Dietary intake of sodium among the general adult U.S. population averages 3,400 mg daily, while federal nutrition policy guidance, the Dietary Guidelines for Americans 2010 (USDA and HHS, 2010a), recommends sodium intake be reduced to less than 2,300 mg daily and to 1,500 mg daily for African Americans, individuals with hypertension, diabetes, or chronic kidney disease (CKD), and individuals 51 years of age and older. As discussed in Chapter 3, current recommendations for dietary sodium intake are based importantly on the use of blood pressure as a surrogate marker for cardiovascular disease (CVD) outcomes (IOM, 2005; USDA and HHS, 2010b). In particular, in establishing Dietary Reference Intake values for sodium, the Panel on Dietary Reference Intakes for Electrolytes and Water (IOM, 2005) found insufficient evidence to derive an Estimated Average Requirement and calculate a Recommended Dietary Allowance, and thus Adequate Intakes (AIs) were set instead, as an amount needed to achieve a diet that is adequate in other essential nutrients and covers sweat losses. In addition, a Tolerable Upper Intake Level (UL) was established for sodium based on evidence showing associations between high sodium intake and risk of high blood pressure.1 Thus, the Institute of Medicine (IOM) panel’s recommendation was based on an AI for sodium intake of 1,500 mg per
1 Applies to individuals aged 14 years and above.
day and projections from effects on blood pressure that dietary sodium intake up to 2,300 mg daily2 is not likely to cause any harm.
The relationship between other electrolytes and changes in blood pressure remains unresolved. Yet, as noted in the Report of the Dietary Guidelines Advisory Committee (DGAC) (USDA and HHS, 2010b), the effects of lowering sodium intake on blood pressure cannot always be disentangled from the effects of total dietary modification. For example, the committee’s review revealed that in a number of studies the effects of dietary sodium on CVD outcomes sometimes persisted even after controlling for blood pressure, suggesting that associations between dietary sodium and risk of CVD may be mediated through other dietary factors (e.g., the effects of other electrolytes), or through pathways in addition to blood pressure. Further, older data indicate that some people in the population may be salt sensitive, while others are not, and that blood pressure response to sodium varies widely (see Chapter 3). In this context, new data have raised questions about the health effects of lowering sodium intake on health outcomes.
Thus, in response to its charge, the committee focused its examination of evidence on the associations between dietary sodium intake and direct health outcomes, not on blood pressure as an indirect or intermediate marker of CVD outcomes (see Appendix D). In deriving its findings and conclusions about the evidence for associations between dietary sodium intake and health outcomes, the committee examined the quality as well as the quantity of the evidence. The conclusions and recommendations drawn from the committee’s findings are described below.
The committee’s assessment of the evidence reviewed was guided by a number of factors. These included the study design, the quantitative measures of dietary sodium intake and confounder adjustment, and the number and consistency of relevant studies available.
From the evidence reviewed on health outcomes, the committee found that a number of the populations evaluated were outside the United States and included groups that consumed mean levels of sodium much higher than 3,400 mg per day, the average amount consumed by adults in the United States (USDA and HHS, 2010b). Thus, the applicability of some of the results to the U.S. population was of concern. For example, in the studies reviewed, “high” sodium intake levels ranged from about 2,700 to more than 10,000 mg per day.
Overall, the committee found both the quantity and quality of relevant studies to be less than optimal. Further, almost all of the evidence on
2 Applies to adults aged 19-50 years.
clinical outcomes identified by the committee was observational, consisting largely of prospective cohort studies. The committee also found important limitations associated with the quantitative measures of sodium intake (see Chapters 2 and 4) and recognized the potential for spurious findings related to incorrect measurement and reverse causality. Specifically, in some studies, low sodium intakes apparently appeared to show an association with risk of disease, when, in fact, the relationship may have been that the disease itself led to low or incomplete measures of sodium among those with preexisting disease (see Chapter 4).
Assessing the impact of sodium intake on health outcomes also was complicated by variability in the types and quality of measures used, so that measures could not be reliably calibrated across studies. It was the consensus of the committee that the lack of consistency among studies in the methods used for defining sodium intakes at both high and low ends of the range of typical intakes among various population groups precluded deriving a numerical definition for high and low intakes in its findings and conclusions. Rather, it could consider sodium intake levels only within the context of each individual study. Likewise, the extreme variability in intake levels between and among population groups precluded the committee from establishing a “healthy” intake range.
Recognizing the limitations of the available evidence, the committee found no consistent evidence to support an association between sodium intake and either a beneficial or adverse effect on health outcomes other than CVD outcomes (including stroke and CVD mortality) and all-cause mortality. Some evidence suggested that decreasing sodium intake could possibly reduce the risk of gastric cancer. However, the evidence was too limited to conclude the converse—that higher sodium intake could possibly increase the risk of gastric cancer. Interpreting these findings was particularly challenging because most gastric cancer studies were conducted outside the United States in populations consuming much higher levels of sodium than those consumed in this country. Thus, the committee focused its findings and conclusions on evidence for associations between sodium intake and risk of CVD-related events and mortality.
Findings and Conclusions for Cardiovascular Disease, Stroke, and Mortality
General U.S. Population
Finding 1: The committee found that the results from studies linking dietary sodium intake with direct health outcomes were highly variable in methodological quality, particularly in assessing sodium intake. The range of limitations included over- or underreporting of intakes or incomplete collection of urine samples. In addition, variability in data collection methodologies limited the committee’s ability to compare results across studies.
Conclusion 1: Although the reviewed evidence on associations between sodium intake and direct health outcomes has methodological flaws and limitations, the committee concluded that, when considered collectively, it indicates a positive relationship between higher levels of sodium intake and risk of CVD. This evidence is consistent with existing evidence on blood pressure as a surrogate indicator of CVD risk.
Finding 2: The committee found that the evidence from studies on direct health outcomes was insufficient and inconsistent regarding an association between sodium intake below 2,300 mg per day and either benefit or risk of CVD outcomes (including stroke and CVD mortality) or all-cause mortality in the general U.S. population.
Conclusion 2: The committee determined that evidence from studies on direct health outcomes is inconsistent and insufficient to conclude that lowering sodium intakes below 2,300 mg per day either increases or decreases risk of CVD outcomes (including stroke and CVD mortality) or all-cause mortality in the general U.S. population.
Finding 1: The committee found that the evidence from multiple randomized controlled trials (RCTs) that were conducted by a single investigative team indicated that low sodium intake (e.g., down to 1,840 mg per day) may lead to greater risk of adverse events in congestive heart failure (CHF) patients with reduced ejection fraction and who are receiving certain aggressive therapeutic regimens. This association also is supported by one observational study where low sodium intake levels in patients with CVD and diabetes were associated with higher risk of CHF events.
Conclusion 1: The committee concluded that the available evidence suggests that low sodium intakes may lead to higher risk of adverse events in mid- to late-stage CHF patients with reduced ejection fraction and who are receiving aggressive therapeutic regimens. Because these therapeutic regimens were very different than current standards of care in the United States, the results may not be generalizable. Similar studies in other settings, and using regimens more closely resembling those in standard U.S. clinical practice are needed.
Finding 2: The committee found that data among prehypertensive participants from two related studies provided some evidence suggesting a continued benefit of lowering sodium intake in these patients down to 2,300 mg per day (and lower, although based on small numbers in the lower range). In contrast, the committee found no evidence for benefit and some evidence suggesting risk of adverse health outcomes associated with sodium intake levels in ranges approximating 1,500 to 2,300 mg per day in other disease-specific population subgroups, specifically those with diabetes, CKD, or preexisting CVD. In addition to inconsistencies in sodium intake measures, methodological flaws included the possibility of confounding and reverse causality. No relevant evidence was found on health outcomes for other population subgroups considered (i.e., persons 51 years of age and older, and African Americans). In studies that explored interactions, race, age, or the presence of hypertension or diabetes did not change the effect of sodium on health outcomes.
Conclusion 2: The committee concluded that, with the exception of the CHF patients described above, the current body of evidence addressing the association between low sodium intake and health outcomes in the population subgroups considered3 is limited. The evidence available is inconsistent and limited in its approaches to measuring sodium intake. The evidence also is limited by small numbers of health outcomes and the methodological constraints of observational study designs, including the potential for reverse causality and confounding.
The committee further concluded that, while the current literature provides some evidence for adverse health effects of low sodium intake among individuals with diabetes, CKD, or preexisting CVD, the evidence on both benefit and harm is not strong enough to indicate that these subgroups should be treated differently from the general U.S. population. Thus, the committee concluded that the evidence on direct health outcomes does not
3 For example, diabetes, CKD, or preexisting CVD, individuals with hypertension, prehy-pertension, persons 51 years of age and older, and African Americans.
support recommendations to lower sodium intake within these subgroups to or even below 1,500 mg per day.
Implications for Population-Based Strategies to Gradually Reduce Sodium Intake in the U.S. Population
As noted in Chapter 1, recommendations of the Panel on Dietary Reference Intakes for Electrolytes and Water (IOM, 2005) of an AI for sodium intake of 1,500 mg per day for all individuals 9 years of age up to 51 years of age was set as an amount necessary to achieve an overall diet that provides an adequate intake of other important nutrients and also covers sodium sweat losses. A UL for sodium was set at 2,300 mg per day based on evidence showing associations between high sodium intakes and risk of high blood pressure and consequent risk of CVD, stroke, and mortality.
Given this background, overall, the committee found that the available evidence on associations between sodium intake and direct health outcomes is consistent with population-based efforts to lower excessive dietary sodium intakes, but it is not consistent with previous efforts that encourage lowering of dietary sodium in the general population to 1,500 mg per day. Further, as noted in the DGAC report, population subgroups, including those with diabetes, CKD, or preexisting CVD, individuals with hypertension, prehypertension, persons 51 years of age and older, and African Americans, represent, in aggregate, a majority of the general U.S. population. Thus, when considered in light of the current state of the evidence on associations between sodium intake and direct health outcomes for those subgroups, except when data specifically indicate they are different, there is not sufficient evidence to support treating them differently from the general U.S. population.
The committee was not asked to draw conclusions about a specific target range of dietary sodium for the general population or for population subgroups. However, the committee notes that there are important factors it considered that preclude such a conclusion. For example, one factor that is often discussed in the context of other health-related questions is the challenge of defining specific intake levels when the variables of interest are continuous. That is an especially difficult issue in the present circumstances, where the target intake level could theoretically differ for different large population subgroups.
Other methodological factors that preclude making conclusions about a specific target range for sodium relate to the variability in approaches and study designs in the literature reviewed. Most importantly, quantitative methods for measuring dietary sodium intake have limitations and there are impedients to calibrating those measures across different methodological
approaches and study designs. Methodologic problems in assessing sodium intake make this particularly challenging.
FUTURE RESEARCH TO ADDRESS GAPS IN DATA AND METHODOLOGY
The committee identified a number of data and methods gaps in studies on sodium intake and risk of adverse health outcomes among population groups. Further research in the areas highlighted below would strengthen the evidence base on the association between lower (1,500 to 2,300 mg) levels of sodium and health outcomes in the general population and population subgroups:
1. standardized methodological approaches to measure sodium intake in population groups. Specific examples include standardizing the use of multiple 24-hour urine collections and validating sodium intake estimates with data on urine volume, urine creatinine, and body weight;
2. approaches using dietary sodium intake levels corresponding to levels in current guidelines (i.e., 1,500 to 2,300 mg per day) when examining associations between sodium intake and health outcomes;
3. analyses examining the effects on health outcomes of dietary sodium in combination with other electrolytes, particularly potassium;
4. methods that account for potential confounding factors in dietary studies, including the influence of reported total daily caloric intake on observational associations between sodium and health outcomes, and methods that clarify attributes of individuals with apparently low sodium intake or excretion; and
5. analyses of interactions with antihypertensive medication and blood pressure in studies examining associations between sodium intake and health outcomes.
In addition, the committee identified a need for RCT research, and observational and mechanistic studies, particularly in population subgroups. Examples of such clinical trials include those to examine
1. effects of a range of sodium levels on risk of cardiovascular events, stroke, and mortality among
a. patients in controlled environments, where randomized trials may be more feasible, such as the elderly in chronic care facilities and other institutionalized individuals; and
b. individuals as part of natural experiments, such as those in other countries where policies affecting sodium consumption are in effect;
2. effects of low-sodium diets on adverse events among CHF patients receiving therapeutic treatment modalities typically used in the United States; and 3.
3. potential beneficial or adverse outcomes of a range of sodium intakes among African Americans, adults 51-70 years of age, 70 years of age and older, and other population subgroups; RCTs may be particularly important within higher-risk patient populations, where reverse causation is a potential limitation of observational studies.
The committee also identified a need for studies to collect and reanalyze
1. data from existing clinical trials that were designed to evaluate sodium and health; and 2.
2. data during extended follow-up periods after completion of a clinical trial to identify health outcomes, such as mortality, that could manifest later in life and after longer follow-up periods. Such trials would not be simple to conduct, however, and careful feasibility assessment is needed first.
In addition to RCT research, mechanistic studies are needed to examine potential physiologic changes associated with lowering sodium intake and adverse health outcomes. Finally, additional observational research is needed to examine associations between sodium intake and cancer, especially gastric cancer in the U.S. population, as well as associations between sodium intake and caloric intake in both short-term and longitudinal studies.
IOM (Institute of Medicine). 2005. Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. Washington, DC: The National Academies Press.
USDA and HHS (U.S. Department of Agriculture and U.S. Department of Health and Human Services). 2010a. Dietary Guidelines for Americans, 2010. 7th ed. Washington, DC: U.S. Government Printing Office. http://www.cnpp.usda.gov/Publications/DietaryGuidelines/2010/PolicyDoc/PolicyDoc.pdf (accessed February 4, 2013).
USDA and HHS. 2010b. Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010, to the Secretary of Agriculture and the Secretary of Health and Human Services. Washington, DC: USDA/ARS. http://www.cnpp.usda.gov/Publications/DietaryGuidelines/2010/DGAC/Report/2010DGACReport-camera-ready-Jan11-11.pdf (accessed February 1, 2013).