets to high blood pressure, elevated LDL (bad) cholesterol, and higher fasting insulin levels, which is a risk factor for diabetes.
To settle the sodium question, a huge international study called INTERSALT—which consisted of many separate, coordinated studies—was launched.
By 1986, when the final data from INTERSALT was analyzed, only three studies demonstrated any sodium-hypertension association. The results were summarized with this succinct statement: “Salt has only small importance in hypertension”.
Incredibly, the U.S. government continued to recommend that everyone should reduce their consumption of sodium, even though the 1988 US Surgeon General's Report admitted that the relationship of universal sodium restriction to lowering blood pressure had never really been tested, much less proved.
The 2005 U.S. Dietary Guidelines, which the imminent 2010 update will replace, urge no more than 2,300 mg daily for healthy adults... and only 1,500mg per day for people with hypertension, African-Americans, and middle-aged and older adults.
Because these higher-risk groups now make up nearly 70 percent of US adults, the advisory committee for the 2010 Dietary Guidelines said the goal should be 1,500 mg per day for all adults.
The 2010 Guidelines won't be issued until December, but the committee's sodium recommendation—only 1,500 daily for all adults—may well become part of them.
Their extreme position is unrealistic, as they admitted:
“Given the current US marketplace [filled with salty processed foods] and the resulting excessively high sodium intake, it will be challenging to achieve the lower level. Thus, the reduction from 2,300 mg to 1,500 mg per day should occur gradually over time.”
No kidding… such a steep drop seems unobtainable. Worse, it could even be harmful, according to sodium experts.
The current U.S. recommended daily maximum sodium intake (2,300mg) is 17 percent lower than the lowest level of sodium intake (2,700mg) found anywhere in the world… and a whopping 38 percent lower than the worldwide average intake (3,700mg).
To help put these doses in perspective, 2,300mg of sodium is the amount in about one teaspoon of table salt (sodium chloride).
A raft of recent findings support the skeptical view expressed by the authors of a 2001 evidence review:
“Funding agencies and medical journals have taken a stronger position favoring the salt hypothesis than is warranted, raising questions about the interaction between the policy process and science” (Freedman DA, Petitti DB 2001).
Let‘s look at some findings on salt intakes in 33 countries, published last year, and then examine similar findings about Americans' salt intakes, published earlier this month by a Harvard team.
Salt intakes in Western countries seen as surprisingly stable
Conventional wisdom holds that salt intake has reached extreme levels in Western societies.
But last year, researchers from UC Davis and Washington University in St. Louis reported compelling evidence that humans naturally self-regulate their salt intake within a narrow range.
The UC-Washington team analyzed the results of urinary excretion tests among 19,151 people in 33 countries, to determine their daily sodium intake as accurately as possible.
The UC-Washington team's analysis showed that daily sodium intake falls within a relatively narrow range—2,700 to 4,900 mg per day—across countries with divergent diets.
Their findings support those of previous studies showing that adult humans naturally seek this range of sodium intake.
And the authors of the new analysis cited animal research demonstrating that sodium intake is tightly controlled by critical pathways in the brain… a mechanism designed to maintain optimal physiological functions.
Harvard study confirms similarly stable salt intake among Americans
A study by two researchers from the Harvard School of Public Health shows that sodium consumption in the United States has remained unchanged for more than 40 years (Bernstein AM, Willett WC 2010).
To draw a picture of sodium intake in the U.S., the Harvard duo analyzed data from 38 studies conducted during the 46 years from 1957 to 2003, which included more than 26,000 participants.
In each study, the participants' sodium intake was monitored by recording levels of sodium in the urine, which is the most accurate indicator.
The Harvard researchers' analysis found no significant change in urinary sodium over those four-plus decades.
In short, sodium consumption did not increase over that time, and the researchers estimated that the average sodium intake from 1957 to 2003 remained relatively constant… at 3,700 milligrams per day.
So much for the common assumption that Americans have been consuming salt at a steadily increasing rate.
Accompanying editorial decries U.S. attempts to force lower salt intake
In a companion editorial in the American Journal of Clinical Nutrition, a trio of academic researchers wrote that the Harvard findings prove that federal efforts to reduce salt intake are both futile and unnecessary (McCarron DA et al. 2010).
The editorial was penned by David McCarron, M.D., of UC Davis—who co-authored the international sodium-intake study described earlier—kidney specialist Tilman Drüeke, M.D., of France's National Institute of Health and Medical Research, and sodium expert Edward Stricker, Ph.D., emeritus chair of neurosciences at the University of Pittsburgh.
As Dr. McCarron said, “The Harvard paper defines a narrow range of human sodium intake over nearly 50 years in the U.S. Combined with our findings, published in 2009, these data should close the debate on whether there should be a recommended level of salt intake for the general population.”
In their editorial, McCarron, Drüeke, and Stricker emphasize that public policy efforts in the U.S. during the past 30 years have had no impact on Americans' average sodium intake. And sodium intake has not increased as processed foods have become saltier.
The authors also noted the importance of sodium in maintaining normal cell function across virtually all organ systems and suggest that the human body has likely evolved internal “fail-safe mechanisms” to insure sufficient sodium in the body.
Professor Stricker—a leading researcher of the processes that balance water and sodium in the body—said this in a press release:
“Human physiology indicates that sodium intake should not fall below 2,300 to 2,760 mgs per day; setting a goal for the general population of a salt intake below that level makes no biological sense” (UCD 2010).
McCarron put his and the two other editorialists' position bluntly:
“Public policy should not try to trump human physiology. Any attempts to do so through well-intended strategies directed at the society at large, such as mandatory sodium labeling of food products and extensive educational and social marketing efforts, are not going to change an intake pattern that reflects human biology. Such efforts also carry potentially substantial risks” (McCarron DA et al. 2010).
Despite the findings on salt consumption by the Harvard and UC-Washington teams, it is expected that the 2010 U.S. Dietary Guidelines will set the safe upper limit for all individuals, regardless of health status, at 1,500 mgs per day, or less than 40 percent of the average intake.
So much for the naïve assumption that science drives America's public health policies!
Bernstein AM, Willett WC. Trends in 24-h urinary sodium excretion in the United States, 1957-2003: a systematic review. Am J Clin Nutr. 2010 Nov;92(5):1172-80. Epub 2010 Sep 8. Review.
Dahl LK. Sodium intake of the American male: implications on the etiology of essential hypertension. Am J Clin Nutr. 1958 Jan-Feb;6(1):1-7
Freedman DA, Petitti DB. Salt and blood pressure. Conventional wisdom reconsidered. Eval Rev. 2001 Jun;25(3):267-87. Review.
McCarron DA, Drüeke TB, Stricker EM. Science trumps politics: urinary sodium data challenge US dietary sodium guideline. Am J Clin Nutr. 2010 Nov;92(5):1005-6. Epub 2010 Oct 6.
Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010. June 15, 2010. Accessed at http://www.cnpp.usda.gov/DGAs2010-DGACReport.htm
University of California Davis (UCD). New study shows science should trump politics in salt debate, editorial says. November 11, 2010. Accessed at http://www.aes.ucdavis.edu/NewsEvents/web-news/2010/11/new-study-shows-science-should-trump-politics-in-salt-debate-editorial-says