samples, samples from the first urination in the morning, and timed overnight samples, but these have not proven satisfactory (Dyer et al., 1997). Even in the case of 24-hour urine collection, as pointed out by Dyer et al. (1997), intake changes from day to day in individuals, and there is large intra-individual variation in salt consumption thereby necessitating large sample sizes, high-quality collection and analysis, and estimates of within-person variability to ensure accurate estimates.
Due to heavy respondent burden and other logistical challenges including costs, 24-hour urine collection has not been a component of the NHANES; therefore nationally representative estimates of sodium intake based on this urinary measure are not available from the data set. However, dietary recalls are also considered to be a valid method for assessing sodium intake (Espeland et al., 2001; Reinivuo et al., 2006). Some information about the U.S. population based on urinary measures is available from two international studies and from a survey of approximately 1,000 persons ages 27–37 years. These estimates are generally consistent with findings based on dietary intake methods and confirm that sodium intake in the United States is above recommended levels.
INTERSALT is the largest study, and among the most often referenced in the literature, relating electrolyte intake to blood pressure. INTERSALT conducted a single 24-hour urine collection from subjects in 32 countries during 1985–1987. Energy intake was not estimated (INTERSALT Cooperative Research Group, 1986; Loria et al., 2001). The study was carried out under the auspices of the Council on Epidemiology and Prevention of the International Society and Federation of Cardiology, with funding from the Wellcome Trust; the National Heart, Lung, and Blood Institute (NHLBI); the International Society on Hypertension; the World Health Organization (WHO); the Heart Foundations of Canada, Great Britain, Japan, and The Netherlands; the Chicago Health Research Foundation; the Belgian National Research Foundation; and Parastatal Insurance Company, Brussels. Field work began in 1984 and was completed in the mid-1980s (INTERSALT Cooperative Research Group, 1988). INTERSALT assessed more than 10,000 men and women ages 20–59 years at 52 centers in 32 countries. Observers were centrally trained, and a central laboratory was used to ensure standardization and quality control. The measures included urinary sodium, blood pressure, and several potentially confounding variables. Sodium intake was determined by a single timed 24-hour urine collection. The INTERSALT Cooperative Research Group, comprised of a number of investigators in participating centers, reported the results.
Among the four centers in the United States, urinary sodium levels