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*Department of Preventive Medicine and Epidemiology and Department of Nephrology, Loyola University Medical Center, Chicago, Illinois;
Channing Laboratory, Brigham and Womens Hospital, and Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts;
General Medicine Unit, Massachusetts General Hospital, Boston, Massachusetts; and
Harvard Medical School, Boston, Massachusetts.
Correspondence to Dr. Gary C. Curhan, Channing Laboratory, 181 Longwood Ave. Boston, MA 02115. Phone: 617-525-2683; Fax: 617-525-2008;
| Abstract |
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| Introduction |
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200 mg/L (5). Thus, even moderate increases in urinary calcium excretion may substantially increase the risk for stone formation. Menopause is associated with an increase in urinary calcium excretion (6,7), which may increase the risk for calcium-containing stone formation. In subjects matched for serum total and ionized calcium levels, urinary calcium excretion was 50% higher (P < 0.001) in postmenopausal women compared with premenopausal women (6). Thus, the onset of menopause may increase urinary calcium excretion and the risk for kidney stone formation. In contrast, postmenopausal hormone (PMH) use has been shown to decrease fasting urinary calcium by 50% compared with baseline in postmenopausal women (8). It is currently not known whether menopause and PMH use are independent risk factors for kidney stone formation in women. The aim of this study was to prospectively study the independent association among menopause, PMH use, and risk of incident kidney stones in women.
| Materials and Methods |
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Menopause and PMH Use
Menopausal status was first determined with the initial questionnaire completed by the NHS subjects in 1976 and was subsequently updated with each biennial questionnaire. The questionnaires specifically asked whether the subjects menstrual periods had ceased permanently, and if so, at what age and for what reason (e.g. natural, surgical, radiation, etc.). If menopause was due to surgery, the subject was asked to report the number of ovaries removed. Self-reported age at menopause and type of menopause has been validated in the NHS and shown to be highly accurate (9).
Women were defined as postmenopausal from the self-reported time of natural menopause or bilateral oophorectomy. We excluded women with indeterminable age at onset of menopause for the analysis of menopause and risk of incident kidney stones. Subjects with indeterminable menopausal status were women who underwent hysterectomy without bilateral oophorectomy, women with incomplete data on the extent of pelvic surgery, women using exogenous estrogen before the onset of menopause, women with permanent cessation of menses due to radiation therapy, and women who did not report the age of onset of menopause.
The analysis of PMH use and incident kidney stones was limited to postmenopausal women. In these analyses, women with indeterminable menopausal status were considered menopausal when they reached the age at which natural menopause occurred in 90% of the cohort (54 yr for smokers and 56 yr for nonsmokers).
In 1976, the Nurses Health Study participants were first queried on the use of PMH therapy, including duration of use. Information on the types of PMH use was subsequently collected beginning in 1978. All of this information was updated with each biennial questionnaire. Duration of PMH use was calculated from information provided on the questionnaires. Each biennial questionnaire asked whether the subject was currently using PMH (within the last month) and the number of months PMH had been used during the past 24 mo since completion of the last biennial questionnaire. Women with missing data on PMH use for a given 2-yr period were assigned to a missing category for that time period.
Kidney Stones
Beginning in 1992, the biennial questionnaire inquired whether the participants had ever been diagnosed with a kidney stone, and new reports of kidney stones were updated with each subsequent questionnaire. A supplementary questionnaire was sent to subjects who reported kidney stones to confirm the diagnosis and ascertain the date of occurrence and other relevant medical information. Subjects had to report associated pain or hematuria on the supplementary questionnaire to be considered cases. To confirm the validity of the participants reports, we obtained the medical records from a random sample of 90 women who reported a kidney stone. The records confirmed the diagnosis for all but one woman (98%) (10).
Assessment of Diet
Participants were asked in 1980, 1984, 1986, 1990, and 1994 to complete semiquantitative food-frequency questionnaires on which they reported the average use of specified foods and beverages during the past year. The 1980 dietary questionnaire contained a list of 61 items, and subsequent questionnaires contained approximately 130 items. 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 (11). The reproducibility and validity of the questionnaires in this cohort have been previously published (11).
Nutrient values were adjusted for total energy intake using a linear regression model with total caloric intake as the independent variable and absolute nutrient intake as the dependent variable (11). Total energy intake for a person tends to be fixed within a narrow range. Thus, variations in nutrient intake are largely due to changes in composition of the diet and not the total amount of food consumed. Energy-adjusted values reflect the nutrient composition of the diet independent of the total amount of food consumed.
Statistical Analyses
The study design was prospective, and information on menopause and PMH use was collected before the onset of the kidney stone. Person-years of follow-up were calculated from the date of the return of the 1980 questionnaire to the date of the first kidney stone, or May 31, 1998. Person-time was allocated to menopausal status or PMH use status according to the 1980 questionnaire and then updated with each subsequent biennial questionnaire. If information on menopausal status, PMH use, or diet was missing at the start of a time period, the subjects were assigned to the missing category for that time period.
Categorical variables were compared using the
2 test, and continuous variables were compared using the t test. We adjusted for age by using 1-yr intervals and computed age-adjusted risks for incident kidney stones according to menopausal status (natural or surgical) with premenopausal status as the reference group. Postmenopausal women were then stratified by type of menopause (surgical versus natural) and age-adjusted risks for incident kidney stones and PMH use (current or past) and duration of current or past PMH use (<1 yr, 1 to 1.9 yr, 2 to 4.9 yr, 5 to 9.9 yr,
10 yr) were calculated with never-use as the reference group.
We used pooled logistic regression to determine the independent association between menopausal status and risk for incident kidney stones while simultaneously adjusting for multiple risk factors (12,13). The following covariates were included in the multivariate pooled logistic regression models: age (1-yr intervals), body mass index (five categories), presence of hypertension (yes/no), supplemental calcium (0 mg/d, 1 to 100 mg/d, 101 to 500 mg/d, and >500 mg/d), alcohol consumption (eight categories), and quintiles of dietary intake of calcium, vitamin B6, animal protein, potassium, sodium, sucrose, magnesium, and total fluid. In the analysis of PMH use and risk of incident kidney stones, we controlled for age at menopause (1-yr intervals) in addition to the demographic and nutritional covariates included in the model of menopausal status and risk of incident kidney stones. All P values are two-tailed and 95% confidence intervals (CI) were calculated for all relative risks (RR).
| Results |
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No significant association between past or current PMH use and risk of incident kidney stones was noted among postmenopausal women with natural or surgical menopause in the age-adjusted or multivariate models (Tables 3 and 4). Furthermore, there was no significant association between the duration of past (data not shown) or current PMH use and the risk of incident kidney stones among postmenopausal women with either natural (Table 3) or surgical menopause (Table 4).
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| Discussion |
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In women with surgical menopause, loss of ovarian estrogen production is sudden and complete (15). The sudden loss of estrogen is compounded by a substantial decrease in circulating testosterone levels (15). Even after menopause, the ovaries normally continue to produce testosterone, and removal of both ovaries after menopause decreases circulating testosterone by 40 to 50% (16). Peripheral tissues metabolize testosterone to estradiol, thus the continued production of testosterone in the ovaries after menopause probably helps to mitigate bone resorption in postmenopausal women (17,18). The sudden loss of ovarian production of estrogens and androgens in women with surgical menopause leads to a more rapid bone loss compared with women with natural menopause (1921). Genant et al. (24) noted a 7 to 9% decrease in vertebral bone mineral density during the first 2 yr after bilateral oophorectomy, a substantially higher rate than the 1 to 2% loss per year noted after natural menopause (22). Although several studies have noted increased urinary excretion of bone resorption markers after surgical menopause (23,24), to our knowledge, no published studies have shown that women with surgical menopause have higher urinary calcium excretion compared with women with natural menopause. However, the higher rate of bone loss after surgical menopause likely reflects a higher urinary calcium excretion and could account for the increased risk of incident kidney stones we noted in this study. Data on the association between surgical menopause and the excretion of other urinary factors that may influence kidney stone formation such as oxalate, uric acid, or citrate in addition to calcium are scant. More studies are needed to determine whether the type of menopause (natural versus surgical) influences the excretion of these urinary factors.
The increase in urinary calcium excretion associated with menopause may be inhibited by estrogen use. After three weeks of orally administered estrogen in postmenopausal women, fasting urinary calcium decreased by 50% compared to baseline (8). In addition, animal studies have shown that estrogen influences the urinary excretion of oxalate. In oophorectomized rats, the administration of estrogen decreased urinary oxalate excretion by over 50% (25). However, we found no association between past or current PMH use or duration of past or current PMH use and incident kidney stones in postmenopausal women. This lack of an association may be partly explained by an increase in urinary uric acid excretion (26,27), which may promote calcium stone formation (28). After the daily administration of oral conjugated estrogen (1.25 mg) for 4 d while being maintained on a purine-restricted diet, urinary uric acid excretion increased by 23% (P < 0.05) compared with baseline in seven postmenopausal women (26). Similar results were also noted in a study of 22 adult transsexual men treated with estrogen where mean uric acid excretion increased by 28% (P < 0.001) (27). Thus the beneficial effects of estrogen use on urinary calcium excretion may be offset by an increase in urinary uric acid excretion leading to an overall null effect on kidney stone formation.
In our study, information on menopause and PMH use was collected before the passage of a kidney stone. In addition, the high level of education and the strong health interest of the subjects are major strengths of this cohort and favorably influence the quality and accuracy of information reported on the questionnaires. However, one cannot determine when the kidney stones were formed, so it is possible that surgical menopause may lead to the movement of preexisting stones, which then become symptomatic, rather than the new formation of stones. It is also possible that the positive association we found between surgical menopause and incident kidney stones could be due to chance.
Self-reported age at onset of menopause may be subject to error (29,30); however, we excluded all women with indeterminate age at menopause (hysterectomy without bilateral oophorectomy, incomplete data on the extent of pelvic surgery, exogenous estrogen use before the onset of menopause, and permanent cessation of menses due to radiation therapy) from all analyses. Age at onset of menopause is, however, highly accurate in women with bilateral oophorectomy (9). Moreover, we noted no association between age at onset of menopause and risk of incident kidney stones among women with either natural or surgical menopause. In addition, the association between PMH use and incident kidney stones did not change substantially when we controlled for age at menopause in women with natural or surgical menopause. It is thus unlikely that the results of this study were biased by possible misclassification of age at onset of menopause.
In summary, we found no association among natural menopause and PMH use and incident kidney stones. However, there may be a modestly increased risk for incident kidney stone disease with surgical menopause. This increased risk may be due to higher excretion rates of urinary calcium after surgical menopause.
| Acknowledgments |
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| References |
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This article has been cited by other articles:
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E. N. Taylor, M. J. Stampfer, and G. C. Curhan Obesity, Weight Gain, and the Risk of Kidney Stones JAMA, January 26, 2005; 293(4): 455 - 462. [Abstract] [Full Text] [PDF] |
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M. S Parmar Kidney stones BMJ, June 12, 2004; 328(7453): 1420 - 1424. [Full Text] [PDF] |
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