To our knowledge, this is the first meta-analysis which investigated the dietary fiber intake in PCOS, and also the first meta-analysis which compare the consumption of specific dietary component in women with and without PCOS.
On pooling the 12 studies which provided absolute value of fiber intake, we confirmed that, while there was no difference in the total energy intake, PCOS women consumed a significantly lower level of dietary fiber compared with the non-PCOS controls. Influence analysis further confirmed the validity and robustness of the main result. Subgroup analyses were conducted for further interpretation. In the subgroup analysis using geographic location, the difference in dietary fiber intake was found significant in Asia with acceptable between-study heterogeneity, but not found in the other continents. Lower fiber intake was also found in studies that used food diary/records or food recall as dietary assessment method. In addition, studies which had a case-control design or cross-sectional design, or studies which acquired Rotterdam criteria for PCOS definition, also showed a significant difference. When the study on adolescent PCOS was not included, the result was still significant. In the subgroup analysis by whether studies were adjusted or matched by BMI or age, we did not find a meaningful influence on the main results.
On the other hand, however, no significant difference was found when pooling the two studies which adjusted fiber intake by total fiber intake, although they both reported a significantly lower fiber intake in PCOS women compared with the controls. The high heterogeneity between the study may explain the inconsistency with the main result. Also, when we pooled the data stratified by BMI and subgroup according to BMI classification, no difference was found in the dietary fiber intake or in total energy intake
A few studies have corroborated an association between inadequate dietary fiber intake and metabolic disturbance in PCOS. For instance, research has reported an inverse correlation between dietary fiber intake and body fat accumulation, insulin resistance, fasting insulin and glucose tolerance in PCOS women 39,48. A recent randomized controlled trial 50 which used the resistant dextrin (a soluble dietary fiber) as intervention in PCOS women, showed significant improvements of metabolic parameters and inflammatory markers including decrease in the serum level of LDL-C, triglycerides, total cholesterol and high sensitivity C-reactive protein. Besides, research has also revealed a possible beneficial effect of fiber intake on hormonal regulation in PCOS. A study 47 from Italy which investigated on 224 women with and without PCOS reported that, following adjustments for BMI and total energy intake, the testosterone level in PCOS women was significantly negative correlated with the adherence to Mediterranean diet (a dietary pattern rich in fiber), or fiber consumption. The clinical trial by Gholizadeh Shamasbi S 50 also reported an improvement in hyperandrogenism and hirsutism as well as menstrual cycle irregularity in PCOS following dietary fiber intervention. Similar results were also found in another study 48.
Since lower dietary fiber intake is indicated to be associated the metabolic and hormonal disturbances in PCOS, and our result has confirmed a significantly lower level of dietary fiber intake in PCOS women compared with controls, it brings up an interesting question on how dietary fiber intake may influence with PCOS. One of the most important physiological roles of dietary fiber in human is that through direct interaction with the gut microbes, it can beneficially shape the microbial ecosystem and enhances the production of key microbial metabolites 51–53. On the contrary, low dietary fiber intake not only lead to progressive loss of microbial diversity 21,54, but also shifts the microbial metabolism towards utilization of less favorable substrates 55,56 and degradation of protective mucin 57, which are detrimental to the hosts. In PCOS, numerous studies have demonstrated a significant decrease in biodiversity in the gut microbiome 58–62. Whether a low dietary fiber intake contribute to the variation in microbial communities remains unclear.
Another mechanism which dietary fiber intake may affect PCOS is the modulation of microbial metabolites. Short-chain fatty acids (SCFAs), which are key microbial metabolites produced in colon through fermentation of dietary fiber by gut microbes 53,63, are famous for possessing functional roles in regulating host metabolism 52,64−66, immune system 65,67,68, and cell proliferation 69,70. Decrease in fiber intake could possibly affected with the production of metabolites especially SCFAs, and finally influence with the overall health and well-being. Consider that PCOS women consume less dietary fiber, whether there is a reduction in SCFA production remain unclear. Furthermore, by increasing the SCFA level through modulation of dietary fiber intake, or through dietary supplements, whether there might be beneficial effects on the disorder warrants further investigation.
Several limitations of the present meta-analysis should be considered. First, the significant heterogeneity detected could not be sufficiently explained by further meta-regression or subgroup analyses. We attributed the heterogeneity to a number of factors which include: the severity or subtypes of PCOS, inconsistent exclusion criteria, inconsistent nutrient analysis method, measurement or reporting inaccuracy of diet. Second, there were limited number of studies included in the certain subgroups, such as the cohort study or adolescent subgroup, making the results lack certain representation. Third, most of the controls in studies were enrolled from outpatient visitors and may result in a lack of representativeness. Fourth, energy adjustment is advantageous in analyses of diet-disease associations since it mitigates the influence of body size, metabolic efficiency, physical activity etc, and also diminish the measurement errors 71,72. However, only two included studies 40,48 conducted energy adjustment. Thus, it was difficult to yield stronger evidence appreciably for result interpretations. In order to diminish the influence of total energy intake and give a more comprehensive interpretations for the results, we also pooled and compared the overall energy intake. Fifth, as only four studies 37–39, 41 presented information based on BMI classification, it was difficult to confirm the dietary fiber intake level in overweight or obese PCOS women, who are the focus group for dietary or lifestyle interventions. How dietary fiber intake differs in this group remain to be evaluated in future investigations.