Beverage Use and Risk for Kidney Stones in Women

An increase in fluid intake is routinely recommended for patients who have had a kidney stone to decrease the likelihood of recurrence. Higher fluid intake leads to increased urinary volume and, in turn, to a decreased concentration of lithogenic factors, presumably decreasing the rate of stone formation. Although it has not been proven successful, this approach is supported by many [1-4] but not all [5] authors. Of particular clinical interest are the risks and benefits conferred by different beverages. Beverages differ in the amount of potentially important constituents, such as caffeine, oxalate, and alcohol, that they contain. Therefore, they may influence stone formation differently. Several studies have examined the effects of particular beverages on changes in urine composition [6, 7], but few have examined the effect of particular beverages on kidney stone formation. In a retrospective study [8], beer and coffee consumption were inversely associated with a history of kidney stones, and carbonated beverage (soda) consumption was positively associated with such a history. No significant associations were found with milk, tea, or water [8]. However, the authors did not adjust for other risk factors for kidney stone formation, such as dietary intake of calcium [9]. In a prospective study of the intake of 21 different beverages and incident kidney stone formation in men [10], we adjusted for nutrient intake and found an increased risk associated with consumption of apple juice and grapefruit juice and a decreased risk associated with consumption of coffee, tea, and alcoholic beverages. These results may not necessarily apply to women because the rate of kidney stone formation in women is one third that in men and the determinants of stone formation may differ by sex. No previous prospective data are available on the relation between beverage use and risk for stone formation in women. We previously demonstrated an inverse association between total fluid intake and risk for incident stone formation in a large prospective study of women [11]. To investigate whether the individual type of fluid ingested is important in women, we examined the relation between 17 different beverages and risk for symptomatic kidney stones in a cohort of 81 093 women in the United States. Methods Study Sample In 1976, 121 700 female nurses 30 to 55 years of age completed and returned the initial questionnaire; these women make up the Nurses' Health Study cohort [12]. The cohort is followed through biennial mailed questionnaires that ask about lifestyle practices, other exposures of interest, and the incidence of newly diagnosed disease. The sample for our current analysis was limited to the women who answered the 1992 questionnaire (which included a question about lifetime history of kidney stones) or the 1994 questionnaire (which asked about kidney stones in the previous 2 years). In addition, it included only participants who had also answered either the 1986 or the 1990 dietary questionnaire, which elicited information on beverage use. Although information on some beverages was obtained on the 1980 and 1984 questionnaires, water had not been included. Because water is the most commonly used beverage, we used only the beverage information from the 1986 and 1990 questionnaires, which included water in the beverage list. We considered only cases of kidney stones that occurred during the 8 years between the return of the 1986 questionnaire and 31 May 1994 in women with no previous kidney stones. After further exclusion of women for whom the date of a kidney stone could not be confirmed, 81 093 women with no history of kidney stones at baseline remained for analysis. Assessment of Diet In 1986 and 1990, participants were asked to complete semiquantitative food-frequency questionnaires on which they reported their average use of 131 specified foods and beverages during the previous year. Nutrient intake was computed from the reported frequency of consumption of each specified unit of food or beverage and from published data on the nutrient content of the specified portions [13]. In addition, information was collected on the amount of supplemental calcium (such as calcium carbonate) ingested, either alone or as part of multivitamin preparations. The reproducibility and validity of the questionnaires in this cohort were documented previously [13-15]. A similar questionnaire was shown to be valid and reproducible in men [16, 17]. Individual beverage use was reported as the number of times that a standard serving size of a given beverage was consumed; nine frequency categories were used, ranging from less than once per month to six or more times per day. Eighteen beverages were included on both the 1986 and the 1990 questionnaires. To calculate the average daily intake of each beverage, we multiplied the reported frequency of used by the serving size for the beverage. Because we had no a priori hypothesis that the various types of wine would differ for risk for stone formation, red and white wine were combined. A food-based validation study showed reasonable levels of reproducibility and validity for reported intake of the 11 individual beverages that were included in the 1980 version of the semiquantitative food-frequency questionnaire [14]. Pearson correlations between the food records and the questionnaire ranged from 0.36 for noncola carbonated beverages to 0.94 for beer, with a mean correlation for the beverages of 0.77. We calculated total daily fluid intake by using data on frequency of use and serving size of individual beverages. In a validation study in men, a high correlation (r = 0.59) between reported fluid intake and 24-hour urinary volume was reported [10]. We adjusted nutrient values for total energy intake by taking the residuals from a linear regression model with total caloric intake as the independent variable and nutrient intake as the dependent variable [18, 19]. Energy-adjusted values reflect the nutrient composition of the diet independent of the total amount of food consumed. In addition, energy adjustment reduces any variation introduced by overall underreporting or overreporting of intake on the food-frequency questionnaire, thus improving the accuracy of nutrient measurements [18, 19]. Calculation of beverage intake was not adjusted for total energy intake. Follow-up and Ascertainment of Cases On the 1992 questionnaire, we asked participants whether they had ever had a kidney stone diagnosed and the date of the first occurrence. The 1994 biennial questionnaire asked about a new diagnosis of a kidney stone since 1992. If a kidney stone was reported, we mailed a supplementary questionnaire to the respondent to confirm the diagnosis and to ascertain the date of occurrence; symptoms; other relevant medical conditions; and the type of stone, if known. The response rate to the supplementary questionnaire was 92%. To confirm the validity of the responders' reports, we obtained the medical records for a random sample of 90 of the women who reported having a kidney stone. The records confirmed the diagnosis in all but one case. Statistical Analysis In this prospective study, information on diet was collected before any diagnosis of kidney stone was made. For each participant, person-months of follow-up were counted from the date of return of the 1986 questionnaire to the date of a diagnosis of kidney stone, death, or 31 May 1994, whichever occurred first. Information on exposures of interest from the 1986 questionnaire was updated in 1990. We allocated person-months of follow-up according to exposure status (the category of individual beverage use) at the start of each follow-up period. If dietary information was missing at the start of a time period, the participant was excluded for that time period. For example, if a participant answered the 1986 questionnaire but not the 1990 questionnaire, she would contribute person-time only for the 1986 to 1989 period. If a participant answered the 1990 questionnaire but not the 1986 questionnaire, she would contribute person-time only for the 1990 to 1994 period. The proportion of participants with information missing for any individual beverage was 5% or less for both the 1986 questionnaire (Table 1) and the 1990 questionnaire (data not shown). A validation study in men showed that missing items were not consumed in most instances; thus, participants with information missing for an individual beverage were assigned to the lowest category of intake [17]. The categories for the beverage items were selected before the analyses on the basis of the frequency response for each individual item. The goal was to have enough participants in each category (particularly the extreme categories) to provide sufficient power to examine associations comparing extremes of intake. For each beverage, we assessed the nine individual response categories (after examining the grouped categories) to determine whether substantial variation was being masked. Because the results were very similar, the a priori selected grouped categories were used to provide more stable estimates. Table 1. Distribution of Beverage Use among 65 584 Women with No History of Kidney Stones in 1986* To formally test the null hypothesis that beverage type does not influence risk for kidney stones, we added intake of all specific beverages (except water) to a multiple logistic model that contained the total fluid intake variable and evaluated the change in total model deviance (likelihood ratio test) with 16 degrees of freedom. To make the results of the multivariate models more clinically useful and to have some standard for direct comparisons of different beverages, we converted the daily use of each beverage into the average number (or fraction) of 240-mL (8-oz) servings consumed per day. This provided an estimate of the effect on risk for stone formation of increasing the consumption of an individual beverage by one unit (2