By utilising three longitudinal cohorts representative of the US, UK, and European samples, this study offered unique insight and extensive evidence on the relationship between obesity and UI in older men and women. Specifically, the higher prevalence of UI in older women is linearly correlated with higher BMI and WC, while there was a U-shaped association between BMI and UI in older men. Our study indicated a revisiting of association between obesity and UI among older men compared to women.
Prior epidemiological studies reported that obesity is a modifiable risk factor for UI development in older women or men5–8, 10,11,14,15,17,18, and some clinical trials provided evidence that weight reduction can reduce the incidence of UI9,12,13,16. However, only seven studies—three longitudinal and four cross-sectional—focused on men and women seperately regarding the relationship between UI and obesity indices19,21−26. In particular, the Subak et al.’s cohort study based on the longitudinal assessment of bariatric surgery 2 revealed that obese participants who lose weight via surgical interventions experience reduced UI24, which was in conflict with the male analysis of our study since this study focused on severely obese people with BMI (kg/m^2) varied from 41.6 to 52.8, whereas our study subjects concentrated on the general elderly population.
In addition, the cohort study by Tsui et al. from a British birth cohort study demonstrated that increased BMI at age 60–64 years was an independent risk factor for urgent urinary incontinence (UUI) in men and women26. Another article by Tennstedt et al. examining the population-based adults aged 30–79 years based on the Boston Area Community Health (BACH) Survey investigated that WC was a risk factor for leakage in women rather than men, with the odds of weekly leakage increasing by 15 percent with each 10-cm increase25. In general, our findings concurred with these investigations. However, the participants in these two research specifically targeted the elderly in Britain and Boston, respectively, rather than the elderly worldwide. Second, only UUI, rather than general UI, was used as the outcome assessment in the Tsui et al. study. Furthermore, we could only be enlightened if obesity was related to UUI or UI rather than the movement of UI as WC grew and the tendency of UUI with growing BMI. Also, Tsui et al.’s study did not take central obesity into consideration.
Moreover, even though there were four cross-sectional studies also considering both genders in regard to the UI-obesity associations19,21−23, one of them from the French 3C study that concentrated on the elderly subjects aged 65–101 suggested that the relationship tended to be linear for UI and obesity in females, whereas the pattern that a higher risk of UI for both underweight and obese subjects was found in males23. This study was in line with our findings but concentrated primarily on French and did not take into account the obesity indices for central adiposity.
In terms of mechanisms, several biological studies have pointed out that obesity is associated with low-grade systemic inflammation and the release of pro-inflammatory cytokines, generating reactive oxygen species and oxidative stress, which alters collagen metabolism. At the same time, intra-abdominal pressure increases with obesity that bears down on pelvic tissues, causing chronic strain, stretching, and weakening of the muscles, nerves, and other structures of the pelvic floor, which thus leads to negatively affecting pelvic organ function. These physical and biochemical stresses predispose obese geriatric patients to develop UI8. Comparably, being underweight also resulted in a higher prevalence of UI for males, according to our study. This could be as a result of the several mechanisms through which being underweight may cause disability, especially a lack of physical activity and a higher risk of falls, increasing the chance of UI incidence33–35. The disability would, in turn, lead to underweight due to partial loss of life skills, thus less accessibility to get nutrition through cooking and etc. Nonetheless, the association between being underweight and UI is still debatable and appears to be weaker than the relationship with obesity. The discovery of permanent rather than reversible alterations to the prostate caused by weight loss once men approach old age11, as well as postmenopausal hormonal changes in women, may be viable explanations in light of the mechanism underlying the gender difference. The type of incontinence, however, was not taken into account in this study's outcome measures, which was likely the cause of the less substantial linear association in men compared to women.
This study delivers distinctive and substantial policy implications. On the one hand, insufficient attention has been paid to the relationship between UI and obesity indices in men. For instance, earlier research has focused less on the patterns between UI and obesity in males than in women. Male participants were not even asked in the Korean Longitudinal Study on Aging (KLoSA) questionnaire if they had ever experienced UI within a certain time period. Although men are less prevalent than women to report UI, it is nevertheless essential to revisit the UI-obesity association in men to better corroborate our findings and thus further improve our present UI prevention and treatment practices. On the other hand, the majority of developed countries examine UI and fecal incontinence separately due to crucial diversity in pathogenic mechanisms, risk factors, and treatment approaches, which HRS, ELSA, KLoSA, and SHARE could support. Contrarily, some nationwide cohort studies of aging, including the China Health and Retirement Longitudinal Study (CHARLS), did not differentiate these two disorders clearly and instead created a single question to target them explicitly. Therefore, it was imperative to distinguish UI and fecal incontinence when developing survey questionnaires to advance pertinent intervention research.
There are several strengths. Firstly, our study's excellent generalizability can be attributed to the utilization of three sizable, diverse, well-characterized, cross-cultural longitudinal studies that included older adults from 22 developed and developing countries on two continents. Second, the validity of our study was strengthened by the use of nationally representative samples for participant recruitment, standardization of the three surveys for database comparisons, and a more extended follow-up period of almost ten years. Third, random-effect logistic models regulate individual-level traits that could alter over longitudinal data waves and thus reducing the likelihood of misestimation. Finally, this study provides a basis for demonstrating the obesity paradox in the relationship between obesity and urinary incontinence in men, showing that those older men with both lower and higher BMI and WC tend to report higher UI prevalence compared to those with BMI or WC in the normal range, although this relationship is not particularly significant.
However, our study has limitations. Firstly, due to data availability for UI measurements, the present study has an early but narrower time period for SHARE compared to HRS and ELSA, which could cause an overestimation or underestimation of the underlying relationships between obesity indices and outcomes. Secondly, this study's UI, BMI, and WC measurements rely on self-report rather than clinician diagnosis, which might be a limitation as responses could be influenced by recall and social desirability biases. Additionally, even though we considered numerous potential confounders that had a significant impact on our findings, there may still be confounding variables that are unaccounted for due to limited information, such as ethnicity in SHARE and residential areas in ELSA. A previous study provided insights that indicators of central obesity, including WC and waist-to-hip ratio, appear to be more sensitive than BMI (a proxy for body fat) in explaining the association between obesity and urine leakage22,25. However, our study did not fully consider the indicators of central obesity due to a lack of comparable data. Finally, the type of incontinence was not specified in the longitudinal cohort, which poses an unavoidable obstacle to further research on its mechanisms.
In summary, the associations between obesity indices and UI is different in older women compared to older men. Therefore, weight loss as a treatment for UI can only be applied to older women but not necessarily to men. With this in mind, developing interventions to address UI among older adults should take sex into account.