Salt and Good Health
Salt is essential not only to life, but to good health. 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 a person's genetic make-up and their lifestyle. 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. Every substance, including water, can be toxic in certain concentrations and amounts; this is not a significant concern for dietary salt.
Most of our salt comes from foods, some from water. Doctors often recommend replacing water and salt lost in exercise and when working outside. Wilderness hikers know the importance of salt tablets to combat hyperthermia. Oral rehydration involves replacing both water and salt. Expectant mothers are advised to get enough salt. Increased salt intakes have been used successfully to combat Chronic Fatigue Syndrome. Dramatic deficiencies (e.g. "salt starvation" in India) or "excessive" sodium intakes have been associated with other conditions and diseases, such as hypertension and stomach cancer. Testing the salinity of perspiration is a good test for cystic fibrosis; scientists suspect that cystic fibrosis is caused by a deformed protein that prevents chloride outside cells from attracting needed moisture. Human blood contains 0.9% salt (sodium chloride), the same concentration as found in United States Pharmacopeia (USP) sodium chloride irrigant commonly used to cleanse wounds. Salt maintains the electrolyte balance inside and outside of cells.
Salt and Cardiovascular Health
For 4,000 years, we have known that salt intake 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 its done.
Some have suggested that since salt intake is 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 persons 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 nine health outcomes studies of sodium reduction, not a single study has found an association in the general population between low-sodium diets and reduced incidence of cardiovascular events like stroke or heart attack.
Heres what scientists have found:
1. 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)
2. An analysis by NHLBIs Dr. Cutler of the first six years data from the MRFIT database documented no health outcomes benefits of lower-sodium diets. (1997)
3. 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)
4. 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 (1998)
5. 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)
6. 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)
7. 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)
8. 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)
9. In September, 2002, the latest and highest-quality meta-analysis of clinical trials was published in the British Medical Journal confirming earlier meta-analyses' conclusions that significant salt reduction would lead to very small blood pressure changes in sensitive populations and no health benefits. (2002)
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, recent Salt Institute comments to the (British) Scientific Advisory Committee on Nutrition.
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 has also reviewed the evidence and concluded:
"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."
The debate has confused the public. Medical journalists from ABC-TVs 20/20 to Americas 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.
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.
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, 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.
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