Introduction

The International Continence Society (ICS) defined nocturia as the condition in which an individual wakes one or more times to void at night [1]. Clinically speaking, nocturia was typically deemed clinically significant if it happened two or more times nightly. Prior studies had associated nocturia occurring more than twice per night with a decline in quality of life [2,3]. Nocturia is the most prevalent and bothersome among all Lower Urinary Tract Symptoms (LUTS), with as many as 60% of elderly individuals (>70 years) and 15–20% of young adults (20–40 years) experiencing voiding two or more times at night [1,4]. Furthermore, nocturia was associated not only with benign prostatic hyperplasia (BPH) [5], obstructive sleep apnea (OSA) [6], coronary artery disease [7], diabetes [8], sleep disorders [6,9] and other conditions, but also with increased all-cause mortality [3]. Therefore, decreasing nocturia could have profound positive effects on quality of life.

The ratio of family income to poverty (PIR) is calculated by dividing family (or individual) income by the poverty threshold specific to each survey year and is an effective measure of income disparity closely related to human life and health [10]. Previous studies showed that lower family income led to poorer health protection, including the inability to access timely and effective preventive and diagnostic measures, which might have led to various diseases [11]. Furthermore, some claimed that PIR was associated with urgent urinary incontinence in adults. As PIR increased, the risk of urinary incontinence significantly decreased [12]. Additionally, research has found that children from lower-income families are more likely to experience bed-wetting at night [13]. Because large-scale cross-sectional studies investigating the relationship between nocturia and PIR levels were scarce, we utilized data from NHANES 2005–2010 to explore a large population of adults aged 20 and above in the United States.

Materials and methods

2.2 Study population and variables

In this survey, we included 31,034 participants from 2005 to 2010, excluding those under 20 years old (n = 13,902), with missing nocturnal voiding data (n = 2,296), missing Poverty Income Ratio (PIR) data (n = 1,087), or meeting specific criteria indicating potential bias (n = 316). Additionally, 7,149 participants had missing data on other covariates. The final sample consisted of 6,662 participants. The participant selection process is outlined in Fig 1.

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Fig 1. Flow chart illustrating the selection of participants in NHANES from 2005 to 2010.

https://doi.org/10.1371/journal.pone.0303927.g001

The PIR was the independent variable, with nocturia as the dependent variable. Based on family income and federally recognized poverty levels, PIR was categorized into three groups: poor (PIR <1), middle class (PIR 1–4), and affluent (PIR > 4) [14]. Nocturia was defined as answering "two times" or "two times or more" to a standardized questionnaire item regarding nocturnal voiding frequency.

We explored several variables routinely collected in NHANES, including age, gender, race, education level, marital status, drinking status, BMI, and serum cotinine concentration. Hypertension and diabetes were defined based on standardized questionnaire responses. Hypertension was determined by systolic blood pressure levels ≥ 130 mm Hg and/or diastolic blood pressure levels ≥ 80 mm Hg or answer "yes" to any of the following questions: "Have you ever been told by a doctor or other health professional that you had hypertension, also called high blood pressure", "Because of high blood pressure/hypertension, have you ever been told to take prescribed medicine?" [15]. Diabetes was defined by glycosylated hemoglobin levels ≥ 6.5% and/or fasting blood glucose levels ≥ 126 mg/dl, or answer "yes" to any of the following questions: "Have you ever been told by a doctor or health professional that you had diabetes or sugar diabetes?" "Are you now taking diabetic pills to lower your blood sugar?" [16].

Results

3.1 Baseline characteristics

We included 6,662 participants from NHANES cycles between 2005 and 2010 and divided the weighted features into three groups based on PIR. (Table 1) The mean age of the participants was 48.35 ± 0.23, with an average PIR of 3.04 ± 024. Compared to low-income individuals (PIR < 1), those with high income (PIR ≥4) were older (PIR < 1: 43.26 years vs PIR ≥4: 48.93 years) and predominantly male (PIR < 1: 43.92% vs PIR ≥4: 53.40%). They were also more likely to be Non-Hispanic White (PIR < 1: 36.23% vs PIR ≥4: 64.64%) and have completed high school or above (PIR < 1: 24.46% vs PIR ≥4: 75.52%). Furthermore, they were more inclined to be married/living with partner (PIR < 1: 47.69% vs PIR ≥4: 74.93%). The number of individuals with serum cotinine concentration >3 was often lower (PIR < 1: 41.85% vs PIR ≥4: 19.04%), while the number of those consuming alcohol more than 12 times a year was higher (PIR < 1: 65.46% vs PIR ≥4: 79.72%).

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Table 1. Baseline characteristics of participants stratified by PIR.

https://doi.org/10.1371/journal.pone.0303927.t001

3.2 Relationship between PIR and nocturia

The results of multivariate linear regression analysis in three models indicated a significant negative correlation between PIR and nocturia (Model 1, OR = 0.868, 95% CI: 0.836–0.900; Model 2, OR = 0.862, 95% CI: 0.828–0.897; Model 3, OR = 0.875, 95% CI: 0.836–0.916). Further analysis treating PIR as a categorical variable, with poor individuals (PIR<1) as reference, revealed a significant reduction in the risk of nocturia for the middle class (PIR 1–4) and affluent individuals (PIR>4). When PIR was divided into quartiles and compared with Q1, a significant negative correlation with nocturia risk was observed in Q2, Q3, and Q4. (Table 2) Additionally, we conducted restricted cubic spline regression analysis in Model 3, indicating a non-linear negative correlation between PIR and nocturia. (Fig 2).

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Fig 2. Restricted cubic splines showing the relationship between PIR and the presence of nocturia.

https://doi.org/10.1371/journal.pone.0303927.g002

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Table 2. Odds ratios for the prevalence of nocturia stratified by PIR.

https://doi.org/10.1371/journal.pone.0303927.t002

3.3 Subgroup analysis

Based on all covariates, we found an unstable correlation between PIR and nocturia in the stratified subgroup analysis. Although a negative correlation between PIR and nocturia was observed in the majority of subgroups, the results of interaction tests provided no evidence of an association between this negative correlation and the variables (Fig 3).

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Fig 3. Subgroup analysis of the association between family PIR and nocturia.

https://doi.org/10.1371/journal.pone.0303927.g003

Discussion

This study investigated the relationship between PIR and nocturia symptoms in individuals aged 20 and above. Our model construction and analysis revealed a significant negative correlation between PIR and nocturia symptoms (PR: 0.875; 95% CI: 0.836–0.91; P < 0.0001). Furthermore, constructing restricted cubic spline (RCS) curves unveiled a notable non-linear relationship, indicating a gradual reduction in the risk of nocturia with increasing PIR (P value < 0.0001, P for non-linear = 0.0093). Although subgroup analyses were conducted, interaction results indicated that various covariates were not significantly dependent on the relationship between PIR and nocturia.

Nocturia was a prevalent chronic condition associated with various ailments, such as cardiovascular diseases, endocrine disorders, chronic respiratory diseases, and mortality, frequently leading to a considerable decline in quality of life [17–21]. Nevertheless, existing treatment and prevention methods were limited, yielding unsatisfactory management outcomes for nocturia [1]. Prior studies identified several nocturia-related factors, including reduced bladder volume, sleep disturbances, and nocturnal polyuria [22–24]. Nevertheless, as research on nocturia advanced, some studies revealed a significant association between economic income and nocturia. Consequently, we conducted a study utilizing the NHANES database to investigate the relationship between PIR and nocturia. Our results aligned with prior findings suggesting that lower household income was often linked with more adverse effects, and there existed an inverse correlation between PIR levels and nocturia. For instance, a recent cross-sectional study conducted among the Danish population corroborated our findings, indicating a significant decrease in the risk of nocturia among individuals with higher household incomes [25]. Additionally, previous studies had underscored obesity, hypertension, and diabetes as pivotal risk factors for nocturia, aligning with our findings [8,26,27]. Building upon these findings, we postulated that the Household PIR could serve as a potential indicator reflecting the overarching state of nocturia. Moreover, as a measure of economic status, PIR may be a helpful tool in assessing socioeconomic disparities, thus offering a valuable reference point for forecasting the likelihood of nocturia occurrence across diverse demographic groups.

Although the precise mechanism by which PIR influences nocturia remained elusive, we offered potential explanations grounded in existing research. Epidemiological studies indicated that low-income families faced environments detrimental to physical health, struggled with healthy food affordability, and lacked adequate medical coverage [28,29]. Additionally, lower family income correlated with poorer family health protection, hindering timely disease prevention, diagnosis and treatment, thus fostering disease occurrence and progression [11,30]. For instance, women with lower family incomes discussed urinary incontinence with doctors less frequently compared to their higher-income counterparts [31]. Conversely, individuals with higher family incomes could promptly intervene in diseases through early-stage examinations, inhibiting disease progression. [32]. Furthermore, individuals of lower socioeconomic status often contended with heightened stress hormone levels due to various social and psychological factors [33,34]. Stress, as shown in previous studies, played a significant role in the pathophysiology of overactive bladder (OAB) and urge urinary incontinence (UUI), both impacting urinations [34,35]. Considering these points, we asserted that the influence of income level on nocturia risk was plausible. The household PIR could reflect the comprehensive state of nocturia, with a discernible decrease in the risk of nocturia occurring as PIR increases. This observation holds significant importance for the clinical management of nocturia, as it suggests that PIR could be employed as a potential indicator for assessing the risk of nocturia. By evaluating patients’ family income status, one can forecast the likelihood of nocturia and intervene with educational measures and treatment in advance to prevent its onset and mitigate its progression.

A significant strength of our study lies in establishing a nationally representative cohort, bolstering the generalizability of our findings. This analysis marked the first attempt to explore the relationship between nocturia and PIR based on the NHANES database. Additionally, our study identified a negative association between household PIR and nocturia, which carried significant clinical implications. Moreover, we controlled for various demographic, socioeconomic, and disease-related variables within the NHANES dataset. Subgroup analyses were performed, stratified by participant characteristics, to assess the impact of different factors on the association between household PIR and nocturia.

Nevertheless, our study possesses certain limitations. Firstly, owing to its cross-sectional design, causal relationships, and underlying mechanisms could not be deduced. Secondly, the diagnosis of nocturia relied on subjective self-reported questionnaire data, which might not have accurately reflected actual conditions and lacked detailed information on nocturnal voiding patterns, potentially introducing information bias. Furthermore, our sample size was relatively small, possibly introducing selection bias. Additionally, despite accounting for numerous potential confounders and exclusion criteria, other unaccounted confounding factors might have affected the experimental outcomes, potentially introducing confounding bias. Considering these limitations, further prospective or longitudinal studies was needed to validate our findings. Furthermore, larger-scale studies, such as those that included more samples and covariates, would have been necessary to obtain more accurate results.

Conclusion

We have established the correlation between PIR and the incidence of nocturia. An elevation in PIR correlates with a reduced likelihood of experiencing nocturia, indicating its potential utility as an indicator for predicting the probability of developing nocturia. Despite offering specific clinical insights, the precise underlying mechanisms remain elusive. Therefore, further randomized controlled trials are warranted to furnish additional evidence for preventing and screening nocturia within the populace.

Acknowledgments

We extend our gratitude to all participants in the US National Health and Nutrition Examination Survey (NHANES) and value the unrestricted access offered to the public by the NHANES database.