{'fraction_non_alphanumeric': 0.055394460553944605, 'fraction_numerical': 0.0382961703829617, 'mean_word_length': 4.712164477441462, 'pattern_counts': {'":': 0, '<': 14, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Association between low-testosterone and kidney stones in US men: The national health and nutrition examination survey 2011–2012

The purpose of this study was to determine the association between low-testosterone (total testosterone ≤3 ng/mL) and prevalence of kidney stones (KS) in men 20 years and older, and whether this varies by comorbidities, and race/ethnicity, and age. This was a cross-sectional study with data from the National Health and Nutrition Examination Survey (NHANES) 2011–2012 cycle. We found that men with low-testosterone had 41% lower odds of KS as compared to men without low-testosterone after multivariable adjustment (OR: 0.59, 95% CI 0.40–0.86). When stratified by obesity, obese men with low-testosterone had 59% lower odds of KS. When stratified by HDL, men with HDL ≥ 40 mg/dL and with low-testosterone had 40% lower odds of KS. When stratified by diabetes, men without diabetes with low-testosterone had 39% lower odds of KS, but the association was not significant in diabetic men with low-testosterone and other comorbidities. There were significant differences when stratified by race/ethnicity. Finally, when stratified by age, only the subgroup of men ≥40–<60 years old with low-testosterone had 68% lower odds of KS (OR: 0.32, 95% CI: 0.16–0.67). The association between low-testosterone and KS was inversed. Similar associations were identified when stratified by obesity, diabetes, dyslipidemia, race/ethnicity and age.

Despite all these relationships, there is a paucity of evidence on the association between testosterone and KS and how this relationship varied when stratified by obesity, diabetes, dyslipidemia, race/ethnicity, and age.

Survey overview, study design, and subjects
This was a cross-sectional study. We analyzed data collected from the National Health and Nutrition Examination Survey (NHANES) 2011-2012. NHANES is a program of studies undertaken by the National Center for Health Statistics (NCHS) of the Centers for Disease Control and Prevention (CDC) to assess the health and nutritional status of the US population. NHANES uses a multistage, stratified and clustered probability sampling in which Hispanics, non-Hispanic Blacks, and the elderly were oversampled to ensure adequate sample sizes and to represent the total US civilian, non-institutionalized population. (NCHS, 1994) The study population consisted entirely of male respondents, 20 years and older.

Outcome, exposure, and covariates
Data on KS, TT concentrations, demographics, comorbidities, and other covariates were gathered from the 2011-2012 cycle of NHANES. Low-testosterone is defined as TT concentrations < 3 ng/mL. Testosterone is the major male androgen, and free circulating concentrations are controlled by SHBG. Bioavailable testosterone is the concentration of non-SHBG-bound testosterone and is comprised of both free and albumin-bound testosterone. TT is the combination of bioavailable and SHBG-bound testosterone. Testosterone was isolated from 100 μL serum by 2 serial liquid-liquid extraction steps and quantified with [ 13 C] stable isotope-labeled testosterone as the internal standard. The method had a nonsignificant bias to established reference methods at National Institute for Standards and Technology and the University of Ghent. Imprecision over 2 years was < 4.8%, and the limit of detection was 0.3 ng/dL (0.01 nmol/L).

Statistical analysis
NHANES analytical reporting guidelines were followed, adhering to the complex survey design. (NHANES, 2015) Sampling weights were applied to account for selection probabilities, over-sampling, non-response, and differences between the sample and the total US population. Descriptive statistics for participants' characteristics were reported with t-tests for continuous variables and Chi-square for categorical. Adjusted odds ratios (OR) and 95% confidence intervals (CI) using logistic regression models were estimated in relation to total testosterone concentrations with multiple covariates, and stratified by age groups and comorbidities (NHANES, 2014). Outcome was defined as Kidney Stones Exposure was TT concentration < 3 ng/mL, (Low-testosterone). Statistical significance of tests is set at p values < 0.05 (Type I Error). Software utilized for analysis was Stata/IC version 12.1.

Results
Descriptive statistics are reported in Table 1. Mean age of men were 48.75 (SD 17.83, for those without KS, and 58.58 (SD 15.89,; p-value < 0.001). We found that men 20 years and older with Low-testosterone had 41% lower odds of KS than men without Low-testosterone after multivariable adjustment (OR: 0.59, 95% CI 0.40-0.86, Table 2).
When stratified by obesity, men with BMI ≥ 30 kg/m 2 and lowtestosterone had 59% lower odds of KS versus men without low-testosterone (OR: 0.41, 95% CI: 0.21-0.786), but this association was not statistically significant in BMI levels < 30 kg/m 2 (Table 3). When stratified by diabetes, there was no association in men with diabetes and low-testosterone, but men without diabetes and with low-testosterone had 39% lower odds of KS (OR: 0.61, 95% CI: 0.42-0.90 (Table 4). When stratified by high density lipid (HDL) levels, men with HDL > 40 mg/dL and low-testosterone had 40% lower odds of have KS (OR: 0.60, 95% CI: 0.40-0.90), but this association was not significant in men with low-testosterone and with HDL ≤ 40 mg/dL ( Table 5). None of the associations was significant in other lipid levels; i.e. low density lipid (LDL) (cut-off = 120 mg/dL), total glycerides (TG) (cutoff = 150 mg/dL), total cholesterol (cut-off = 200 mg/dL or 240 mg/ dL) (Table 5).
When stratified by race/ethnicity, white men with low-testosterone had 48% lower odds of KS (OR: 0.52, 95% CI: 0.30-0.89) ( Table 6). Hispanic men with low-testosterone had 2.36 times higher odds of KS (OR: 2.36, 95% CI: 1.03-5.38), and there was no significant association in Black (non-Hispanic) men (Table 6). There was no other significant association when stratified by other comorbidities and prescription drug use, (Supplementary tables).
Age was a statistically significant factor in the association between testosterone and KS (Supplementary tables). When stratified by age, men in the age group ≥40-< 60 years and with low-testosterone had 68% lower odds of KS (OR: 0.32, 95% CI: 0.16-0.67, Table 7) but this association was not statistically significant in other age groups, i.e., ≥20-< 40, ≥60-< 80, and ≥80 years.

Discussion
We found that men who were 20 years and older and with Lowtestosterone had 41% lower odds of KS as compared to men without low-testosterone. Contrary to our finding, Otunctemur et al. (Otunctemur et al., 2015) found that that men with testosterone concentrations < 2.85 ng/mL had three time higher odds of KS (OR: 2.93, 95%CI: 1.88-4.56). This could be due to insufficient adjustment for confounders, small sample size, and Berksonian bias in Otunctemur et al.'s study (Otunctemur et al., 2015). Furthermore, our finding was consistent with earlier studies. For example, higher testosterone levels were found in KS patients (3.30 ng/mL, SE: 2.50) as compared to controls (2.41 ng/mL, SE: 1.06; p = 0.003) (Naghii et al., 2014). There was a significant and positive Pearson correlation between free serum testosterone levels and urinary oxalate concentrations (R = 0.297, p- p-value threshold set at 0.05; ⁎ α < 0.05, (ng/mL: nanograms per milliliter), Men (≥20 years), Adjusted for the same number of confounders as in model presented in Table 2. ⁎ Denotes statistical significance at p < 0.05.   Table 2. ⁎ Denotes statistical significance at p < 0.05. value = 0.040) (Nath et al., 2013). Serum testosterone levels were significantly associated with higher urinary excretion of uric acid in patients with KS versus those without (Shakhssalim et al., 2011). Furthermore, kidney stones were consistently reported more often in men than women, and men biologically carry higher testosterone levels than women (Scales et al., 2012;Stamatelou et al., 2003). The association between testosterone and KS in men could be explained by the upregulation of calcium oxalate biosynthesis by androgen receptors (Li et al., 2010;Liang et al., 2014), and testosterone could have a biologic role in KS formation (Naghii et al., 2014). We found that the association between low-testosterone and KS differed by race/ethnicity. Our findings were consistent with the recent findings. For example, Vesper et al. (2015) demonstrated significant variation in total testosterone levels among race/ethnicity. Non-Hispanic Black and Hispanic were 63% and 40% less likely to have KS as compared to Non-Hispanic White, respectively (Scales et al., 2012).
Despite the strengths of our study (e.g.; NHANES is a validated representative sample of the civilian noninstitutionalized U.S. male population, and there is oversampling of minorities and the elderly), our findings should be evaluated with caution because NHANES is a crosssectional study; therefore, causality cannot be inferred in any of the observed associations or suggest any clinical practice change. There could be an inherent bias in the use of surveys, and measurements were largely obtained at a single-time point.
In conclusion, we found men with low-testosterone had significantly 41% lower odds of KS. This finding should be further studied in cohort studies that control for confounding and bias and with longitudinal follow-up of patients.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This study complies with current ethical considerations. No informed consent was required for this investigation because all data is from the publicly available database of the National Health and Nutrition Examination Survey, which complied with all of the Code of Ethics of the World Medical Association (Declaration of Helsinki) and the privacy rights of human subjects.

Conflict of interest
Authors have no conflict of interest for this manuscript.