This study demonstrates that vitamin D deficiency is an independent and important factor in POP in a female postmenopausal population as hypothesised. Significantly lower vitamin D levels were found in POP group and a significant prevalence of vitamin D deficiency was found in patients with POP compared to controls. A significant correlation was found between vitamin D deficiency and cystocele, but not vitamin D deficiency and rectocele. Among the possible factors affecting the pelvic floor, parity and vaginal deliveries showed significant importance as expected, since caesarian section appears to be protective against POP.
Our results regarding vitamin D are consistent with some previous publications. Badalian and colleagues [9] found that higher vitamin D levels are associated with decreased risk of pelvic floor dysfunction in women. The study included women of all ages; PFD was evaluated by anamnestic questionnaire. Based on a cohort of 349 participants Parker-Autry et al. [10] concluded that insufficient vitamin D was associated with increased colorectal symptoms and greater impact of urinary incontinence on the quality of life. However, no significant correlation was found between lower urinary symptoms and vitamin D deficiency in the research by Aydogmus and coworkers [11], but they stressed the necessity of further investigation of pelvic floor integrity and functions. Interestingly, the study of Vaughan and colleagues [18] showed a potential association between vitamin D and the development of urinary incontinence in a racially diverse cohort of older men and women. A very recent study by Kaur et al. [19] confirms that vitamin D levels were associated with a decreased risk of pelvic floor disorders in geriatric females. In their research, Hyung Ahn et al. [20] examined vitamin D levels and performed a VDR genotype analysis. The presence of a certain sequence of the vitamin D receptor (VDR) polymorphism (Apal and Bsmi) was associated with PFD in vitamin D deficient subjects.
To avoid seasonal variations in vitamin D concentrations we opted for the determination of 25-OH-D during winter time. Our study and control groups were meant to be of comparable age and they were weight-matched, both being important factors in vitamin D concentrations. Although the mean age of patients and controls was 62.6 years and 58.7 years, which was statistically significantly different (p < 0.05), this difference was not unexpected given since our study age span included postmenopausal women between 50 and 75 years. Of the most importance is that our participants were weight-matched, because of the known significant inverse correlation between vitamin D levels and BMI [21, 22]. Since vitamin D is soluble in body fat, the reduced bioavailability of vitamin D results in obesity [23].
A strength of our study is that both of our groups were comparable in behavioural factors impacting the pelvic floor: physical and sexual activity, habits (coffee and smoking) and pelvic floor muscle training. Considering reproductive parameters that might influence pelvic floor, parity—being a known risk factor for POP, differed among the two groups (p < 0.05), whereas caesarean section did not. It is known that the odds for POP increase tenfold with single vaginal birth and additional vaginal births cause no significant further increase in risk for POP [24]. However, logistic regression analysis in our study revealed a significant association with vitamin D even after adjustment for parity and other predictors for POP. Vitamin D and parity were significant predictors in the logistic regression model for POP, being predictive in opposite directions: higher vitamin D levels – lower risk for POP, higher parity – higher risk for POP.
PFD-POP was objectified using the POP-Q examination system in the present study. The numerical quantification of the disorder yielded a homogeneous recruitment and classification of participants, where subjective factors were minimized. Previous investigations were not based on POP-Q assessment [9, 10], except for the study of Kaur and coworkers [19], and Adognymus and coworkers [11]. However, none of these studies assessed possible differences in risk among the different compartments of prolapse.
In our study cystocele was associated with vitamin D deficiency, whereas rectocele was not. This was not necessarily a surprise taking into account that abnormal pelvic floor muscles are observed more often in women with anterior prolapse than with posterior prolapse [25], and that anterior prolapse in a great number of cases is also linked to the descent of the apical vaginal support, hence sharing a more complex mechanism in terms of different pelvic support structures involved [26]. Additionally, women with cystocele have the most compliant anterior and posterior vaginal wall support systems when compared to women with rectocele and normal support [4]. As vitamin D plays a role in different support tissues (i.e. striated and smooth muscles, tendons, fascias and connective tissue), we speculate that the likelihood of clinical manifestation of its deficiency is more likely to be pronounced in pelvic floor conditions where many of those support tissues and structures are involved.
We strongly believe that the results of our study are clinically important: for vitamin D deficiency/insufficiency, low cost and effective vitamin D supplementation treatment is available. In an Indian study [19], the regression of symptoms of urinary incontinence as well as the positive effect on the symptoms of PFD were observed after 6 months of treatment. Further clinical studies on the effect of supplementation are needed.