At the Tipping Point: Accomplishing Population‐Wide Sodium Reduction in the United States

A convergence of recent policy recommendations and scientific advances has set the stage for renewed, aggressive, and hopefully effective efforts to reduce dietary sodium intake in the general population. The objective of this paper is to summarize the scientific rationale for population-wide sodium reduction, review recent scientific and policy developments, and offer guidance to health care providers. On average, as dietary salt (sodium chloride) intake increases, so does blood pressure (BP). Evidence includes results from animal studies, epidemiologic studies, migration studies, randomized trials (including several dose-response studies), and meta-analyses of clinical trials. To date, more than 50 randomized trials have been conducted. In a recent meta-analyses,1 a median reduction in urinary sodium of approximately 1.8 g/d (78 mmol/d) lowered systolic/diastolic BP by 2.0/1.0 mm Hg in nonhypertensive and by 5.0/2.7 mm Hg in hypertensive individuals. A few caveats are noteworthy. First, the BP response to sodium reduction, while direct and progressive, is nonlinear. Specifically, in dose-response studies, the slope of BP by sodium intake is steepest at sodium levels <100 mmol/d.2,3 Second, the BP response to changes in dietary sodium intake is heterogeneous,4 as is the BP response to other lifestyle interventions. The terms salt-sensitive and salt-resistant have been used to classify individuals in research studies; however the change in BP in response to a change in salt intake has a continuous, rather than binary, distribution.5 In general, the BP effects of sodium reduction tend to be greater in blacks, middle- and older-aged persons, and individuals with hypertension, diabetes, or chronic kidney disease. These groups tend to have a less responsive renin-angiotensin-aldosterone system.6 Dietary factors also influence the response to sodium reduction. For example, the Dietary Approaches to Stop Hypertension (DASH) diet3 and a high potassium intake7,8 blunt the rise in BP for a given increase in sodium intake. In addition to lowering BP, clinical trials have documented that a reduced sodium intake can prevent hypertension,9 lower BP in the setting of antihypertensive medication.10,11 and facilitate hypertension control.12,13. In observational studies, reduced sodium intake is associated with a blunted age-related rise in systolic BP,14 a reduced risk of atherosclerotic cardiovascular events,15,16 and a reduced risk of developing congestive heart failure.17 Accomplishing sodium reduction is similar to achieving other lifestyle modifications: a major public health approach is required in addition to changes in individual behavior. The need for a public health approach is even greater with sodium reduction than with other lifestyle modifications. In contrast to cigarette smoking, where usage is evident to the consumer, the sodium content of our diets is not readily apparent. Furthermore, more than 75% of consumed salt comes from processed foods.18 Hence, any meaningful strategy to reduce salt intake must involve the efforts of food manufacturers and restaurants. The need for public health approaches is also apparent in view of the global burden of hypertension (estimated worldwide prevalence of 972 million persons in 2000)19 and the limited success of individual-based sodium reduction interventions. Such interventions have been notoriously difficult to implement, especially in the setting of the current food supply, which is replete with “hidden” salt. In clinical trials, intensive interventions that focused just on salt reduction were able to shift mean intake to approximately 100 mmol/d (left panel of Figure 1).9,13,20 When efforts to reduce sodium intake were combined with weight loss9 or were part of a comprehensive lifestyle intervention program,21 sodium reduction was more modest (right panel of Figure 1), likely because of the complexity of making multiple lifestyle changes and potential tradeoffs when there are multiple lifestyle goals. Mean pre- and post-levels of urinary sodium excretion (a proxy of sodium intake) in 3 trials that tested interventions that focused only on salt reduction (left panel).9,13,20 Mean pre- and post-levels of urinary sodium excretion in 2 trials that combined sodium (Na) reduction with other lifestyle interventions (right panel). Trials of Hypertension Prevention, Phase 2 (TOHP2) tested an intervention combining sodium reduction with weight loss.9 PREMIER tested a comprehensive intervention combining sodium reduction with weight loss, increased physical activity, and adoption of the Dietary Approaches to Stop Hypertension (DASH) diet.21 TOHP1 indicates Trials of Hypertension Prevention, Phase 1; TONE, Trials of Nonpharmacologic Interventions in the Elderly. The scientific rationale for sodium reduction has been strong, and several recent advances have strengthened the case for population-wide sodium reduction. To date, policy recommendations have relied on evidence that sodium reduction lowers BP; elevated BP is an etiologically relevant, well-accepted, and modifiable cardiovascular risk factor. Elevated BP is not a cardiovascular event, however, and some individuals have called for large-scale, long-term trials of sodium reduction with clinical outcomes. Three trials (2 trials of behavioral interventions to lower sodium intake11,22 and 1 trial of a reduced sodium/high potassium salt diet23) have now reported clinical outcomes (Table). In each instance, there was a reduction in clinical cardiovascular disease (CVD) events in patients with a reduced sodium intake. In aggregate, these trials reinforce dietary recommendations to lower sodium intake as a means of preventing CVD in the general population and should dispel any residual concern that sodium reduction might be harmful.24 Recent evidence supports sodium reduction as a means to lower BP in children. Like many chronic diseases and conditions, the genesis of the BP problem begins early in life. Figure 2 displays BP data by age in US children25 along with mean BP levels in adult Yanomami Indians, a population with an extremely low intake of sodium and also no age-related rise in BP.26 The pattern of increasing systolic BP with age, a phenomenon that is well-recognized in US adults and other populations, is also evident in US children and is rapid (≈1.9 mm Hg/y in boys and ≈1.5 mm Hg/y in girls vs ≈0.6 mm Hg/y in US adults). Of importance, by approximately age 7 years in US girls and age 11 years in US boys, mean BP levels in US children exceed the mean BP of Yanomami adults aged 20 to 59 years. Such data highlight the need to initiate hypertension prevention efforts early in life. To this end, sodium reduction is one such therapy. In a recent meta-analysis, a reduced sodium intake lowered mean systolic/diastolic BP by 1.2/1.3 mm Hg in children and adolescents and lowered systolic BP in infants by 2.5 mm Hg.27 Mean blood pressure (BP) in US children aged 1 to 17 years (left)25 and mean BP in Yanomami adults aged 20 to 59 years.26 Mean BP in US children is for those at 50th percentile of height. In the United States, advice to reduce salt intake has been in place for about 30 years, despite persistent and highly aggressive attempts by commercial interests to roll back recommendations. With publication of the 2005 US Dietary Guidelines,28 the currently recommended upper limit of sodium intake is 100 mmol/d in the general population and 65 mmol/d in persons who are especially sensitive to the adverse effects of sodium (blacks; middle- and older-aged individuals; and persons with hypertension, diabetes, or chronic kidney disease). Likewise, other reports and recommendations advocate sharp reductions in salt consumption, largely because of its adverse effects on BP.28–32 Indeed, there is widespread acceptance of the need for population-wide sodium reduction, typically with recommended upper limits of no more than 100 mmol/d. While the United States has been at the forefront of dietary sodium recommendations, it has lagged behind on implementation efforts. In contrast to several other countries, particularly the United Kingdom, there is no comprehensive strategy to achieve meaningful, population-wide reduction in sodium intake. In June 2006, the American Medical Association (AMA) took a major step in that direction. Its membership voted to do the following: Urge the US Food and Drug Administration (FDA) to revoke the Generally Recognized as Safe status of salt and develop regulatory measures limiting the amount of salt in processed and restaurant foods Establish quantifiable milestones, specifically, a 50% reduction over the next decade in the sodium content of processed foods, fast-food products, and restaurant meals Partner with organizations to educate consumers about the benefits of long-term sodium reduction Work with the FDA to improve food labeling and develop warning labels for foods high in sodium The AMA proposal is a logical, coherent, and likely effective framework to accomplish this vitally important public health objective. With momentum shifting, food manufacturers are changing their foods, often silently reducing the sodium content of their foods with little fanfare. For example, several soup manufacturers have reduced the sodium content of their products from >1000 mg per serving to <500 mg per serving. The public response to such changes has been positive. No longer is low sodium equated with poor taste. Still, in contrast with the United Kingdom and other countries, there is no master plan or strategy to lower sodium intake. In the United Kingdom, the Food Standards Agency is actively engaged with industry to reduce the sodium content of most products, not just those that are highest in sodium. Concurrently, there are effective nonprofit organizations in the United Kingdom that promote sodium reduction (Consensus Action on Salt and Health [CASH] and its worldwide counterpart World Action on Salt and Health [WASH]). For their individual patients, physicians should be strong advocates of sodium reduction, even if the available means to counsel patients, especially in the office setting, are limited. Physician-directed and office-based treatment of high BP using non-pharmacologic strategies can be accomplished, but success is dependent upon several factors including the skills of the physician and staff, available resources, organizational structure of the office, and the patient's willingness to change. When possible, physicians should refer their prehypertensive and hypertensive patients to a dietitian or skilled health educator. Accomplishing dietary changes that reduce BP (eg, reduced sodium intake and the DASH diet) often requires lifestyle changes that are best facilitated by a skilled professional therapist. In addition to counseling individual patients, health care providers carry enormous influence in terms of guiding public health policy. Indeed, it is reasonable to posit that physician actions were an integral if not essential component of early tobacco control efforts in the United States. Likewise, health care providers and their leadership can provide substantial momentum in promoting sodium reduction. I view the bold proposal of the AMA as a tipping point, more than just a symbol, in efforts to accomplish population-wide sodium reduction in the United States. We are well past the debate on whether sodium should be reduced. Indeed, a compelling and still increasing body of evidence supports population-wide sodium reduction as an integral component of public health efforts to lower BP and prevent its CVD complications. The current challenge in the United States is developing and implementing a comprehensive strategic plan to accomplish sodium reduction.