Salt: More confirmation bias for your preferred narrative

Death in a bottle? Or harmless food enhancer?

Judging by the recent press reports, the latest Cochrane review reveals that everything we’ve been told about eating salt, and cardiovascular disease, is wrong:

The New York Times: Nostrums: Cutting Salt Has Little Effect on Heart Risk

The Daily Mail: Cutting back on salt ‘does not make you healthier’ (despite nanny state warnings)

Scientific American: It’s Time to End the War on Salt

Sometimes it’s possible to completely miss this point. And that’s what’s happened here.

When it comes to health, it’s the hard outcomes we care about. We pay attention to measures like high blood pressure (hypertension) because of the relationship between hypertension and events like heart attacks and strokes. The higher the blood pressure, the greater the risk of these events. The relationship between the two is well established. So when it comes to preventive health, we want to lower blood pressure to reduce the risk of subsequent effects. Weight loss, diet, and exercise are usually prescribed (though often insufficient) to reduce blood pressure. For many, drug treatment is still required.

There is reasonable population-level data linking higher levels of salt consumption with higher blood pressure. From a population perspective, interventions that dramatically lower salt intake result in lower blood pressure. Not everyone responds in the same way — many people with normal blood pressure can regularly consume a high salt load without any apparent change in blood pressure. But not everyone, and not forever. Salt sensitivity seems to increase with age and is more pronounced in some ethnic groups, as well as in those with salt-sensitive conditions such as kidney disease. And chronic high levels of salt consumption may be associated with the subsequent emergence of hypertension. There may be additional effects, unrelated to blood pressure, too. However, the causality between salt consumption, and all of these negative effects, is less clear.

So does reducing dietary salt reduce cardiovascular events? That’s the key question. To definitively answer the question, we’d randomize patients to high- and low-sodium diets, force them to follow these diets for years or decades, and monitor consumption, blood pressure, and cardiovascular events. We’d also want to explore the factors that seem to make some more sensitive to the effects of salt than others. To ensure we could see a difference (if it exists), we’d need a large sample size — hundreds or thousands of people, ideally. See any problems with the feasibility? Like any dietary intervention trial, this type of study would be exceptionally difficult to do — forcing dietary changes is very difficult, and cannot be done in a blinded manner. Even randomization is unlikely to be effective in ensuring there’s adherence — established dietary habits don’t lend themselves to long-term change easily. So we must look to lower-quality evidence — inferences from observational studies that have tracked consumption, or indicators like blood pressure and salt consumption in the short-term. And there are fair criticisms of the data. Some see relationships, and others dismiss them.

When it comes to clinical practice guidelines, low salt diets are the mainstays of pretty much every set of guidelines on the management of high blood pressure. The evidence supporting the relationship with hard outcomes is robust, but not rock-solid. We don’t have causal data, but we do have considerable epidemiologic evidence to suggest that reducing dietary salt consumption is likely to offer net benefits in the management of hypertension.

And that’s where the recommendations to cut salt come from. The vast majority of the salt we eat (75%) is from processed foods. Restaurants are a large source, too. Few foods in their original state are naturally high in salt, and in general, we don’t add that much at the table. Interestingly, when foods are reduced in sodium, we don’t tend to add the same amount back at the table. So public health initiatives have concentrated on a few strategies: education on how to reduce your own salt consumption, and putting pressure on packaged food manufacturers to reduce the amount of sodium that they use in their products. But reducing salt may hurt sales: if we’re accustomed to eating salty foods, low-salt foods taste unpalatable. Just last week Campbell Soup Company announced that it’s raising the salt content in its products in an attempt to boost sagging sales.

So do dietary intervention strategies work? That’s what a recent Cochrane review attempted to answer. But you wouldn’t know it, based on the headlines above. Scientific American described the paper as:

This week a meta-analysis of seven studies involving a total of 6,250 subjects in the American Journal of Hypertension found no strong evidence that cutting salt intake reduces the risk for heart attacks, strokes or death in people with normal or high blood pressure.

The Daily Mail?

Eating less salt will not prevent heart attacks, strokes or early death, according to a major study.
Its findings contradict all recommendations by the Government and medical profession urging the public to reduce the amount of salt they consume.

Neither statement accurately describe the findings. Rod Taylor and colleagues set out to do a meta-analysis of dietary intervention studies. They analyzed only studies that measured the effects of dietary interventions that restricted salt consumption, or where the intervention was advice to reduce salt consumption. This was an update of a prior analysis.

Seven studies made up this meta-analysis, including 6,489 patients in total. Three studies looked at those with normal blood pressure, two included patients with high blood pressure, and one was a mixed population, including patients with heart failure. The overall effect? Interventions had small effects on sodium consumption, which led to small effects on blood pressure. There was insufficient information to analyze the effects on cardiovascular disease endpoints.

The authors go on to make the following point, which was ignored in the media coverage:

Our findings are consistent with the belief that salt reduction is beneficial in normotensive and hypertensive people. However, the methods of achieving salt reduction in the trials included in our review, and other systematic reviews, were relatively modest in their impact on sodium excretion and on blood pressure levels, generally required considerable efforts to implement and would not be expected to have major impacts on the burden of CVD.

The authors did not conclude that reducing salt consumption is ineffective. They concluded that interventions such as dietary advice, do not result in substantial reductions in consumption. As expected, blood pressure didn’t change much as a consequence. This finding should not be a surprise. Given the vast majority of salt is consumed via processed foods, it should come as no surprise that dietary approaches are modestly effective at reducing consumption.

Despite the modest and equivocal results, the authors seem to have lost the narrative on their own research findings:

Professor Rod Taylor, the lead researcher of the review, is ‘completely dismayed’ at the headlines that distort the message of his research published today. Having spoken to BBC Scotland, and to CASH, he clarified that the review looked at studies where people were advised to reduce salt intake compared to those who were not and found no differences, this is not because reduced salt doesn’t have an effect but because it’s hard to reduce salt intake for a long time. He stated that people should continue to strive to reduce their salt intake to reduce their blood pressure, but that dietary advice alone is not enough, calling for further government and industry action.

Conclusion

The true finding from the Cochrane review is that dietary interventions to reduce salt intake are largely ineffective at reducing salt consumption. Salt’s impact on cardiovascular events is less clear than its effects on blood pressure. And the long-term benefits of population-level interventions to reduce dietary salt consumption are not yet well established. Until the data are more clear, you can find the data to support whatever narrative you believe. If you want to demonize salt and ignore other factors that contribute to poor cardiovascular outcomes, you can do that. And if you believe that interventions to reduce salt consumption are misguided and unwarranted, and symptomatic of an overreaching nanny state, then you can find data to support that position, too.

My personal take is that most of us will ultimately end up with salt-sensitive conditions. Odds are good we’ll be hypertensive, too. Gradually reducing our chronic salt consumption would seem to be a conservative approach — not by focusing strictly on the salt, but by working to reduce the consumption of salty, processed foods, and substituting healthier, more nutritious choices instead. But I won’t worry when I finish an entire bag of chips — I’ll consider it in the context of an overall strategy: a diet that minimizes processed foods, maintaining an appropriate weight, and getting regular physical exercise.

Reference
ResearchBlogging.org
Taylor RS, Ashton KE, Moxham T, Hooper L, & Ebrahim S (2011). Reduced dietary salt for the prevention of cardiovascular disease: a meta-analysis of randomized controlled trials (cochrane review). American journal of hypertension, 24 (8), 843-53 PMID: 21731062

This post is a cross-post from Science-Based Medicine. Photo from Flickr user TheGiantVermin used under a Creative Commons License.

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