Approach to Evidence Review

Brian L. Strom; Ann L. Yaktine; Maria Oria; Committee on the Consequences of Sodium Reduction in Populations; Food and Nutrition Board; Board on Population Health and Public Health Practice; Institute of Medicine

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Committee on the Consequences of Sodium Reduction in Populations; Food and Nutrition Board; Board on Population Health and Public Health Practice; Institute of Medicine; Strom BL, Yaktine AL, Oria M, editors. Sodium Intake in Populations: Assessment of Evidence. Washington (DC): National Academies Press (US); 2013 Aug 27.

Sodium Intake in Populations: Assessment of Evidence.

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2Approach to Evidence Review

This chapter briefly summarizes the methodological challenges and the approach the committee took to assess the evidence on relationships between sodium intake and health outcomes. The chapter begins with a description of the approach used by the committee to identify relevant evidence for consideration in response to its task. The sources of evidence, questions that guided the literature search, the literature search strategy, and the process of selecting studies for detailed review are described, as are the criteria to critically appraise the individual studies. Finally, the chapter summarizes the advantages and limitations of different approaches used to estimate sodium intake, an important criterion that the committee used to assess the quality of the studies.

GENERAL APPROACH OF THE COMMITTEE

The committee focused its review and assessment on evidence for direct associations between sodium intake and risk of adverse health outcomes. Although intermediate markers serve as a means of tracking and predicting health status, the scientific community continues to debate their use. A recent Institute of Medicine (IOM) report, Evaluation of Biomarkers and Surrogate Endpoints in Chronic Disease (IOM, 2010), highlights the concerns about the use of intermediate markers and provides a framework to evaluate them. The drug development literature contains numerous examples, including with cardiovascular drugs, where treatment based on biomarkers did not correctly predict the ultimately studied clinical outcomes (e.g., Nissen and Wolski, 2007).

However, in the case of blood pressure, the recent report on biomarkers (IOM, 2010) points to its wide acceptance as a surrogate marker and to the recent evidence attributing 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). Nevertheless, the committee recognizes that cardiovascular effects do not occur only as a result of blood pressure. For example, and as described in the 2010 IOM report, it is known that different classes of drugs can have multiple cardiovascular outcomes but not all of them are a result of their blood pressure lowering effects. For example, despite their effects on blood pressure, alpha blockers have been shown to have a higher risk of heart failure compared to diuretics (ALLHAT Collaborative Research Group, 2000). The committee also recognizes that, in addition to blood pressure effects, diets modified in sodium may execute their effects on health outcomes through other factors, including other dietary constituents. The committee did not address questions related to such potential additional mechanisms because such questions were not part of its statement of task.

The committee's assessment of the evidence reviewed was influenced by a number of factors. These included the variability in methodological approaches used to evaluate relationships between sodium intake and risk of health outcomes, study design, limitations in the quantitative measures of both dietary intake and urinary excretion of sodium, confounder adjustment, and the number of relevant studies available. Assessing the impact of sodium intake on health outcomes was further complicated by wide variability in intake ranges among studies. For example, in the studies reviewed, high sodium intake levels for examining associations with health outcomes ranged from about 2,700 to more than 10,000 mg per day. The lack of consistency between studies in defining sodium intakes at both high and low ends of the range of typical intakes among various population groups meant that the committee could not derive a numerical definition for high or low intakes in its findings and conclusions. Rather, it could consider sodium intake levels only within the context of an individual study. Thus, in its findings and conclusions, the committee's use of “high” or “low” sodium intake indicates levels in the ranges described in the evidence reviewed.

LITERATURE REVIEW

Sources of Information

The committee obtained data and information for its conclusions from several sources. The main source of information came from a review of the evidence in the scientific literature from 2003 through 2012. For this review, scientific literature searches were conducted by the study staff in consultation with National Academy of Sciences librarians and described below in detail and in Appendix E. The review was conducted by adhering closely to the recommendations of the 2011 IOM report Finding What Works in Health Care: Standards for Systematic Reviews (IOM, 2011). The committee also reviewed and considered summaries of the scientific information about the association of sodium dietary intake and direct health outcomes in recent reports from authoritative sources (HHS and USDA, 2005, 2010; IOM, 2005). Additional information was gathered during its public workshop (see Appendix C for workshop agenda). Invited presentations included a range of perspectives about relationships between sodium intake and health outcomes. In addition, stakeholders were invited to present their views on the topics presented. The committee considered unpublished data when provided by the public. Unpublished data, however, were not used as principal evidence for findings and conclusions; these data were used only as supportive evidence for committee's findings and conclusions, when appropriate. In a few circumstances, the committee concluded that additional analysis of the published data would add important information to a specific topic. In those cases, the authors of the study were contacted by staff and the analysis requested. As with other information, these additional analyses were included in the study public access file.

Questions

Based on the statement of task, the committee formulated the following questions to guide its literature search:

Question 1. What is the effect of reducing dietary sodium intake in all individuals compared to habitual intake on health outcomes (cardiovascular disease, heart failure, myocardial infarction, diabetes, mortality, stroke, bone disease, fractures, falls, headaches, kidney stones, skin reactions, immune function, thyroid disease, or cancer)?

Question 2. What is the effect of reducing dietary sodium intake in individuals with hypertension, prehypertension, those aged 51 years and older, African Americans, and individuals with diabetes, chronic kidney disease, or congestive heart failure, compared to habitual intake on health outcomes (cardiovascular disease, myocardial infarction, diabetes, mortality, stroke, bone disease, fractures, falls, headaches, kidney stones, skin reactions, immune function, thyroid disease, or cancer)?

Literature Search Strategy

Literature searches were conducted to identify studies to answer the committee's questions. The online databases used for these searches were the Cochrane Database of Systematic Reviews Embase, MedLine, PubMed, and Web of Science. A broad search was first performed to include all health outcomes. In addition, a number of searches targeted at specific outcomes identified by the committee were conducted. The specific outcomes were cardiovascular disease, congestive heart failure, hypertension, myocardial infarction, diabetes, mortality, stroke, bone disease, fractures, falls, headaches, kidney stones, chronic kidney disease, skin reactions, immune function, thyroid disease, and cancer. Table E-1 in Appendix E presents the search conducted in the MedLine database as an example of the searches conducted.

The searches were limited to peer-reviewed original research studies, systematic reviews, and meta-analyses published from January 1, 2003, through December 18, 2012, in the English language. Studies in all countries, of all sample sizes and of all follow-up periods were included. In addition, studies with all populations irrespective of health status, ages, races, and ethnicities were included. Case studies and case series as well as animal or in vitro studies were excluded in the search strategies. The studies included in relevant systematic reviews and meta-analyses were used only as background and to cross-check the references used so as to ensure the most complete review of the literature.

Selection of Studies

The abstracts of all studies identified by the targeted searches were reviewed by the IOM staff. A diagram illustrating the selection process is shown in Figure 2-1. Conference abstracts, nonsystematic reviews, public statements, policy studies, modeling studies, mechanistic studies, and studies where data on dietary sodium intake or a health outcome of interest were not collected were excluded. Studies with only intermediate outcomes were also excluded. Such studies, however, were summarized and are discussed in Chapter 3. The method used to estimate sodium intake was a key exclusion criterion. Only studies where a food frequency questionnaire (FFQ), a 24-hour recall, food record, or urinary excretion methods were used to estimate sodium assessment were selected for review. Among those, studies were excluded if the method to estimate sodium intake was not described in sufficient detail or for which numerical sodium levels were not calculated. In addition, studies that analyzed only the association between sodium/potassium ratio and a health outcome or that did not analyze the independent effect of sodium intake were excluded.

FIGURE 2-1

Flow diagram depicting the literature search strategy. NOTE: CVD, cardiovascular disease; RCT, randomized controlled trial. ^a See Chapter 2 for a description of the inclusion and exclusion criteria.

EVALUATION OF STUDIES

Review Process

Three committee members reviewed each of the studies individually and then as a group to deliberate and reach consensus on their strengths and weaknesses. Further, all studies were individually presented and discussed as a committee. The committee used summary tables to present details of the study designs (see evidence tables in Appendix F), the strengths and weaknesses based on the generalizability of the study population to the U.S. population and population subgroups of interest, and the methodological appropriateness of the studies reviewed (see Tables 4-1 through 4-8). The committee consulted the IOM report Finding What Works in Health Care: Standards for Systematic Reviews (IOM, 2011) to establish the overall review process, including determining the quality of the studies. The 2011 IOM report recommended that three elements be used to critically evaluate individual studies: methodological appropriateness (i.e., the risk of bias); relevance of the study's population, interventions, and outcomes measures; and the fidelity of implementation of interventions. The committee did not formally rate the studies for methodological quality because upon its review, it found the study designs were too varied and no generally accepted approach to performing such ratings for studies of dietary sodium existed. Instead, the studies were reviewed and assessed individually for their strengths and weaknesses. Evidence about associations between sodium intake and health outcomes was considered in its totality.

The committee also chose not to perform a formal meta-analysis of the available results. First, such a meta-analysis was deemed inappropriate for this review because of the wide differences in the methodologies of the studies, especially with respect to measurements of sodium intake and adjustment for confounders. Second, the studies were very different in the ranges of populations and sodium intakes examined, and the measures of dietary sodium could not readily be calibrated from study to study. Thus, aggregating them statistically would lead to a misleadingly simplistic summary statistic, rather than a meaningful synthesis. Instead, the committee evaluated each study in the context of its population, dietary sodium intake range, and methodological strengths and weaknesses in order to synthesize its findings and conclusions.

Criteria for Evaluating Evidence

Following identification of studies for review, the committee determined two broad criteria to critically appraise each study: generalizability to the populations of interest and methodological appropriateness (i.e., risk of bias).

Generalizability to Populations of Interest

The report Evaluation of Biomarkers and Surrogate Endpoints in Chronic Disease (IOM, 2010) recommends using relevance to the study population, interventions, and outcomes measures as one element to evaluate each study. In this evaluation, only relevant interventions and outcomes were selected for the final review. The relevance of the population under study to populations of interest in the statement of task (i.e., individuals with hypertension or prehypertension, those 51 years of age and older, African Americans, and individuals with diabetes, chronic kidney disease, and congestive heart failure) was used as an element of the evaluation.

Methodological Appropriateness

The body of evidence selected, based on the inclusion/exclusion criteria, included studies with wide diversity in methodological approaches. As discussed above, the committee did not rate studies for quality because, upon review, it found that the study designs, including randomized controlled trials (RCTs) and observational studies, were too varied. In addition, a consistent lack of calibration of approaches used to estimate sodium intake precluded a uniform rating system. Instead, each study was reviewed and critically appraised individually for its strengths and weaknesses (see Tables 4-1 through 4-8). Thus, the design and methodological approach of each study was of critical importance to the committee's evidence review.

The committee conducted a qualitative evaluation of the strengths and weaknesses of each eligible study based on the following criteria: size and characteristics of the study population, risk of bias, and fidelity of implementation of the intervention. The committee considered RCTs as the most valuable for determining the effect of sodium on health outcomes. Among observational studies, prospective cohort studies were considered the highest quality of design. Case-control studies were considered of lower quality as a study design, and the findings from these studies were given a lower relative weighting. Cross-sectional studies by themselves were included only as an indicator of a potential association between sodium intake and health outcome or to support results from other studies.

For RCTs, the committee identified specific criteria to assess risk of bias, including blinding, method of randomization, size and characteristics of the study population, drop-out rate, or relevance of the sodium intake level (intervention). The conformity of the method of implementation also was considered in appraising the quality of RCTs.

For observational studies, the committee evaluated risk of bias by evaluating strengths and weaknesses based on study design and length, method of measuring sodium intake as well as intake levels, and adjustment for confounders. To assess sodium intake measures, the committee used two broad categories: dietary intake and urinary excretion. Weaknesses in dietary intake measures included limitations in FFQs (e.g., few food items included, lack of validation with other sodium intake measurement methods, inaccurate food composition tables) and poor or missing validation of 24-hour food recalls.

Examples of weaknesses in urinary excretion measures included potential systematic errors in measuring sodium intake due to incomplete 24-hour urine collections, lack of reporting urine creatinine levels or body weight, and lack of validation of spot urine collection methods. Examples of weaknesses in adjustment of confounders included exclusion of appropriate confounders in the analytical model (e.g., age, gender, use of antihypertensive medications, caloric intake, dietary potassium intake, or other risk factors for the relevant health outcome) and inclusion of factors in the causal pathway of the effect as confounders in the analytical model. Interpretation of observational studies also took into account the possibility of reverse causality (i.e., the fact that an individual with lower sodium intake might experience an adverse health outcome, not because of sodium intake, but because of their health status, which in turn might lead to a low measured sodium intake).

METHODOLOGICAL ISSUES IN SODIUM RESEARCH: SODIUM INTAKE ASSESSMENT

The most common methods used to measure sodium intake are discussed below along with weaknesses and strengths. The descriptions are intended to illustrate that the appropriateness of the methodological approach depends on the application of the method and, most importantly, on the quality of its implementation. For example, although the 24-hour urine collection is portrayed as the “gold standard,” it is important to assess the quality of its implementation.

Dietary Intake Assessment

Data on nutrient intakes among free-living individuals are collected most commonly by one or more of three different methods: 24-hour dietary recall, FFQ, or food record. Each method has its own advantages and disadvantages and, to be useful, must be selected to satisfy the research objectives. Factors that are considered in selecting a method to collect nutrient intake data include

level of detail needed;
research design/protocol;
funding/resources (staff qualifications, resources for analyses);
participant burden; and
population characteristics (geography, ethnicity, education, health/cognitive status).

24-Hour Dietary Recall

The 24-hour dietary recall is typically an interviewer-administered survey that asks respondents to recall everything they ate or drank in the previous 24 hours. In addition, the respondent is probed for such supplemental information as timing of meals, eating occasions, place where the meal is eaten, recipes used, and additions to foods. The 24-hour recall has the advantage that it can be used with most population groups because a high level of literacy is not required. Respondents are generally compliant because only a small amount of information is requested in the interview. This approach also does not affect eating behavior because it is retrospective. More accurate information about “usual” eating habits can be obtained when more than 1 day of recall data are collected (Conway et al., 2004).

A general limitation to the 24-hour recall is that respondents may underreport and/or underestimate foods or intake amounts. To minimize this problem, it is important that the recalls be collected by trained interviewers who follow a systematic protocol to elicit information. This requirement makes use of 24-hour recalls typically more costly than other diet assessment methods.

The National Health and Nutrition Examination Survey (NHANES) uses 24-hour recalls to assess dietary intake. Before 2003, a second dietary recall was collected for only a subset of respondents for quality assurance or to permit more accurate assessment of usual dietary intake. Beginning in 2001, a second recall was collected for a subset of respondents to permit more accurate assessment of usual dietary intake. Since 2003, two recalls have been collected for all respondents. The first recall is collected in person and the second recall is collected 3 to 10 days later by telephone.

Over the years, the methods used to estimate sodium intake in NHANES have been adjusted. As of 2005, NHANES surveys have included sodium consumed from tap and bottled water in the individual intake estimates. In earlier years, data on intake of tap and bottled water were collected, but the sodium contribution of water was not included in estimates of sodium intake. The methods used to capture contributions from salt used in cooking also have changed over time (Cogswell et al., 2012).

Food Frequency Questionnaire

The FFQ includes a finite list of foods and beverages and responses to indicate how often each item was consumed by the respondent over a specified period of time. FFQs are often used when a relatively low respondent burden is desired and/or when available resources preclude use of interviewer-administered methods. It has the advantage that it can be self-administered and, in some cases, responses can be optically scanned for ease of data entry and analysis. In addition, FFQs can capture usual intakes, rather than a single-day “snapshot” of intake. When applied to sodium intake analyses in U.S. populations, the FFQ generally does not include questions about salt added in cooking or at the table. Additionally, if a limited selection of foods is available to the respondent, the FFQ can also be limited in the detail and quantitative intakes it captures. Sodium levels in particular may differ widely by brand of processed foods, and these may be inadequately captured within the broad categories commonly used in these questionnaires. Overall, the FFQ serves best to rank order individuals rather than to estimate absolute levels of sodium intake (Boeing et al., 1997; Carithers et al., 2009; Fayet et al., 2011). Commonly used FFQs include the Block FFQ (Block et al., 1990), the Harvard FFQ (Feskanich et al., 1993; Oh et al., 2005), and the Diet History Questionnaire developed at the National Cancer Institute (Subar et al., 2001).

Food Records

A food record is a detailed description, usually in the form of a diary, of the types and amounts of foods, beverages, and/or supplements consumed over a specified time period, such as 3 days. Respondents are asked to record detailed information about the foods and beverages they consumed each day. The information requested can include food preparation methods, recipes, brand names of products, and portion sizes. Other information includes the type of meal consumed, and the time of day and location of consumption. Accurate responses require a high level of knowledge and motivation and may require training in how to record intakes accurately. The burden placed on respondents is often a disincentive to use food records and thus, compared to the FFQ and the 24-hour recall, it is infrequently used in intake studies. Another but important limitation is the potential for inaccuracy in recording actual intake of items due to respondents' awareness of or desire to exhibit behavior change (Freudenheim et al., 1987).

In summary, dietary sodium intake is a complex exposure, but it is possible to obtain useful information provided the right measures are used. FFQs, 24-hour recalls, and diet records each have measurement errors that must be taken into consideration in assessing and analyzing the data collected.

Use of Urine Samples to Estimate Dietary Sodium Intake

In addition to the 24-hour recall, FFQs, and diet records, another common method of assessing dietary sodium intake is through measuring urine sodium excretion. General considerations when analyzing urine samples for biochemical indicators of dietary intake, and particularly for sodium intake, are described in other publications (Bernstein and Willett, 2010). This section presents a brief comparison of the methods most commonly used in the papers that the committee reviewed (i.e., 24-hour urine collection, timed overnight urine collection, and spot urine analysis) as a demonstration of the challenges they present and factors that the committee considered when attempting to interpret and compare results among studies using different methods. In addition to the obvious sources of error, such as whether the individual is under pharmacological therapies that affect sodium excretion (e.g., diuretics when evaluating spot urine specimens), sources of systematic and random errors were considered when interpreting urine sodium data. The most common systematic errors in 24-hour urine specimens involve undercollection of specimens and the day specimens are collected (Dyer et al., 1994). In spot urine specimens, errors involve the substantial variations of the sodium concentration in the urine depending on the time of the collection (related to timing of meals) (Mann and Garber, 2010). When evaluating spot urine specimens and indexing to urine creatinine, differences in muscle mass are another important consideration, as this is a major determinant of urine creatinine concentration and may therefore influence the spot urine sodium-to-creatinine ratio (Heymsfield et al., 1983).

24-Hour Urine Sodium

As more than 90 percent of consumed sodium is absorbed and excreted in the urine (Holbrook et al., 1984; Pitts, 1974), an accurately collected 24-hour urine sodium excretion has been considered the clinical gold standard method to assess an individual's dietary sodium intake on the same day (Clark and Mossholder, 1986; Luft et al., 1982; McCullough et al., 1991; Schachter et al., 1980). A study by Luft and colleagues (1982) showed a correlation of 0.75 between actual sodium intake measured from the diet and average 24-hour urine sodium excretion over a 9-day collection. This method minimizes errors in sodium measurement due to changes in tonicity that occur throughout the day, and on different days. To estimate an individual's usual dietary intake, multiple urinary measurements of sodium are required over a 24-hour period (He et al., 1993; Liu et al., 1979, 1986, 1987). Collecting 24-hour urine specimens is cumbersome and challenging, and frequently inaccurate unless conducted in a monitored setting, such as an inpatient clinical research unit. In unmonitored settings, individuals may be less able to collect all of their urine due to physical limitations, urinary incontinence, neurological problems, or other health reasons. Thus, 24-hour urine sodium assessment is mostly available in populations with modest sample sizes. Outpatient 24-hour urine collections are available in some studies, but systematic undercollection is considered a major threat to validity. When accurately collected, 24-hour urine sodium measurements have particular advantages. They capture sodium from salt in food and salt added during the preparation of meals as well as at the table. They objectively measure dietary sodium intake, and they are independent of participant recall. On the other hand, systematic undercollection may introduce bias if those subjects with the greatest degree of comorbidity (and thus, highest risk of adverse events) are also those most likely to collect an insufficient amount of their urine. Undercollection will, of course, incorrectly classify individuals into the low-sodium categories.

Methods are available to assess and improve the accuracy of outpatient timed urine collections. Some studies average 24-hour urine sodium over multiple collections, which is likely to improve accuracy as the measure of usual sodium intake. Another method is to assess the 24-hour urine creatinine excretion. Because 24-hour urine creatinine excretion is not materially influenced by kidney function, and is tightly correlated to muscle mass, the total amount excreted over 24 hours should be relatively constant within an individual. Thus, undercollected 24-hour urine specimens may have lower-than-expected creatinine excretion. Conversely, overcollection will have higher-than-expected creatinine excretion. Prior studies suggest that usual 24-hour urine creatinine excretion is between 15-25 mg/kg/day in men and 10-20 mg/kg/day in women (Walser, 1987). Some studies have conducted sensitivity analyses that limited analysis to the subset with consistent 24-hour urine creatinine excretion measurements within individuals when multiple 24-hour urine collections were available. Others have excluded 24-hour urine specimens when 24-hour urine creatinine excretion measurements were implausibly low or high. Such maneuvers should decrease bias due to inaccurate collections and improve precision.

Alternative Urine Methods of Estimating Sodium Dietary Intake

Given the burden of collecting 24-hour urinary specimens and threats to validity due to collection errors, alternative methods of estimating sodium analysis have been developed, such as timed overnight (usually 8-hour) urine collections. Correlations between 24- and 8-hour overnight urine sodium are generally high. Various studies have shown good correlations (r=0.75 to 0.94) between overnight and 24-hour urine collections (He et al., 1993; Liu et al., 1979, 1986, 1987). Despite these high correlations, some authors have shown that the intra- and interindividual coefficients of variation were consistently greater for the 8-hour urine sodium than for the 24-hour urine sodium. Moreover, some studies suggest that individuals may excrete more sodium overnight than during the day, so 8-hour urine collections may introduce some systematic error (Dyer et al., 1987).

The ease of collecting spot urine sodium specimens provides an attractive third alternative. However, as specimens are collected at only one time point, they may introduce an important source of error because of temporal changes in urine tonicity as well as changes in urine sodium excretion that occur throughout the day and relative to the timing of meals. Use of spot specimens requires correction for urine tonicity, most commonly by indexing to urine creatinine (Godevithanage et al., 2010; Imai et al., 2011). A recent systematic review of studies comparing spot and 24-hour urine analysis for estimating sodium intake revealed substantial variations in the correlations (ranges=0.28-0.67) depending on the methods used (Ji et al., 2011). The authors highlighted the need for studies validating methods of sodium urine analysis as alternatives to the more cumbersome 24-hour urine sodium collection.

As discussed previously, urine creatinine excretion (a marker of collection accuracy) is influenced not only by urine tonicity, but also by muscle mass. Creatinine excretion is lower in persons with lower muscle mass. Thus, women, older individuals, those with lower body weight, and those of non-African descent have lower urine creatinine excretion, on average (Ix et al., 2011). A number of investigators have developed equations to estimate creatinine excretion on the basis of these demographic variables for various clinical indications (Cockcroft and Gault, 1976; Ix et al., 2011; Kawasaki et al., 1991). Kawasaki and colleagues (1982) developed one such equation. Subsequently, they multiplied spot urine sodium-to-creatinine ratios to the estimated creatinine excretion derived from their equation, in an attempt to improve the correlation of spot urine sodium-to-creatinine ratio with 24-hour urine sodium. They demonstrated that the correlation of this estimate to 24-hour urine sodium was 0.73. However, the equation to estimate creatinine excretion was derived in a Japanese population with a mean dietary sodium intake of approximately 5,000 mg per day, which is substantially higher than the typical sodium intake in the United States. O'Donnell and colleagues (2011) recently evaluated this equation in 105 participants in the Prospective Urban Rural Epidemiology study, a multinational study focused on noncommunicable disease epidemiology. In these individuals, the construct of spot urine sodium-to-creatinine ratio multiplied by the estimated creatinine excretion using the Kawasaki formula was correlated with measured 24-hour urine sodium excretion with a correlation coefficient of 0.55 (O'Donnell et al., 2011). This correlation is reasonably strong. Moreover, studies are consistent in demonstrating that correcting the spot urine sodium-to-creatinine ratio for creatinine excretion improves the correlation with measured 24-hour urine sodium compared to using spot urine sodium-to-creatinine ratio alone (Mann and Gerber, 2010). Further, despite reassuring correlation coefficients, the calibration of this technique in relation to actual dietary sodium intake is uncertain. The committee agreed that while the spot urine sodium-to-creatinine ratio multiplied by the estimated creatinine excretion construct appears useful to rank order persons with respect to their sodium intake, the absolute level of estimated dietary intake is uncertain in non-Asian populations.

In summary, multiple well-done 24-hour urine collections remain the gold standard method to assess dietary sodium intake, but bias due to inaccurate collection represents a major threat to validity. Moreover, most studies using 24-hour urine specimens do not provide data on urine volume, urine creatinine, and body weight, variables that would allow readers to evaluate whether 24-hour urine creatinine excretion is similar among sodium intake categories. Similar to survey methods such as the 24-hour diet recall, a single 24-hour, overnight 8-hour, or spot urine sodium measures are unlikely to capture habitual dietary sodium intake accurately. The correlation of spot urine sodium-to-creatinine ratio with 24-hour urine sodium is improved when adjusted for estimated creatinine excretion, which may provide a useful method to rank order participants based on dietary sodium intake in large epidemiologic studies. However, whether or not the estimated value of 24-hour urine sodium by this method is accurately calibrated to dietary sodium intake remains uncertain.

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