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3 Methodological Considerations
Pages 61-98

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From page 61...
... A number of complex methodological considerations are integral to the critical evaluation and interpretation of studies that examine these relationships. This chapter summarizes the committee's review and interpretation of four methodological considerations related to deriving the DRIs for potassium and sodium: relevant biological roles of potassium and sodium, methods for estimating potassium and sodium intake, interactions of potassium and sodium, and evidence on subpopulations.
From page 62...
... . Aldosterone promotes potassium excretion, sodium reabsorption, and hydrogen ion 1  Estimates of mean usual potassium intake for U.S.
From page 63...
... When this is achieved, sodium excretion is approximately equivalent to intake. In general, sodium intake does not affect potassium excretion, but net losses of potassium have been documented at very high levels of sodium intake (6,900 mg/d [300 mmol/d]
From page 64...
... Studies with 23Na-MRI have shown that skin sodium content is related to the blood pressure levels in patients with resistant hypertension (Kopp et al., 2013) .2 Furthermore, recent data suggest that urinary sodium excretion does not mirror sodium intake on a day-to-day basis (Kopp et al., 2013; Lerchl et al., 2015; Rakova et al., 2013; Weaver et al., 2016)
From page 65...
... Implications for the Committee's Review of the Evidence Potassium and sodium's physiological functions appear to be primarily mediated through blood pressure, which has a strong relationship with cardiovascular disease. Accordingly, the committee focused the indicator review on relationships between potassium and sodium intakes and blood pressure and cardiovascular disease outcomes.
From page 66...
... Thus, the accurate assessment of usual dietary intake -- the long-run average daily nutrient intake -- is applicable to multiple steps in deriving DRI values. The accuracy of potassium and sodium intake estimates is critical, as it can affect the strength of diet–indicator relationships, the strength of intake–response relationships, the accuracy of quantitative estimates of the intake–response relationship, and accuracy of the estimation of usual intake distribution for a population.
From page 67...
... If no statistical adjustments are applied, a large number of samples (10 or more for each individual) , collected on both weekdays and weekends, may be needed to obtain an accurate estimate of the distribution of usual sodium intake in the group that has the correct variance (Dyer et al., 1997; Liu and Stamler, 1984; Luft et al., 1982)
From page 68...
... . Although cautious interpretation of these results is needed because there is a large amount of unexplained heterogeneity, the meta-analysis provides support for using 24-hour urine collections to estimate average sodium intake and recommends multiple 24-hour urine samples to determine an individual's usual sodium intake (Lucko et al., 2018)
From page 69...
... . Various equations exist to estimate 24-hour potassium or sodium excretion from spot urine samples, including Tanaka, INTERSALT, Kawasaki, Mage, Nerbass, Arithmetic, PAHO, and Danish (Brown et al., 2013; Ji et al., 2014; Kawasaki et al., 1993; Mage et al., 2008; Nerbass et al., 2014; Tanaka et al., 2002; Toft et al., 2014; WHO/PAHO, 2010)
From page 70...
... To accurately measure sodium intake, consideration must be given to the variability in sodium content across specific brands of foods, limited information for restaurant-­ prepared food, and the precision and currency of food composition databases to estimate intakes, as well as systemic underreporting of total intake. Further­more, self-reported dietary assessment methods do not always capture sodium added during cooking or at table, which is estimated to account for 6 and 5 percent of intake, respectively (Harnack et al., 2017)
From page 71...
... . Thus, if the mean of 24-hour dietary recalls for an individual is used to estimate usual intake, more replicate recalls would be necessary for the measurement of sodium intake than for potassium intake, preferably including both weekend and weekday data.
From page 72...
... The committee was provided with distributions of usual potassium and sodium intake based on the 24-hour urinary samples and 24-hour dietary recall data from the NHANES 2014 subsample (n = 779) .4 Figures 3-1 and 3-2 summarize the estimated usual intake distributions for potassium and sodium, respectively, obtained by three measures of daily nutrient intake: 24-hour dietary recalls, 24-hour urinary excretion, and 24-hour urine excretion adjusted for rate of recovery.
From page 73...
... The light blue bars represent excretion after correction for percent recovered. The red bars represent sodium intake measured using 24-hour dietary recalls.
From page 74...
... population that it is appropriate to use 24-hour dietary recalls to describe the usual intake distributions of potassium and sodium for comparing established population intake levels, as prescribed in the third step of the DRI organizing framework. Measurement error in potassium and sodium intake generally attenuates the diet–health relationship.
From page 75...
... Food records are infrequently used and studied compared with 24-hour dietary recalls and food frequency questionnaires, which may be attributable, in part, to respondent and investigator burden. Compared with 24-hour urinary excretions in adults, food records have underestimated mean sodium intake by 2 to 8 percent and varied from mean potassium intake by –4 to +3 percent (Lassale et al., 2015)
From page 76...
... . Sodium intake estimated from food frequency questionnaires compared with urinary measures is generally reported to be underestimated by 4–42 percent (Freedman et al., 2015; Kelly et al., 2015; Li et al., 2014; Pereira et al., 2016; Trijsburg et al., 2015)
From page 77...
... . The evidence suggests that 24-hour dietary recalls are more accurate than food frequency questionnaires for estimating the absolute intakes of both potassium and sodium intake (Ferrari et al., 2009; Freedman et al., 2015; Trijsburg et al., 2015)
From page 78...
... Other methods for assessing potassium and sodium intake had higher risk-of-bias ratings for this domain. Annex C-1 in Appendix C presents the full list of the risk-of-bias domains and criteria used in the AHRQ Systematic Review.
From page 79...
... Some studies have reported that consumption of potassium-containing salts increase urinary sodium excretion and that blood pressure is more highly correlated with the sodium-to-potassium ratio than to intake of
From page 80...
... . The AHRQ Systematic Review concluded that, based on a low strength of evidence, there is no significant moderating effect of potassium intake on the effects of sodium intake on systolic or diastolic blood pressure.
From page 81...
... Changing the diet to increase potassium intake requires changes that likely increase the consumption of nutrient-rich foods that will increase intakes of other nutrients while also possibly resulting in lower sodium intakes, thus contributing to an improvement in blood pressure. With these types of interventions, it is not possible to discern the independent contribution of the sodium-to-potassium ratio on changes in blood pressure, because multiple, often undefined, dietary changes (e.g., other food components, concomitant dietary compensations)
From page 82...
... 8  For additional information regarding sources of evidence for potassium and sodium intake distributions, see Appendix G
From page 83...
... . The AHRQ Systematic Review found insufficient evidence to assess the moderating effects of calcium or magnesium on the effects of potassium or sodium intake with any of the indicators reviewed (i.e., systolic and diastolic blood pressure, cardiovascular disease morbidity and mortality, and kidney disease)
From page 84...
... With the introduction of DRIs based on chronic disease, opportunity exists to further specify the applicable population group or groups. The AHRQ Systematic Review included subquestions to determine if characteristics such as sex, age, race/ethnicity, or comorbidity affected the relationship between sodium or potassium intake and chronic disease outcomes and risk.
From page 85...
... The existing criteria have varying ranges of high and low sodium intake levels, durations, and sequences of approach. There continues to be a lack of reproducibility of the acute blood pressure responses to sodium challenges that are indicative of salt sensitivity.
From page 86...
... Throughout its evidence review, the committee notes where there is evidence of effect modification by a population characteristic. SUMMARY In preparation for its review of the evidence, the committee examined a range of methodological considerations that are central to evaluating and interpreting studies assessing the relationship between potassium and sodium intake and indicators.
From page 87...
... • Bone health, kidney disease, and glycemic control are also potentially informa tive indicators. Methods for Estimating Potassium and Sodium Intake • The most accurate measure of sodium or potassium intake for the purposes of informing a Dietary Reference Intake (DRI)
From page 88...
... 2013b. Effect of lower sodium intake on health: Systematic review and meta-analyses.
From page 89...
... 2015. Use of urine biomarkers to assess sodium intake: Challenges and opportunities.
From page 90...
... 2016. Spot urine sodium measurements do not accurately estimate dietary sodium intake in chronic kidney disease.
From page 91...
... 2018. Errors in estimating usual sodium intake by the Kawasaki formula alter its relationship with mortality: Implications for public health.
From page 92...
... 2016. Mean population salt intake estimated from 24-h urine samples and spot urine samples: A systematic review and meta-analysis.
From page 93...
... 1984. Assessment of sodium intake in epidemiological studies on blood pressure.
From page 94...
... 2017. Assessment of dietary sodium intake using a food frequency questionnaire and 24-hour urinary sodium excretion: A systematic literature review.
From page 95...
... 2014. Devel­ opment of a formula for estimation of sodium intake from spot urine in people with chronic kidney disease.
From page 96...
... 2003. Validity of a self-administered food frequency questionnaire in the 5-year follow-up survey of the JPHC Study Cohort I to assess sodium and potassium intake: Comparison with dietary records and 24-hour urinary excretion level.
From page 97...
... 2013. Effects of sodium intake and diet on racial differences in urinary potassium excretion: Results from the Dietary Approaches to Stop Hypertension (DASH)
From page 98...
... American Journal of Physiology 268(4 Pt 2)


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