Despite the finding in cross-cultural comparisons that habitual sodium intake correlates with levels of blood pressure, similar studies from within population groups have yielded inconsistent results. The data presented in this report indicate that in industrialized societies the high degree of intra-individual variability of sodium intake, compared to much smaller inter-individual differences, may obscure potential biological correlations. A quantitative statistical method is presented to assess and minimize the effect of the large intra-individual variation in daily urinary sodium excretion.

SUMMARY This study explored the association between sodium excretion and blood pressure (BP). A new method was used to minimize the measurement error introduced by the large intrinsic variability of 24-hour sodium excretion. The ratio of intra-to interindividual variation was used to estimate the number of measurements needed to characterize the individual. When seven consecutive 24-hour samples were collected from 73 children, ages 11-14 years, a significant correlation between mean individual sodium excretion and BP was demonstrated. The independent relationship persisted when controlling for height, weight, pulse, age, sex and race (p = 0.045), but was eliminated by simultaneously considering mean creatinine excretion. Although the cross-sectional association described is quantitatively weak, a linear relationship between BP and sodium over the range consumed in this society could be important for prevention.CHRONIC HIGH SODIUM INTAKE has long been thought to play a role in the development of high blood pressure.' Cross-cultural comparisons support the hypothesis of a sodium-blood pressure relationship, and extremes of sodium intake, both high and low, have been shown to alter blood pressure.1-2 The association within a population, however, over the range usually ingested by industrialized societies, has been difficult to demonstrate."1-8 Using a new approach, this study shows a positive cross-sectional association between sodium excretion and blood pressure in children ages 11-14 years.'9' 20 MethodsThe study was an extension of a blood pressure survey among children in the parochial schools of Chicago (Levinson S, Berkson D: manuscript in preparation). Through contact with interested science teachers and school personnel, two schools were chosen for an intensive study of the relation of sodium excretion and blood pressure. The proposed study was presented as part of the curriculum to science classes of the sixth through eighth grades and the children were asked, with parental consent, to volunteer. Previous experience and statistical analysis indicated that the crucial methodologic problem involved minimizing sizable measurement error introduced by a high ratio of intra-to interindividual variation in daily sodium excretion.19 Based on findings in adults, we estimated that seven 24-hour urinary sodium determinations were needed to characterize the individual.19Groups of six to 10 children were carefully instructed on the procedure for collection of 24-hour urine samples. The sample was divided into an overnight portion, defined as any urine voided after going to bed at night and first morning void, and daytime sample, the remainder of the day. Carrying cases and plastic bottles were provided each student on a daily basis. Every effort was made to collect the seven samples on consecutive days. When a child was ill, missed school or accidently discarded urine, an additional 24-hour sample was obtained the following week. Although not necessarily the same day of the week, repeat weekday samples were collected ...

SUMMARY How many 24-hour urine sodium measurements are adequate for characterizing a child's salt intake? Can overnight urine specimens accurately replace 24-hour collections for salt assessment? A sample of 73 6tb-8th grade children was taken from two parochial schools in Chicago to inrestigate systematically these questions. Seven consecutive 24-hour-urine specimens were collected from each child. The estimated ratio of intra-to inter-individual variances was 1.94 for 24-hour-urine sodium. Based on this value, eight 24-hour specimens are necessary to limit to 10% the diminution of the estimated correlation coefficient between 24-hour-urine sodium and blood pressure. Six measurements are required to reduce to 0.01 the probability of misclassifying a child in tertile 1 versus fertile 3.The overnight specimens show a moderate consistency with the 24-hour collections in detecting children with high or low salt intake. For example 92% and 85% of children in the fifth quintile and the third tertile respectively of the true mean overnight sodium hare their true mean 24-hour Na In the upper half of the distribution. These results suggest that in a large scale epidemiologic study, overnight specimens may be reasonable alternatives when 24-hour-urine sodium is practically very difficult to collect. (Hypertension 1: 631-636, 1979) KEY WORDS • sodium excretion • children • urine sampleA CAUSAL relationship between excess sodium intake and high blood pressure has been inferred from the findings of epidemiological, clinical and animal experimental studies. In order to study this matter accurately, an appropriate method is required for characterizing each child's mean urinary Na output, as an index of Na intake. A recent study by our group on sodium excretion in healthy American adults indicated that the intraindividual variation in the individual's daily output is about three times the inter-individual variation.13 This phenomenon can result in serious underestimation of the true association between habitual sodium intake and blood pressure within a population unless several 24-hour urine specimens are collected. 'u For example, in the adult population studied, fourteen 24-hour urine samples are needed to limit to less than 10% the error in the correlation between true mean 24-hour urine Na and blood pressure. Therefore, as a first step in studying this relationship in children, it was deemed necessary to examine the ratio of intra-to interindividual variation in 24-hour Na excretion, and to assess the number of urine specimens required for accurately characterizing each child's sodium intake.Collection of large numbers of 24-hour urines per person, adult or child, is very difficult practically. This difficulty can be reduced if overnight urine collection can replace 24-hour specimens for Na assessment.

CEREBROVASCULAR diseases rank as the third most common cause of death in the United States. In recent years the long-term downward trend in stroke mortality appears to have accelerated; death rates for stroke have declined more rapidly than for any other component of cardiovascular mortality. This paper presents data on the trends in stroke mortality for the 4 major sex-color groups and discusses possible causative mechanisms. Methods Data sources.Age-specific mortality data by cause for each of the 4 major sex-color groups were obtained from the monograph of Moriyama, Krueger and Method of analysis. U.S. mortality data for the entire age range were age-adjusted to the 1970 U.S. population by the direct method for each sex-color group. Death rates for persons age 35-74 were examined to assess the impact of premature mortality from stroke; age-adjustment was accomplished by averaging rates for the 4 decades for each sex-color group. Ratios of white to non-white mortality were based on the age-adjusted rates for the entire age range.Mortality data from each of the 50 states were combined to form the 9 standard geographic regions routinely presented by the National Center for Health Statistics (Appendix I). The 7 even numbered years between 1960 and 1974 were analyzed for the 9 regions. Complete data were not available on the population base by region and age adjustment was based on 3 age categories: 0-44, 45-64, 65 and over. Population data by sex and color were also unavailable for the geographic regions.

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