The National Academy of Sciences recommends that Americans consume a minimum of 500 mg/day of sodium to maintain good health. Individual needs, however, vary enormously based on their genetic make-up and the way they live their lives. While individual requirements range widely, most Americans have no trouble reaching their minimum requirements. Most consume “excess” sodium above and beyond that required for proper bodily function. The kidneys efficiently process this “excess” sodium in healthy people. Experimental studies show that most humans tolerate a wide range of sodium intakes, from about 250 mg/day to over 30,000 mg/day. The actual range is much narrower. Americans consume about 3,500 mg/day of sodium; men more, women less. The very large percentage of the population consumes 1,150- 5,750 mg/day which is termed the “hygienic safety range” of sodium intake by renowned Swedish hypertension expert Dr. Björn Folkow. Chloride is also essential to good health. Every substance, including water, can be toxic in certain concentrations and amounts; this is not a significant concern for dietary salt.
Salt and Cardiovascular Health
For 4,000 years, we have known that salt intakes can affect blood pressure through signals to the muscles of blood vessels trying to maintain blood pressure within a proper range. We know that a minority of the population can lower blood pressure by restricting dietary salt. And we know that elevated blood pressure, “hypertension,” is a well-documented marker or “risk factor” for cardiovascular events like heart attacks and strokes, a “silent killer.” Cardiovascular events are a major cause of “premature” death and cost Americans more than $300 billion every year in increased medical costs and lost productivity. Reducing blood pressure can reduce the risk of a heart attack or stroke – depending on how it’s done.
Some have suggested that since salt intakes are related to blood pressure, and since cardiovascular risks are also related to blood pressure, that, surely, salt intake levels are related to cardiovascular risk. This is the “salt hypothesis” or “sodium hypothesis.” Data are needed to confirm or reject hypotheses.
Blood pressure is a sign. When it goes up (or down) it indicates an underlying health concern. Changes result from many variables, often still poorly-understood. High blood pressure is treated with pharmaceuticals and with lifestyle interventions such as diet and exercise. The anti-hypertensive drugs are all approved by regulatory authorities such as the U.S. Food and Drug Administration. To be approved, these drugs must prove they work to lower blood pressure. Whether they also work to lower the incidence of heart attacks and strokes has not been the test to gain approval (it would take too long to develop new drugs), but the National Heart, Lung and Blood Institute has invested heavily in such “health outcomes” studies.
The ALLHAT study was funded by the National Heart, Lung and Blood Institute (NHLBI) to compare the health outcomes of four classes of anti-hypertensive drugs, all of which had demonstrated their ability to reduce blood pressure in relative safety. The idea is that blood pressure is only a “surrogate outcome,” and we should be more concerned with clinically meaningful endpoints. Dr. Jeffrey R. Cutler of the National Heart, Lung and Blood Institute (NHLBI) has supervised the study and explains its importance: “Trials are based on the notion that different antihypertensive regimes, despite similar efficacy in lowering blood pressure, have other beneficial or harmful effects that modify their net effect on cardiovascular or all-cause morbidity and mortality.”
Lifestyle interventions are “antihypertensive regimes” too. For years, the same situation prompting the ALLHAT trial applied to lifestyle interventions designed to improve blood pressure — they were untested regarding health outcomes. Certain dietary and lifestyle interventions reduced blood pressure, at least in sensitive sub-populations. Whether they also reduced the incidence of heart attacks and strokes had never been tested. Thus, until the 1990s, scientists had never tested the “salt hypothesis” by documenting whether reducing dietary salt actually reduces a person’s chances of having a heart attack or a stroke. As in the drug “health outcomes” trials, this is now changing. The results have vast public health policy implications. We should not be recommending that everyone change their diets without evidence of some overall health benefit.
Even documenting an association of, for example, low-sodium diets with reduced incidence of heart attacks would only be the first step. Association is not the same as causation. Nevertheless, unless an association is established, we have no reason to think that a causal link is possible. Of the first fifteen “health outcomes” studies of sodium reduction, three have found an association in the general population between low-sodium diets and reduced incidence of cardiovascular events like stroke or heart attack (and two of those were in exceptionally high salt-consuming societies). Three others have identified health risks of low-salt diets. Here’s what scientists have found:
1. A ten-year study of nearly 8,000 Hawaiian Japanese men concluded: “No relation was found between salt intake and the incidence of stroke.” (1985)
2. An eight-year study of a New York City hypertensive population stratified for sodium intake levels found those on low-salt diets had more than four times as many heart attacks as those on normal-sodium diets – the exact opposite of what the “salt hypothesis” would have predicted. (1995)
3. An analysis by NHLBI’s Dr. Cutler of the first six years’ data from the MRFIT database documented no health outcomes benefits of lower-sodium diets. (1997)
4. A ten-year follow-up study to the huge Scottish Heart Health Study found no improved health outcomes for those on low-salt diets. (1997)
5. An analysis of the health outcomes over twenty years from those in the massive US National Health and Nutrition Examination Survey (NHANES I) documented a 20% greater incidence of heart attacks among those on low-salt diets compared to normal-salt diets ( 1 2 ) (1998)
6. A health outcomes study in Finland, reported to the American Heart Association that no health benefits could be identified and concluded “…our results do not support the recommendations for entire populations to reduce dietary sodium intake to prevent coronary heart disease.” (1998)
7. A further analysis of the MRFIT database, this time using fourteen years’ data, confirmed no improved health benefit from low-sodium diets. Its author conceded that there is “no relationship observed between dietary sodium and mortality.” (1999)
8. A study of Americans found that less sodium-dense diets did reduce the cardiovascular mortality of one population sub-set, overweight men – the article reporting the findings did not explain why this obese group actually consumed less sodium than normal-weight individuals in the study. (1999)
9. A Finnish study reported an increase in cardiovascular events for obese men (but not women or normal-weight individuals of either gender) – the article, however, failed to adjust for potassium intake levels which many researchers consider a key associated variable. (2001)
10. In September, 2002, the prestigous Cochrane Collaboration produced the latest and highest-quality meta-analysis of clinical trials. It was published in the British Medical Journal and confirmed earlier meta-analyses’ conclusions that significant salt reduction would lead to very small blood pressure changes in sensitive populations and no health benefits. (2002)
11. In June 2003, Dutch researchers using a massive database in Rotterdam concluded that “variations in dietary sodium and potassium within the range commonly observed in Westernized societies have no material effect on the occurrence of cardiovascular events and mortality at old age.” (2003)
12. In July 2004, the first “outcomes” study identifying a population risk appeared in Stroke magazine. Researchers found that in a Japanese population, “low” sodium intakes (about 20% above Americans’ average intake) had one-third the incidence of fatal strokes of those consuming twice as much sodium as Americans. (2004)
13. A March 2006 analysis of the federal NHANES II database in The American Journal of Medicine found a 37% higher cardiovascular mortality rate for low-sodium dieters (2006). See their university’s news release. Hear a podcast.
14. A February 2007 reported in the International Journal of Epidemiology studied 40,547 Japanese over seven years and found “the Japanese dietary pattern was associated with a decreased risk of CVD mortality, despite its relation to sodium intake and hypertension.” (2007)
15. An April 2007 article in the British Medical Journal found a 25% lower risk of CV events in a group which years earlier had achieved significant sodium reduction during two clinical trials (TOHP I and TOHP II). (2007)
For many years, the intense public controversy that has characterized the public policy debate over public health nutrition recommendations on salt intake has focused on the wrong question. Medical experts, public health policy-makers and the public, trying to sort out the issues reading the consumer press – have all focused on the relationship of sodium intake to blood pressure instead of the relevant question of whether changing intake levels of dietary sodium results in improved health outcomes. See, for example, Salt Institute comments to the (British) Scientific Advisory Committee on Nutrition. The (British) Salt Manufacturers Association has further information (including its comments to SACN).
There is no evidence that reducing dietary sodium improves the risk for heart attacks or strokes for the general population. In 1999, the Canadian Hypertension Society, the Canadian Coalition for High Blood Pressure Prevention and Control, the Health Canada Laboratory Centre for Disease Control and the Heart and Stroke Foundation of Canada issued a joint statement opposing general recommendations for sodium reduction.
The U.S. Preventive Services Task Force Recommendations for a healthy diet, chapter 56 on “Counseling to Promote a Healthy Diet,” at page 634 states:
There is insufficient evidence that, for the general population, reducing dietary sodium intake or increasing dietary intake of iron, beta-carotene, or other antioxidants results in improved health outcomes (”C” recommendation); recommendations to reduce sodium intake may be made on other grounds, including potential beneficial effects on blood pressure in salt sensitive persons.”
The debate has confused the public. Medical journalists from ABC-TV’s 20/20 to America’s pre-eminent scientific journal, Science, published by the prestigious American Association for the Advancement of Science, have investigated the source of this confusion. The report in Science won author Gary Taubes a top prize from the National Association of Science Writers and has also been translated into French. Taubes concluded:
“After interviews with some 80 researchers, clinicians, and administrators around the world, it is safe to say that if ever there were a controversy over the interpretation of scientific data, this is it…. After decades of intensive research, the apparent benefits of avoiding salt have only diminished. This suggests either that the true benefit has now been revealed and is indeed small or that it is non-existent and researchers believing they have detected such benefits have been deluded by the confounding of other variables.”
In letters to Science, NHLBI contested Taubes’ conclusions, but others found them valid and valuable.
The Salt Institute is confident that the higher standards of evidence-based medicine will reduce the ongoing controversy, better inform public policy and reduce consumer confusion. For more information about the importance of evidence-based health, you may wish to visit the Cochrane Collaboration (particularly consider the 2003 Cochrane Reviews “Reduced dietary salt for prevention of cardiovascular disease” and “Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterols, and triglyceride”), Oxford University (UK) Centre for Evidence-based Medicine, the Health Information Research Unit (McMaster University) or the Canadian Centres for Health Evidence. Using the latest science, we can create better public health nutrition policy and avoid sending confusing messages to consumers.
There is a lot of current activity on this issue, in medical research, public health policy and popular media ( 1 2 3 ). For further information contactthe Salt Institute (or see an informative presentation by the Institute), federal Centers for Disease Control and Prevention, Journal of the American Medical Association (JAMA), The New England Journal of Medicine, American Journal of Hypertension, The Annals of Internal Medicine, The Lancet, British Medical Journal, Public Library of Medicine, Federation of American Societies of Experimental Biology (FASEB), American Heart Association, American Society of Hypertension (ASH), National Heart, Lung and Blood Institute (NHLBI), American Dietetic Association (ADA), Society for Nutrition Education (SNE), The American Society for Clinical Nutrition, National Food Processors Association, American Council for Science and Health and (British) Salt Manufacturers Association. Information about clinical trials sponsored by the U.S. federal government is also online.