TCS Daily

A Grain of Salt

By John Luik - November 15, 2005 12:00 AM

For Susan Jebb, head of nutrition and health research for the UK's Medical Research Council, the problem and the solution is really quite simple: Reducing salt intake can reduce blood pressure and dramatically improve Britain's health. Commenting at a recent press conference launching the MRC's report on salt, Jebb noted that among "independent scientists, there is a clear consensus on the evidence of the link between health and blood pressure." While that's true of course, it is not at all the same thing as saying that there is a clear consensus on the evidence of a link between population-wide health benefits and reduced salt intake. This is the real question at the heart of the revived debate about salt.

This new campaign against salt is not limited to the UK; it is also found across the Atlantic. In the US, the Center for Science in the Public Interest has also decided to target salt. Despite the fact that for the past 20 years the US government has issued a blanket warning about salt intake, urging Americans to reduce their salt consumption, CSPI now wants the FDA to classify salt as a food additive to "establish the basis for regulatory efforts by the FDA to limit the sodium content of processed foods."  

We've known for a long while that salt is linked to blood pressure and more recently we have understood that reducing salt intake by two-thirds will lower average blood pressure by a few millimeters. And we've also learned that populations whose average blood pressure is lower will have fewer strokes and heart attacks. Salt does raise blood pressure and blood pressure is one risk factor for cardiovascular disease.  

So it appears quite logical to assume, as do Dr. Jebb and CSPI, that reductions in dietary salt intake will result in reduced risk for strokes and heart attacks. As CSPI's report "Salt: The Forgotten Killer" claims, reducing US dietary salt intake would save about 150,000 lives a year. That's equivalent to the total US under-age-65 mortality from heart disease, a figure which alone makes the claim suspect as surely not all of those people died from a silent salt epidemic.  

The problem with this "salt hypothesis", however, is that salt science is much more complicated than the simple picture suggested by the MRC's Jebb and CSPI. Indeed, it fails to support such claims, particularly those about salt reduction leading to reductions in mortality.  

To begin with, there is no study showing population-wide net health benefits from low-sodium diets. Since 1995 at least 10 studies have looked at whether reduced salt diets provide a population health benefit. All of these studies have found that in the general population there is no health benefit from salt reductions, even though some sub-groups can benefit from reduced salt intake. That's why across the board salt reductions such as the MRC and CSPI call for make no scientific sense.  

Instead, the science, taken in its entirety, suggests that population-wide dietary salt reductions do not improve health outcomes, such as the number of strokes, heart attacks or the risk of premature mortality. In fact for some groups they actually increase certain risks. For example, analysis of the MRFIT (Multiple Risk Factor Intervention Trial), which followed the lives and deaths of 12,866 American males for an average of 12 years, found there were no health benefits from low-sodium diets.  

Nor is the MRFIT analysis an anomaly. A meta-analysis of randomized controlled trials of dietary salt reduction, published in the British Medical Journal in 2002, found significant salt reduction led to only very small reductions in blood pressure; the degree of salt reduction and change in blood pressure were not related and there were no health benefits. The researchers did find certain risks associated with reduced salt intake, including effects on vascular endothelium and on serum total and low-density lipoprotein cholesterol. As they noted, "lower salt intake in people with hypertension has been associated with higher [my emphasis] levels of cardiovascular disease and in general populations with greater all-cause mortality."  

This absence of benefit is also found in the Dietary Approaches to Stop Hypertension (DASH, 1997) study. In the original study, subjects consumed a diet high in fruits, vegetables and low-fat dairy products in which the salt content remained the same. After three weeks the DASH diet reduced blood pressure by 5.5/3.0 mmHG in mild hypertensives and 11.4/5.5 mmHG in those with extreme hypertension. Since the salt content was constant it had nothing to do with the blood pressure reductions.  

In the second DASH study (DASH-Sodium, 2000) researchers examined the effects of the DASH diet and a control diet at three levels -- 8, 6, and 4 grams a day -- of salt intake. For the hypertensives in this study the DASH diet combined with a sodium restriction of 4 grams reduced systolic blood pressure by 11.5 millimeters as compared to the original DASH study with normal sodium levels which reduced systolic blood pressure for this group by 11.4 millimeters, a non-significant difference. And for those with normal blood pressure, eating the DASH diet with a low salt intake made little difference in blood pressure.  

In an article published earlier this year in the American Journal of Hypertension which looked at data from the US National Health and Nutrition Examination Survey (NHANES), researchers found that top-number hypertensives -- those with systolic blood pressure more than 140 mm/HG -- already have the lowest intakes of salt, calcium, potassium and magnesium. What this group needs is a diet that increases these minerals. Telling them to reduce salt is obviously unwarranted and shows just how dangerous indiscriminate population-wide health advice -- like that offered by CSPI -- can be. Clearly, dietary advice is something that needs to be tailored to individuals, not populations. As the authors note "... the emphasis of national nutrition policy on sodium restriction for hypertension is not consistent with these findings." That's about as clear a recommendation against across the board salt reductions as you will find.  

These findings are also reflected in government advice, even within the US. For instance, the US Preventive Services Task Force Recommendations on healthy diets notes that "There is insufficient evidence, that, for the general population, reducing dietary sodium intake... results in improved health outcomes."  

None of this makes much difference to CSPI's anti-salt crusade. But for those who care about science-based public policy it should inspire an interest not only in opposing poorly evidenced population-wide measures to reduce salt intake but also in supporting long-term trials that look at whether salt-reduced diets really result in longer lives with fewer strokes and heart attacks.  

John Luik is writing a book on health policy.

TCS Daily Archives