The biggest improvements in human health occur with systematic changes. Clean water systems make a huge differences. Wide-spread immunization requirements conquered illnesses.
For a more recent example, I present Back-to-Sleep, a recommendation from the American Academy of Pediatrics in 1992. Placing most infants on their backs for sleep seemed to reduce the risk of sudden infant death syndrome (SIDS), also known as crib death. In October, about the time my son came into the world, they began a blast of public health announcements to put infants on their backs unless your pediatrician instructed otherwise. The risks of positioning infants this way seemed minimal, and the costs negligible. The effects have been impressive, with a 50% reduction in deaths over a decade (see graph).
The message was simple and easily followed. Statistical modeling suggested a major effect from the intervention, and we see it in the data. Unfortunately, preventative measures for other disorders do not meet these criteria.
A recent article examines salt and public health, an area of intense debate.
Ronald Bayer, Daveid Merritt Johns, and Sandro Galea. Salt and Public Health: Contested Science and the Challenge of Evidence-Based Decision Making. Health Affairs 31:2738, 2012 DOI: 10.1377/hlthaff.2012.0554
The authors describe the issue at hand:
For more than four decades, starting in the late 1960s, a sometimes furious battle has raged among scientists over the extent to which elevated salt consumption has adverse implications for population health and contributes to deaths from stroke and cardiovascular disease.
In 2011 two authors involved in the conduct of systematic reviews on salt declared, “It is surprising that many countries have uncritically adopted sodium reduction, which probably is the largest delusion in the history of preventive medicine.”3 Concurrently, a group of scientists long associated with studies on the harmful consequences of salt consumption wrote, “Denial and procrastination about dietary salt reduction will be costly in terms of avoidable illness and costs; it will also be ethically irresponsible.”4
The article discusses the evidence regarding sodium and blood pressure over the past 40 years, including systematic reviews of these data. Weighing both quality and quantity of data, the Cochrane group reported in 2011:
The second 2011 Cochrane report went further. After examining the potential impact of salt reduction on hormones and lipids in people with normal blood pressure, it concluded that the available evidence did not permit a conclusion as to whether low-salt diets improved or worsened health. It was possible, the authors concluded, that further research might be able to detect the beneficial impact of salt reduction, but “after more than 150 RCTs and 13 population studies without an obvious signal in favor of sodium reduction, another position could be to accept that such a signal may not exist.”55(p18)
Clearly, many people with hypertension have salt-sensitivity and could benefit from reduced sodium intake; however, we cannot see the benefits when large general populations are examined. Just as clinicians have to make decisions about individual patients despite scientific uncertainty, policy makers must do the same for large groups. Several considerations must be weighed:
- What are the benefits of reducing salt intake?
Those who have salt-sensitive hypertension have received the most attention in this debate; however, the reduction in blood pressure from sodium restriction alone amounts to only 1-2 mmHg in most people. Other populations might also benefit from global reduction in food salt content. Chronic kidney disease rates are rising as our population ages, and high salt intake complicates treatment of this group. High sodium intake can also induce kidney stones, and may be a major factor in this condition in the US.
- Are there risks to health of reducing salt intake?
The answer here is about as clear. Some studies have suggested that patients with a number of chronic disorders may not fare as well with severe salt restriction. Just as there are biologically plausible hypotheses for the benefits of salt restriction, the activation of the renin-angiotensin-aldosterone system by low salt intake provides fodder for the other side. As the authors note, "medicine and public health are replete with examples of seemingly sound ideas that had devastating unintended consequences. One hundred percent oxygen for newborns can cause blindness. Extensive use of x-rays for screening purposes is associated with greater risk of cancer. The risk of unintended consequences grows dramatically when interventions are translated to a populationwide scale."
- What is the cost of reducing salt intake?
The short answer: I have no idea. Salt initially served a preservative function in our food, but we no longer need it for that reason. However, people's palates have grown accustomed to its presence. Try a can of no-salt green beans sometime; they taste wrong, even when salted at the table. Manufacturers could begin cutting salt out of their processes and slowly getting us accustomed to its absence; some have started to do this with a variety of products. However, I have no idea what this change may cost at the factory or in the store.
The authors of the Health Affairs article do not solve this big hairy dilemma for us; they wrote this piece to demonstrate "the role that judgment and values must play in evidence-informed policy making."
As Roger Chou, a central figure in the conduct of systematic reviews for the US Preventive Services Task Force, has stated, “The evidence can tell us the likely benefits and likely harms, burdens and costs, but it does not directly tell us how to weigh all of these factors.”60(p10) Policy makers must ask: Are the burdens of public health interventions too great, and for whom? Are the expected benefits sufficient given the potential costs? These are not questions that can be answered in the absence of normative judgments.
As a doctor whose patients must often restrict salt intake, I know it would be in their best interest to systematically reduce salt in foods in the US. I do not know if it would benefit the general population enough to be considered successful on the same level as Back-to-Sleep.
The debate rages on. More data will be published.