Friday, April 9, 2010

The great salt war; population-wide salt reduction

Population-wide salt reduction is gaining increased media and political attention over the last few years. The most notable example is the National Salt Reduction Initiative (NSRI) spearheaded by New York City Mayor Michael Bloomberg. This initiative was unveiled in January of 2010 in New York City and currently multiple cities around the US have joined forces. Public policy to limit salt intake is not a new concept around the world as the NSRI was modeled after the Food Standard Agency's efforts to reduce salt intake in the UK which started in 2003. Canada, Australia, Finland, France, Ireland and New Zealand have launched similar initiatives. This effort is not without its detractors. An increasingly skeptical US public is enduring an era of less and not more government involvement. Weighing the available evidence has always been an important part of medical advancement. When this in juxtaposed with public policy and politics is when these debates begin to get controversial. The benefits of salt reduction in lowering blood pressure and reducing cardiovascular risk have been consistently seen in the medical literature both in randomized clinical trials (DASH diet) and observational studies (INTERSALT). The US diet is extremely high is salt. The average American man and woman is estimated to consume 10.4 and 7.3 grams of salt per day respectively. The US Department of Agriculture and Health and Human Services recommends 5.8 g of salt (2.3 g sodium), with a lower target of 3.7 g of salt per day for most adults (people over 40, blacks, patients with hypertension). A recently published paper by Bibbins-Domingo et al. in the February 18 NEJM attempts to answer important questions about the cardiovascular and economic benefits of population-wide salt reduction.

This study is not a randomized clinical trial nor is it a observational cohort. This study utilizes a computer-simulated cohort called a "Markov" cohort. "Markov" chains are named after Andrey Markov and are commonly used for statistical modeling in finance, economics, social sciences, physics and medicine. Bibbins-Domingo utilized the Coronary Heart Disease (CHD) model which has been used to describe trends in CHD and the effects of interventions intended to reduce the risk of CHD and cost associated with treating CHD. This model inputs known data in regards to most accurate epidemiological data in the US (census, death rates, CHD rates, heath care expenditure, risk reduction estimates etc.) into a complex "Markov" chain to determine the cardiovascular and economic benefit of reducing the salt consumptions by 3 grams per day.


Reducing the number of new cases/year of
  • Coronary heart disease (CHD) by 60,000 to 120,000
  • Stroke by 32,000 to 66,000 (750,000 per year in US)
  • Myocardial infarction by 54,000to 99,000 (1.25 million per year in US)
Reduce the annual number of deaths from any cause by 44,000 to 92,000.
Save 194,000 to 392,000 quality-adjusted life-years

Save $10 billion to $24 billion in health care costs annually

The results of this "study" are provocative and intriguing. Would this "Markov" cohort play off as predicted in the "Real" world? If so, the reduction of 3 grams of sodium from the diet would have a huge economic and medical impact. These efforts would be akin to weight reduction and smoking cessation initiatives. Even if this reduction of sodium intake was only by 1 gram per day we would likely see huge benefits. However, achieving salt reduction will be a major feat. Lowering salt intake would require change on two important fronts; the public policy approach and the individual approach. The later will likely prove to be the most difficult to curtail as the "salt appetite" of the US population is increasingly fed with processed food products. An interesting editorial exploring this phenomenon by Philip Klemmer appeared in the April 2010 AJKD entitled "salt appetite". He subjected himself (and several UNC renal fellows) to an extremely low salt diet. Being from North Carolina myself I know first hand that this is an extremely difficult task. They each had an average drop in weight of 1.4 kg with a drop in blood pressure. The group finished the "experiment" and went back to their normal eating habits. Salt has endured as an important part of our culture. The word "salary" was used to describe how Roman soldiers were paid for their duties with salt. Salt was used as a way to preserve foods before the widespread use of refrigerators, but its usage has persisted and has perhaps grown in popularity.

For now, recommending a low salt diet is an important part of blood pressure control. But, one can argue that this is an important part of each of our diets. We will see how the public policy debate plays out in this "great salt war". Any decrease in "added" salt to food will be a welcomed improvement. Curtailing the "salt appetite" will likely prove to be more challenging than any spirited political debate.

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