BACKGROUND

Sodium is an essential nutrient and, as the primary cation in extracellular fluid, is required for a number of physiologic activities, including maintenance of extracellular volume and plasma osmolality, cellular membrane potential, and functioning of active transport systems. About 95 percent of total sodium is in extracellular fluid. Sodium balance is regulated collectively by the renin-angiotensin-aldosterone system, the sympathic nervous system, atrial natriuretic peptide, the kalikrein-kinin system, intra-renal mechanisms, and other factors. The majority of ingested sodium (more than 90 percent) is excreted in the urine (unless sweating is excessive).

Previously established evidence strongly supports that consumption of excessive sodium is a risk factor for high blood pressure, which in turn is a strong risk factor for consequent health outcomes, including cardiovascular disease (CVD), stroke, and mortality. However, emerging evidence suggests sodium intakes below 2,300 milligrams (mg) per day may increase risk of adverse health outcomes, at least in some population subgroups. Thus, debate has emerged about the level of dietary sodium intake that is associated with risk of adverse outcomes. The following discussion briefly summarizes current recommendations for sodium intake for the healthy U.S. population and for at-risk populations, namely African Americans, those 51 years of age and older, and those with hypertension or prehypertension, diabetes, or chronic kidney disease (CKD).

The Institute of Medicine (IOM) Panel on Dietary Reference Intakes (DRIs) for Electrolytes and Water was charged with establishing DRIs for sodium. DRIs comprise a set of nutrient reference values for assessing and planning diets for healthy people. These reference values replace and expand upon the previous Recommended Dietary Allowances (RDAs) for the United States and the Recommended Nutrient Intakes for Canada. DRIs encompass RDAs (derived from the Estimated Average Requirements, EARs) and Tolerable Upper Intake Levels (ULs) for life stages and genders. Detailed information about the process of establishing DRIs can be found in the report Dietary Reference Intakes: The Essential Guide to Nutrient Requirements (IOM, 2006). A brief summary of the DRIs is found in Box 1-1.

BOX 1-1

Definition of Dietary Reference Intakes. Adequate Intake (AI): The recommended average daily intake level based on observed or experimentally determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people (more...)

In establishing DRI values for sodium, the Panel on Dietary Reference Intakes for Electrolytes and Water (IOM, 2005) found insufficient evidence to derive RDAs. Instead Adequate Intakes (AIs) were set for all life stage and gender groups. For example, an AI of 1,500 mg per day was set for all children 9 years of age and older, adolescents, and adult men and women up to 51 years of age to ensure that the overall diet provides an adequate intake of other important nutrients and also to cover sweat losses in unacclimated individuals who are exposed to high temperatures or who become physically active (IOM, 2006, p. 388). The AIs for those 51 to 70 years of age and 70 years or older were set at 1,300 and 1,200 mg per day, respectively. A UL was established for sodium based on evidence showing associations between excessive sodium intake and risk of high blood pressure and consequent risk of CVD, stroke, and mortality (see Chapter 3 for a detailed discussion). Evidence from intervention studies showed no threshold in the dose-response curve of the relationship between sodium intake level and blood pressure, and therefore a no-observed adverse effect level (NOAEL) could not be established. Instead, a UL of 2,300 mg per day for males and females 14 years and above was established based on the lowest observed adverse effect level (LOAEL) from the dose–response evidence for blood pressure. For some population subgroups, namely older individuals, African Americans, and those with hypertension, diabetes, or CKD, the report suggested the UL should be lower than 2,300 mg/day although a defined level was not determined (see IOM, 2005, pp. 387–394, for more detailed information).

The Dietary Guidelines for Americans (DGA) are the basis for federal nutrition policy and nutrition programs. The goals of the DGA have evolved since the first DGA were published in 1980, reflecting changes in the science of nutrition and understanding of the role of nutrition in health. Every 5 years the DGA are revised, drawing from a technical review of evidence by a panel of experts, the Dietary Guidelines Advisory Committee (DGAC). In 2005, the DGAC concluded that the general adult population reduce sodium intake to less than 2,300 mg of sodium per day (HHS and USDA, 2005). The committee further concluded that individuals with hypertension, African Americans, and middle-aged and older adults (51 years of age and older) would benefit from reducing their sodium intake even further (HHS and USDA, 2005).

Because these latter groups together now comprise a majority of U.S. adults, the 2010 DGAC technical report concluded that a goal for sodium intake reduction should be 1,500 mg per day for the general population (HHS and USDA, 2010a). The Dietary Guidelines for Americans, 2010 (HHS and USDA, 2010b), the federal nutrition policy document, recommended retaining the goal that the general adult population should reduce consumption to less than 2,300 mg per day, but only that individuals 51 years of age and older, African Americans, and those with hypertension, diabetes, and CKD should reduce intake to 1,500 mg per day.

As with the DRI report (IOM, 2005), the rationale for the DGAC (HHS and USDA, 2010b) conclusion is grounded in evidence for a relationship between sodium intake and blood pressure as a surrogate marker of disease, and supported by additional evidence that decreasing blood pressure is associated with decreased risk of adverse health outcomes, particularly stroke and ischemic heart disease. Although the scientific community continues to debate the use of biomarkers in general and surrogate indicators of health outcomes, recent evidence attributes 35 percent of myocardial infarction and stroke events, 49 percent of heart failure episodes, and 24 percent of premature deaths to high blood pressure (Lawes et al., 2008).

Based on analyses of intake data from the 2003–2006 What We Eat in America dietary interview component of the National Health and Nutrition Examination Survey (NHANES), the 2010 DGAC report (HHS and USDA, 2010b) found that usual intakes of sodium were excessive for most age and gender groups in the U.S. population. Specifically, usual sodium intake exceeded the AI for more than 97 percent of all age and gender groups and exceeded the UL for more than 90 percent of boys older than 9 years of age and adult men up to 70 years of age. Among women, usual sodium intakes exceeded the UL in 84 and 75 percent of girls older than 9 years of age and women 19 years and older, respectively (ARS, 2010).

As a result of the persistent evidence that excessive sodium intake increases blood pressure, a risk factor for CVD, stroke, and mortality, federal nutrition policy now includes an emphasis on decreasing sodium intake in the general population as a preventive measure against risk of these adverse health outcomes. Based on the same evidence, various other domestic (e.g., American Heart Association) and international (e.g., World Health Organization) organizations have published recommendations for sodium consumption. Although they may differ from the DGA recommendations, they generally agree that sodium consumption is excessive among populations worldwide, and that it should be gradually reduced. As with the findings from the evidence reviews in IOM (2005) and DGAC (HHS and USDA, 2010a), these recommendations are grounded in evidence showing associations between excessive sodium intake and risk of high blood pressure and the strong association between hypertension and risk of CVD, stroke, and mortality.

Efforts to reduce sodium intake in the population have included educating the public about risks associated with excessive sodium consumption, and encouraging the food industry to reduce the sodium content of processed foods or develop alternate low-sodium products. The IOM report Strategies to Reduce Sodium Intake in the United States (IOM, 2010) reviewed indicators of dietary sodium intake and assessed a range of efforts aimed at reducing sodium consumption (e.g., consumer education, product labeling, and product reformulation) in both the public and private sectors. The report noted that consumers live in a food environment in which social, environmental, and macrolevel factors influence the types of foods consumed, highlighted the difficulties consumers face in consuming no more than 2,300 mg of sodium per day in this environment, and concluded that current strategies to reduce sodium consumption are ineffective. It also emphasized that the food supply is a key obstacle to reducing sodium consumption even when a myriad of strategies have been implemented.

To illustrate, NHANES 2009–2010 data show mean daily sodium intakes among U.S. adults 20 years of age and older are 2,980 and 4,243 mg for women and men, respectively. A salt adjustment is not applied to the 2009–2010 or later surveys; estimates of sodium intake include salt added in cooking and food preparation as assumed in the nutrient profiles for foods. These results are similar to the data from NHANES 2007–2008 (3,000 and 4,224 mg for women and men, respectively), indicating that sodium consumption patterns have changed little. A recent analysis of data from NHANES 2003–2008 showed that 90.7 percent (confidence interval [CI]: 89.6-91.8) of adults 20 years of age and older consumed more than 2,300 mg per day. Moreover, the same analysis showed that among U.S. subpopulations at risk (i.e., African Americans, those 51 years of age and older, or persons with hypertension, diabetes, or CKD), 98.8 percent (CI: 98.4-99.2) overall consumed more than 1,500 mg per day (Cogswell et al., 2012). Collectively, these findings pointed to the need for a gradual reduction in sodium consumption in the U.S. population.

THE COMMITTEE'S TASK

Despite the evidence that decreasing sodium intake reduces risk of high blood pressure, which in turn is strongly linked to CVD and other adverse health outcomes, new evidence has emerged that raises questions about potential adverse effects, specifically effects on insulin resistance and risk of CVD, associated with reducing sodium intake levels below 2,300 mg per day. Whether this evidence refutes previous evidence must be considered in light of the approaches used in these studies to examine health effects mediated directly through interactions with sodium, rather than examining blood pressure as a surrogate indicator of disease risk.

The controversy about possible adverse consequences to some individuals of low sodium intakes, and therefore of population strategies to reduce sodium consumption, prompted the Centers for Disease Control and Prevention (CDC) to ask the IOM to convene a committee to review the scientific evidence published since 2003 about potential benefits and adverse effects on health outcomes of sodium intake, particularly in the ranges of 1,500 to 2,300 mg per day. The committee also was asked to comment on the implications that this new evidence will have on population-based strategies to reduce sodium intake in the population and to identify research gaps. The statement of task is shown in Box 1-2. In its task, the committee was asked to consider potential benefits and adverse outcomes in the U.S. population generally and for relevant subsets of the population, namely those 51 years of age and older, African Americans, and those with hypertension, prehypertension, diabetes, CKD, and congestive heart failure.

BOX 1-2

Statement of Task. An IOM committee will be convened to evaluate the results, study design, and methodological approaches that have been used to assess the relationship between sodium and health outcomes in literature published since the IOM Dietary Reference (more...)

The 2010 DGA (HHS and USDA, 2010b) identified individuals of any age with hypertension, adults 51 years of age and older, and African Americans as at-risk subgroups within the general population. Specifically, the prevalence of hypertension is greater among African Americans than whites (Fields et al., 2004; Gillespie et al., 2011); this population group also is at greater risk of complications related to hypertension, such as stroke (Ayala et al., 2001; Giles et al., 1995) and kidney disease (Klag et al., 1996). Individuals who are 51 years of age and older and adults of any age who have prehypertension also are at greater risk of developing hypertension than are those who are younger and those with normal blood pressure (Gillespie et al., 2011; Lloyd-Jones et al., 2005; Vasan et al., 2001).

Finally, some evidence suggests that individuals with hypertension, diabetes, and CKD may be more sensitive to sodium than are nonaffected groups (Lifton et al., 2002) and, thus, are at greater risk of developing adverse outcomes such as CVD and stroke. Given the disproportionate level of risk of adverse health outcomes among these population subgroups, the committee identified for further evaluation evidence that evaluated sodium intake in relation to either benefit or risk for these groups and considered them as a subset apart from the overall U.S. population.

NHANES data indicate that these at-risk subgroups now comprise a majority of the U.S. population (HHS and USDA, 2010). Further, more than 90 percent of U.S. adults 50 years of age or older will develop hypertension in their lifetime (Vasan et al., 2002), and nearly 70 percent of men and 49 percent of women who develop hypertension in middle age will experience a CVD event by 85 years of age (Allen et al., 2012). Even though some population subgroups stand out because they are disproportionately affected by hypertension and its adverse health outcomes, they represent a large proportion of the general population. Thus, the committee considered that part of its task was to examine the effects of dietary sodium on health outcomes in the general population. Separately, the committee considered the data in these subgroups of special interest, both whether separate data were available on individuals in these subgroups, and whether the general population studies included analyses of specific subgroups.

THE STUDY PROCESS

The IOM established a committee of 12 members with expertise in nutrition, CVD, hypertension, diabetes, kidney disease, epidemiological studies, clinical trial design and data analysis, biostatistics, and evidence-based reviews. The committee met in closed session and by conference calls, and held a 2-day public workshop to gather information pertinent to the task.

To address its task, the committee first formulated a process to identify and weigh studies identified from its search of literature published from 2003 through 2012. The committee did not rate the studies for quality because the broad range of study designs and sodium intake assessment precluded the application of a uniform rating system. Instead, the papers were reviewed and assessed individually. The committee developed a strategy to qualitatively assess each relevant study and the totality of the evidence based on the variability of methodological approaches, study designs, the method to estimate sodium intake, confounder adjustment, and the number of relevant studies available. The committee's findings and conclusions are derived from its assessment of the evidence.

ORGANIZATION OF THE REPORT

This report reviews and evaluates the evidence on the potential for adverse health effects from reducing sodium intakes and discusses the findings, conclusions, and implications of the committee's assessment. The report is organized into five chapters. Chapter 1 describes the background for the study and the statement of task. Chapter 2 describes in detail the literature search, including the process to identify relevant reports for review and the committee's methodological approach to assessing the quality of the evidence. The committee describes first the potential methodological issues in the studies reviewed. Then a detailed description of the literature search and process that the committee followed to review and assess the scientific literature is provided. Chapter 3 includes a summary of studies that examine the association between sodium intake and intermediate health outcomes, including blood pressure as a surrogate indicator of disease. The review includes an overview of studies that are representative of the current body of evidence. Chapter 4 describes the studies reviewed by the committee examining the association between sodium intake and direct health outcomes, identified from its search, and determined from its criteria as relevant for further review and assessment. Studies that did not meet the criteria for further consideration are summarized at the end of the chapter. Chapter 5 presents the committee's overarching findings, and the findings and conclusions for specific health outcomes, as well as the implications of its conclusions for reducing sodium intake in the population, and recommendations for future research.

Appendix A contains acronyms and abbreviations used throughout the report. Appendix B contains the committee members' biographical sketches. Appendix C presents the agendas of the open session and public workshop, respectively. Appendix D contains a depiction of direct and indirect pathways to adverse health effects related to sodium intake. Appendix E describes the literature search strategy. Appendix F contains tables with the summary of evidence for CVD, mortality, cancer, heart failure, metabolic syndrome, and diabetes as health outcomes.

REFERENCES

• Allen N, Berry JD, Ning H, Van Horn L, Dyer A, Lloyd-Jones DM. Impact of blood pressure and blood pressure change during middle age on the remaining lifetime risk for cardiovascular disease: The cardiovascular lifetime risk pooling project. Circulation. 2012;125(1):37–44. [PMC free article: PMC3310202] [PubMed: 22184621]
• ARS (Agricultural Research Service). Sodium (mg): Usual intakes from food and water, 2003–2006, compared to adequate intakes and tolerable upper intake levels. 2010. [March 18, 2013]. http://www​.ars.usda.gov​/SP2UserFiles/Place​/12355000/pdf/0506​/usual_nutrient_intake_sodium_2003-06​.pdf.
• Ayala C, Greenlund KJ, Croft JB, Keenan NL, Donehoo RS, Giles WH, Kittner SJ, Marks JS. Racial/ethnic disparities in mortality by stroke subtype in the United States, 1995–1998. American Journal of Epidemiology. 2001;154(11):1057–1063. [PubMed: 11724723]
• Cogswell ME, Zhang Z, Carriquiry AL, Gunn JP, Kuklina EV, Saydah SH, Yang Q, Moshfegh AJ. Sodium and potassium intakes among US adults: NHANES 2003–2008. American Journal of Clinical Nutrition. 2012;96(3):647–657. [PMC free article: PMC3417219] [PubMed: 22854410]
• Fields LE, Burt VL, Cutler JA, Hughes J, Roccella EJ, Sorlie P. The burden of adult hypertension in the United States 1999 to 2000: A rising tide. Hypertension. 2004;44(4):398–404. [PubMed: 15326093]
• Giles WH, Kittner SJ, Hebel JR, Losonczy KG, Sherwin RW. Determinants of black-white differences in the risk of cerebral infarction: The National Health and Nutrition Examination Survey Epidemiologic Follow-up Study. Archives of Internal Medicine. 1995;155(12):1319–1324. [PubMed: 7778964]
• Gillespie C, Kuklina EV, Briss PA, Blair NA, Hong Y. Vital signs: Prevalence, treatment, and control of hypertension—United States, 1999–2002 and 2005–2008. Morbidity and Mortality Weekly Report. 2011;60(4):103–108. [April 10, 2013]; http://www​.cdc.gov/mmwr​/preview/mmwrhtml/mm6004a4.htm. [PubMed: 21293325]
• HHS and USDA (U.S. Department of Health and Human Services and U.S. Department of Agriculture). The report of the Dietary Guidelines Advisory Committee on Dietary Guidelines for Americans, 2005. Washington, DC: HHS; 2005. [April 26, 2013]. http://www​.health.gov​/dietaryguidelines/dga2005/default​.htm#1.
• HHS and USDA. 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; 2010a. [February 1, 2013]. http://www​.cnpp.usda​.gov/Publications/DietaryGuidelines​/2010​/DGAC/Report/2010DGACReport-camera-ready-Jan11-11.pdf.
• HHS and USDA. Dietary Guidelines for Americans, 2010. 7th. Washington, DC: U.S. Government Printing Office; 2010b. [February 4, 2013]. http://www​.cnpp.usda​.gov/Publications/DietaryGuidelines​/2010​/PolicyDoc/PolicyDoc.pdf.
• IOM (Institute of Medicine). Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. Washington, DC: The National Academies Press; 2005.
• IOM. Dietary reference intakes: The essential guide to nutrient requirements. Washington, DC: The National Academies Press; 2006.
• IOM. Strategies to reduce sodium intake in the United States. Washington, DC: The National Academies Press; 2010. [PMC free article: PMC3042781] [PubMed: 22043452]
• Klag MJ, Whelton PK, Randall BL, Neaton JD, Brancati FL, Ford CE, Shulman NB, Stamler J. Blood pressure and end-stage renal disease in men. New England Journal of Medicine. 1996;334(1):13–18. [PubMed: 7494564]
• Lawes CM, Hoorn SV, Rodgers A. Global burden of blood-pressure-related disease, 2001. The Lancet. 2008;371(9623):1513–1518. [PubMed: 18456100]
• Lifton RP, Wilson FH, Choate KA, Geller DS. Salt and blood pressure: New insight from human genetic studies. Cold Spring Harbor Symposia on Quantitative Biology. 2002;67:445–450. [PubMed: 12858570]
• Lloyd-Jones DM, Evans JC, Levy D. Hypertension in adults across the age spectrum: Current outcomes and control in the community. Journal of the American Medical Association. 2005;294(4):466–472. [PubMed: 16046653]
• Vasan RS, Larson MG, Leip EP, Evans JC, O'Donnell CJ, Kannel WB, Levy D. Impact of high-normal blood pressure on the risk of cardiovascular disease. New England Journal of Medicine. 2001;345(18):1291–1297. [PubMed: 11794147]
• Vasan RS, Beiser A, Seshadri S, Larson MG, Kannel WB, D'Agostino RB, Levy D. Residual lifetime risk for developing hypertension in middle-aged women and men: The Framingham Heart Study. Journal of the American Medical Association. 2002;287(8):1003–1010. [PubMed: 11866648]